Abstract

Pharmaceutical particle engineering does not simply aim to minimize particle size. Its practical objective is to produce a reproducible particle or vesicle population that is physically stable, manufacturable, compatible with the intended route of administration, and capable of delivering the required dose and release profile.

9. Hydrodynamic Cavitation in Pharmaceutical Particle Engineering, Crystallization, and Drug Delivery

9.1 Introduction and Design Basis

Pharmaceutical particle engineering does not simply aim to minimize particle size. Its practical objective is to produce a reproducible particle or vesicle population that is physically stable, manufacturable, compatible with the intended route of administration, and capable of delivering the required dose and release profile.

Median particle size alone can be misleading. A formulation may still fail when the coarse tail, fines fraction, morphology, polymorphic form, surface chemistry, or free drug fraction remains uncontrolled. Development should therefore connect material attributes and process parameters to clinically and operationally relevant quality attributes [7-16].

This chapter examines four process families used in pharmaceutical particle engineering:

Top down processing: Techniques that reduce existing particles, droplets, agglomerates, or vesicles through fracture, deagglomeration, shear, impact, and interfacial breakup.

Bottom up processing: Methods that create particles directly from solution through controlled supersaturation, nucleation, growth, and stabilization.

Controlled hydrodynamic cavitation: An intensive flow process that uses pressure reduction, cavity formation, and collapse to promote mixing, particle breakage, nucleation, and surface treatment.

Lipid based delivery systems: Methods that use bilayer self assembly and controlled mechanical remodeling to create and stabilize liposomes and related vesicles.

Across these process families, hydrodynamic cavitation and fixed geometry microchannel processing concentrate mechanical energy within a small liquid volume. Fluid acceleration, pressure reduction and recovery, turbulence, shear, impact, and cavity collapse can be used to connect raw material attributes with particle, crystal, droplet, or vesicle quality.

9.1.1 Critical Quality Attributes

Critical quality attributes should be defined before operating conditions are optimized. A target such as D50 is rarely sufficient by itself. D10, D90, distribution span, polydispersity, number weighted and volume weighted distributions, morphology, surface charge, crystal form, free drug, encapsulation efficiency, and release behavior may all be decisive. For sterile products, particulate burden, bioburden, endotoxin, container compatibility, filterability, and aseptic processing controls must also be incorporated [7-14,36-38].

Table 9.1. Representative critical quality attributes for pharmaceutical particles and liposomes. Source: compiled from [7-16,36-38].

Attribute	Why it matters	Representative methods
D10, D50, D90, span, PDI	Controls dissolution, filtration, injectability, sedimentation, optical appearance, and dose uniformity.	Laser diffraction, DLS, nanoparticle tracking analysis, microscopy
Morphology and aspect ratio	Affects flow, packing, breakage, filtration, syringeability, and aerodynamic behavior.	Optical microscopy, SEM, TEM, image analysis
Crystal form and crystallinity	Controls solubility, dissolution, stability, mechanical response, and possible conversion during processing.	Powder X ray diffraction, DSC, Raman, solid state NMR
Surface chemistry and zeta potential	Affects wetting, electrostatic stabilization, protein adsorption, aggregation, and biological interaction.	Electrophoretic mobility, XPS, surface tension, contact angle
Encapsulation efficiency and free drug	Determines dose distribution between carrier and continuous phase, toxicity, and release.	Chromatographic separation, ultrafiltration, HPLC or LC MS
Residual solvent and impurities	May limit safety, stability, and regulatory acceptability.	Gas chromatography, HPLC, ICP MS, extractables and leachables studies
Release profile and leakage	Determines exposure, shelf stability, and functional performance of the delivery system.	In vitro release, dialysis, flow through cells, accelerated leakage studies
Microbial attributes and sterility assurance	Essential for parenteral and ophthalmic products.	Bioburden, endotoxin, sterility testing, media fills, environmental monitoring

9.1.2 Product Performance Begins with Particle Geometry and Interfaces

For equal mass of spherical particles, reducing diameter increases the total surface area in direct proportion to the inverse of diameter. The idealized gain can be large. Reducing a 100 µm particle to 1 µm increases specific surface area by a factor of 100, provided that the particles remain discrete and the surface is accessible to the dissolution medium. In practice, aggregation, poor wetting, hydrophobic surfaces, a diffusion layer, or crystal growth can partially eliminate this theoretical advantage [6].

The Noyes and Whitney form expresses the central relation between accessible surface area and dissolution rate [6]:

$$\frac{dM}{dt}=kA(C_s-C)$$

(9.1)

where M is the dissolved mass; t is time; k is the overall mass transfer coefficient; A is the accessible particle surface area; Cₛ is the concentration at the particle surface; and C is the concentration in the bulk liquid.

For monodisperse spherical particles, the specific surface area is:

$$\frac{A}{m}=\frac{6}{{\rho }_p{d}_p}$$

(9.2)

where A is total particle surface area; m is particle mass; ρₚ is particle density; and dₚ is the diameter of a spherical particle.

Figure 9.1. Idealized influence of particle diameter on specific surface area and dissolution capacity for a fixed particle mass. Original calculation based on Equations (9.1) and (9.2).

Very small crystals can exhibit curvature dependent solubility. The Ostwald and Freundlich relation indicates that the apparent saturation concentration rises as particle radius decreases. This effect can help explain improved dissolution, but it can also drive Ostwald ripening because the smallest particles are thermodynamically less stable and may dissolve while larger particles grow.

$$\ln \left(\frac{C_s(r)}{C_{s,\infty }}\right)=\frac{2\gamma V_m}{rRT}$$

(9.3)

where Cₛ(r) is the saturation concentration for a particle of radius r; Cₛ,∞ is the saturation concentration at a planar interface; γ is interfacial energy; Vₘ is molar volume; R is the gas constant; and T is absolute temperature.

Brownian diffusion also becomes more important as hydrodynamic diameter decreases. The Stokes and Einstein expression is useful for estimating molecular or nanoparticle diffusivity in dilute Newtonian systems:

$$D=\frac{k_{B}T}{3\pi \mu d_h}$$

(9.4)

where D is the translational diffusion coefficient, $k_B$ is the Boltzmann constant, T is absolute temperature, μ is dynamic viscosity, and dₕ is hydrodynamic diameter.

Engineering implication: Particle size reduction creates new surface area and new interfacial energy. The stabilizer system must be developed at the same time as the energy input. A process that creates small particles faster than the formulation can stabilize them may produce temporary size reduction followed by aggregation, ripening, or leakage.

9.2 Scientific Foundation of Hydrodynamic Cavitation and High Pressure Microchannel Processing

9.2.1 Hydrodynamic Cavitation Sequence

Hydrodynamic cavitation occurs when local static pressure falls to or below the effective vapor pressure of the liquid, allowing cavities to form. The cavities grow in the low pressure region and collapse when they enter a region of pressure recovery. Bubble collapse can create pressure pulses, liquid microjets, high strain rate deformation, localized energy dissipation, and steep gradients in temperature and concentration. These effects can promote particle fracture, droplet breakup, deagglomeration, surface cleaning, primary nucleation, secondary nucleation, and micromixing [1-5,29-32].

Figure 9.2. Idealized sequence of controlled hydrodynamic cavitation in a flow through constriction. Source: based on [1-5,29-32].

A convenient dimensionless indicator is the cavitation number. Definitions vary with device geometry and reference pressure, so the selected definition must be reported. Lower values generally indicate a greater tendency toward cavitation, but the cavitation number alone does not define collapse intensity or product response. Back pressure, dissolved gas, vapor pressure, temperature, viscosity, surface tension, nuclei population, residence time, and geometry remain important [1,3]-[5].

$$\sigma =\frac{p_2-p_v}{\frac{1}{2}\rho u^2}$$

(9.5)

where σ is the cavitation number; p₂ is downstream or recovery pressure; pᵥ is liquid vapor pressure; ρ is liquid density; and u is the selected characteristic velocity.

The Reynolds and Weber numbers help separate viscous, inertial, and interfacial effects. They are especially useful when comparing devices or scaling a process across channel dimensions and flow rates.

$$\text{Re=}\frac{\rho uL}{\mu }$$

(9.6)

where Re is the Reynolds number; ρ is density; u is characteristic velocity; L is characteristic length; and μ is dynamic viscosity.

$$We=\frac{\rho u^{2}d}{\gamma }$$

(9.7)

where We is the Weber number; d is particle or droplet diameter; and γ is interfacial tension. A higher Weber number indicates greater inertial stress relative to interfacial restoring stress.

9.2.2 Energy Input, Dissipation, and Residence Time

Pressure alone is not a transferable measure of process severity. The specific pressure energy of an incompressible stream provides a first approximation of the maximum mechanical energy available from a pressure drop. Actual useful energy is lower because of pump losses, heat generation, flow bypass, incomplete cavitation, and energy dissipation outside the active zone.

$$e_p=\frac{\mathrm{\Delta }p}{\rho }$$

(9.8)

where eₚ is the ideal specific pressure energy; Δp is pressure drop; and ρ is density.

For a defined active processing volume, mean energy dissipation rate can be estimated from power input. Local dissipation in constrictions, jets, and collapse regions can be orders of magnitude higher than the volume average. A design report should state whether a value is local, volume averaged, or system averaged.

$$\epsilon =\frac{P}{\rho V}$$

(9.9)

where ε is the mean energy dissipation rate; P is power dissipated in the selected volume; ρ is density; and V is the selected active volume.

The Kolmogorov length is a turbulence scale rather than a direct particle size prediction. It indicates the smallest scale of turbulent eddies in an idealized isotropic field. When formulation structures are larger than the local dissipation scales, strong deformation and rapid micromixing become more likely.

$${\eta }_K={\left(\frac{{\nu }^3}{\epsilon }\right)}^{1/4}$$

(9.10)

where ${\eta }_K$ is the Kolmogorov length scale; ν is kinematic viscosity; and ε is energy dissipation rate.

Residence time and exposure history are equally important. A short intense exposure may preserve a sensitive payload better than a longer moderate exposure, even when the total energy per unit mass is similar. Recirculation creates a distribution of pass histories unless the loop volume, mixing, and withdrawal protocol are controlled.

$$\tau =\frac{V}{Q}$$

(9.11)

where τ is nominal residence time; V is process or hold volume; and Q is volumetric flow rate.

9.2.3 Bubble Collapse Models and Their Limits

The Rayleigh collapse time gives an ideal estimate for an empty spherical cavity in an infinite inviscid liquid. It is useful for scale reasoning but does not include noncondensable gas, heat transfer, viscosity, compressibility, wall effects, neighboring cavities, or nonspherical collapse [2,3].

$$t_c=0.915R_0\sqrt{\frac{\rho }{p_{\infty }-p_v}}$$

(9.12)

where $t_c$ is the ideal collapse time; R₀ is initial cavity radius; ρ is liquid density; p∞ is far field pressure; and pᵥ is vapor pressure.

The Rayleigh and Plesset equation provides a more complete radial force balance. Even this equation is an idealization when applied to dense cavity clouds, high speed jets, or complex pharmaceutical suspensions. Nevertheless, it clarifies how pressure, viscosity, interfacial tension, and bubble wall acceleration interact [2-5].

$$\rho (R\ddot{R}+\frac{3}{2}{\dot{R}}^2)=p_B(t)-p_{\infty }(t)-\frac{2\gamma }{R}-\frac{4\mu \dot{R}}{R}$$

(9.13)

where R is bubble radius; dR/dt and d²R/dt² are bubble wall velocity and acceleration; $p_B$ is pressure inside the bubble; p∞ is far field pressure; γ is surface tension; and μ is dynamic viscosity.

9.2.4 Microfluidizer Processing

Microfluidizer processors use a constant pressure pumping system and a fixed geometry interaction chamber. The supplied technical documentation reports fluid velocities of up to approximately 500 m/s and identifies hydrodynamic cavitation within the interaction chambers. The principal processing mechanisms include high shear, impact against channel walls or opposing streams, turbulence within microliter volumes, and bubble collapse [17,18,33-35].

Y type chambers: These chambers are commonly promoted for oil in water emulsions, liposomes, and polymer encapsulation. The supplied documentation lists representative microchannel dimensions in the 75 to 125 µm range [33-35].

Z type chambers: These chambers are primarily used for intensive particle processing in which acceleration, turning, and impact contribute to deagglomeration and size reduction [17,18,35].

Figure 9.3. Simplified principles of Y type and Z type interaction chambers. Source: based on [17,18,35].

9.3 Processing Platform Selection

9.3.1 Top Down and Bottom Up Strategies

Top down processing begins with a coarse solid, agglomerate, droplet, or vesicle and applies mechanical stress until the structure reaches a limiting size. The limit may be imposed by crystal strength, primary crystal dimensions, interfacial tension, stabilizer coverage, viscosity, channel size, or a balance between breakup and recoalescence. Bottom up processing begins with dissolved species and controls the rate of supersaturation generation, nucleation, growth, and stabilization. Bottom up crystallization can create particles below the primary crystal size of the feed because the particles are formed rather than broken [26,27,29,30].

Figure 9.4. Top down size reduction and bottom up crystallization as complementary particle engineering strategies. Source: based on [17,18,26,27].

A hybrid route is often preferable. For example, controlled antisolvent crystallization can create a narrow nanosuspension, followed by a short low severity pass to break weak agglomerates and establish final dispersion. A liposome process can use self assembly to create vesicles, then high pressure processing to reduce lamellarity and size, followed by extrusion or sterile filtration. A cavitation crystallizer can use cavity collapse both to intensify nucleation and to fragment crystals that become too large, thereby increasing secondary nucleation [1,29,30].

9.3.2 Technology Selection Criteria

Technology selection should begin with the product target and failure modes. Dry jet milling is attractive when a dry powder is required and heat or solvent exposure must be avoided, but it may generate electrostatic charging, broad tails, or amorphous content. Wet media milling can reach the nanometer scale but introduces media wear, separation, and cleaning considerations. Rotor stator systems are simple and scalable but usually have lower local energy density. High pressure homogenizers and Microfluidizer processors are established for emulsions, nanosuspensions, and liposomes. Controlled hydrodynamic cavitation is attractive when pressure recovery and cavity collapse can be engineered for continuous processing, particle breakage, nucleation, or surface treatment. MRT and related impinging jet crystallizers are particularly useful when fast antisolvent mixing is the controlling step [17-35,40,41].

Figure 9.5. Qualitative technology selection matrix for common pharmaceutical particle engineering methods. Original synthesis based on the sources cited in Section 9.3.

Selection rule: Choose the mechanism that directly controls the dominant product risk. Use top down processing when the feed crystal is suitable and fracture is predictable. Use bottom up processing when nucleation and growth control are necessary. Use a hybrid route when formation, deagglomeration, and surface stabilization must be separated.

9.4 Top Down Particle Size Reduction and Nanosuspension Processing

9.4.1 Breakage, Deagglomeration, and Limiting Size

Solid particles can be reduced by brittle fracture, fatigue, abrasion, interparticle collision, impact with equipment surfaces, and cavitation induced pressure pulses or microjets. Weak agglomerates usually break before primary crystals. Once the agglomerates are removed, further reduction depends on crystal mechanical properties and defect structure. Needle and plate crystals may break anisotropically, so D50 can decrease while aspect ratio or one dimension remains relatively unchanged. Modern hydrodynamic cavitation work on pharmaceutical crystals has reported a rapid initial breakage regime followed by a slower regime and a limiting size, reinforcing the need to model pass count rather than assume linear reduction [40,41].

For suspensions, surface coverage by stabilizer is a kinetic constraint. A newly fractured surface that is not rapidly wetted and protected can reagglomerate. Stabilizer concentration should therefore be related to created surface area, not only to bulk solids concentration. Temperature and ionic strength can change adsorption and electrostatic screening. When organic solvents are present, solvent quality can alter both cavitation behavior and surface stabilization [1,17-28].

9.4.2 Florfenicol Example from the Supplied Cavitation Material

The supplied hydrodynamic cavitation article presents florfenicol as a particle size reduction example. The starting commercial material had a broad distribution with a reported mean near 100 µm. Processing at a reported working pressure of 70 MPa and a crystal concentration of 12 wt% reduced the mean to approximately 10 µm and narrowed the distribution [1].

Figure 9.6. Florfenicol before and after cavitation processing, with illustrative particle size distributions. Source: [1].

9.4.3 Pressure, Pass Count, and Liquid Phase Effects

The supplied data indicate that higher operating pressure can reduce the mean particle size of selected active pharmaceutical ingredients. In one example, material with an unmilled mean size of approximately 109 µm was compared with material processed at 0.7 MPa and 10.3 MPa. The higher pressure condition produced a reported mean particle size of approximately 6.3 µm [28].

Figure 9.7. Comparison of unmilled, low pressure, and high pressure cavitation milled active ingredient particle size distributions. Source: [28].

A related pressure plot shows a decrease in mean volume diameter as pressure increases. This trend is physically plausible because pressure drop and pressure recovery influence bubble formation and collapse intensity. In pharmaceutical development, this relationship should be evaluated together with temperature rise, equipment wear, impurity formation, and potential changes in particle form [2-5,17].

Figure 9.8. Mean active ingredient particle size as a function of cavitation processing pressure. Source: [1]

Pass count controls cumulative exposure to the cavitation zone. Multiple cycles can reduce particle size and narrow the distribution until the system approaches a limiting condition, after which additional processing provides little benefit. Figure 9.9 compares feed material, a single pass, and continuous cavitation milling. In the reported experiments, near steady conditions were reached after approximately 60 to 90 cycles at 0.7 to 7 MPa, 15 to 30 cycles at 34.4 MPa, and 2 to 10 cycles at 70 to 100 MPa. The required cycle number depends on material properties, concentration, formulation, and device design. The cited experiments were conducted without the surfactants and dispersants commonly used to assist particle size reduction [28].

Figure 9.9. Particle size distributions for feed, single pass, and continuous cavitation milling. Source: [28

The solvent or liquid phase can influence cavitation because viscosity, vapor pressure, surface tension, and dissolved gas content affect bubble behavior. In one API experiment, two organic solvents produced similar particle size trends.

Figure 9.10. Influence of solvent on active ingredient crystal size as a function of processing pressure. Source: [1]; graph recolored for consistent presentation.

Solids concentration is an important variable in particle reduction because each milling or dispersion method has a practical concentration range. In high pressure homogenization, solids concentration is often limited by the viscosity increase that accompanies higher solids loading, which can reduce turbulence and impair dispersion.

The concentration study indicates that increasing solids loading improved dispersion within the investigated range.

Figure 9.11. Influence of crystal concentration on particle size distribution at constant cavitation number. Source: [28]

9.4.4 Reproducibility and Scaling

For production use, cavitation processing must be reproducible between batches and transferable from laboratory to pilot and commercial scales. Particle size variability can affect product performance and may create batch rejection, regulatory, and economic risk.

Three pilot scale batches processed at 11.3 L/min produced closely overlapping particle size distributions. The supplied scale comparison also shows similar distributions at two flow rates under 1000 and 2000 psi conditions [28].

Figure 9.12. Reproducibility of three pilot scale cavitation milling batches and comparison of particle size distributions at two flow rates under 1000 and 2000 psi conditions. Source:[1], [28]

9.4.5 Impinging Cavitating Jet Examples for Pharmaceutical Suspensions

The Microfluidics data provide several useful examples of how pressure, chamber geometry, and pass count were applied to pharmaceutical suspensions. A penicillin suspension processed in an M-210B at 18,000 psi used an upstream H230Z auxiliary module and downstream J210Z interaction chamber. One pass reduced the reported mean size from 53 µm to 12.1 µm [19]. An ophthalmic suspension processed in an M-110EH at 25,000 psi with H30Z and H10Z chambers was reduced from a reported 5 µm mean diameter to 0.75 µm after 35 continuous passes [20]. A 25% oral drug slurry processed in an M-110Y at 18,000 psi with H30Z and H10Z chambers was reduced from 28.407 µm to 0.831 µm after ten passes [21].

The BioPharmaceutical Formulations brochure provides additional laboratory cases. A drug nanosuspension for irritable bowel syndrome decreased from a reported 9.089 µm to 0.385 µm after 25 passes at 22,000 psi. A crystalline general anesthetic dispersion decreased from 29.141 µm to 0.142 µm after two passes at 18,000 psi. A crystalline organic salt decreased from 18.003 µm to 0.573 µm after 20 passes at 20,000 psi [24]. These examples demonstrate strong formulation dependence in pass efficiency.

Figure 9.13. Selected particle and vesicle size changes in the supplied laboratory examples. Source: compiled from [1,19-24].

Table 9.2. Selected pharmaceutical particle size reduction cases. Source: Microfluidics data [19-21,24].

Product	Processor and conditions	Starting size	Processed size	Interpretation
Penicillin suspension	M-210B; 18,000 psi; H230Z plus J210Z; 1 pass	53 µm mean	12.1 µm mean	Large one pass reduction, but final product remains in the low micrometer range.
Ophthalmic suspension	M-110EH; 25,000 psi; H30Z plus H10Z; 35 continuous passes	5 µm mean	0.75 µm mean	Submicron target required high cumulative exposure.
25% oral drug slurry	M-110Y; 18,000 psi; H30Z plus H10Z; 10 passes	28.407 µm mean	0.831 µm mean	High solids loading was processed to submicron mean size.
IBS drug nanosuspension	22,000 psi; 25 passes	9.089 µm	0.385 µm	Progressive reduction with diminishing returns.
General anesthetic dispersion	18,000 psi; 2 passes	29.141 µm	0.142 µm	Rapid reduction in the supplied laboratory case.
Crystalline organic salt	20,000 psi; 20 passes	18.003 µm	0.573 µm	Strong early reduction, followed by slower approach to a limiting size.

9.4.6 Pressure, Chamber Geometry, and Pass Count

Higher pressure generally increases fluid velocity and local stress for a fixed chamber, but pressure response is not universal. The same pressure can produce different results when chamber geometry, fluid viscosity, temperature, surface tension, vapor pressure, solids concentration, or stabilizer system changes. The supplied Microfluidics pharmaceutical emulsion chart shows a decline in mean particle size from approximately 268 nm at 10,000 psi to approximately 127 nm at 20,000 psi after one pass. The curve also flattens at higher pressure, which is consistent with a practical limiting size [25].

Figure 9.14. Effect of pressure on mean particle size for a pharmaceutical emulsion after one pass. Source: Microfluidics data [25].

Pass count and chamber geometry interact. The supplied three percent oil emulsion example shows that both 17,000 psi and 23,000 psi conditions decreased size with additional passes. The two chamber configurations approached similar final sizes after sufficient recirculation even though their early trajectories differed. This indicates that chamber choice can alter pass efficiency and that an apparently superior early pass may not produce a different steady condition [25].

Figure 9.15. Combined influence of pass count, chamber geometry, and pressure in a pharmaceutical emulsion example. Source: Microfluidics data [25].

Figure 9.16. Diminishing returns with cumulative passes in two supplied pharmaceutical suspension examples. Source: [24].

9.4.7 Naproxen Cavitation Milling Example

The supplied study reports cavitation processing of a 7 wt% naproxen suspension containing hydroxypropyl cellulose, sodium lauryl sulfate, and water. The control D50 was 8.27 µm. At 62.0 MPa and 102.3 cumulative passes, D50 decreased to 2.41 µm. At the same pressure and 204.7 passes, D50 decreased further to 2.10 µm. The additional reduction between 102.3 and 204.7 passes was substantially smaller than the initial reduction. At 172.4 MPa and 35 passes, D50 was 2.41 µm, showing that pressure and pass count cannot be optimized independently [1].

Table 9.3. Naproxen cavitation milling results. Source: supplied cavitation article [1].

Pressure, MPa	Cumulative passes	D[4,3], µm	D10, µm	D50, µm	D90, µm
Control	Not applicable	10.43	3.07	8.27	20.28
17.2	50.7	5.66	2.16	4.73	10.37
62.0	102.3	2.76	1.18	2.41	4.83
62.0	204.7	2.31	1.10	2.10	3.85
172.4	35.0	2.83	1.19	2.41	5.03

9.4.8 Comparison with Conventional Particle Reduction Technologies

Conventional pharmaceutical particle size reduction technologies include rotor stator wet milling, dry jet milling, wet media milling, and high pressure homogenization. Each method has valid applications in development and manufacturing. Selection depends on active ingredient properties, dosage form, target size distribution, sterility requirements, excipient and solvent systems, batch size, cleaning burden, and acceptable risks of degradation or contamination [9-15,18-25,28].

For the same active ingredient in the supplied comparison, cavitation milling produced the smallest mean particle size and a relatively narrow distribution compared with dry milling and rotor stator milling [28].

Figure 9.17. Comparison of cavitation, dry, and rotor stator milling for the same active ingredient. Source: [28]

Media milling and cavitation processing without milling media should be compared with respect to contamination, degradation, scale up, cleaning, processing time, and particle size distribution. Eliminating milling media removes one potential source of particulate contamination and can simplify cleaning, but these advantages must be balanced against cavitation equipment wear, heat generation, pump requirements, and the need to demonstrate product performance with validated analytical methods [18-22].

Media milling may introduce contaminants from the milling media and may require long processing cycles, media separation, and extensive cleaning. A media free cavitation process removes that specific contamination source and may reduce processing time. However, cavitation systems still require control of equipment wear, heat removal, pressure history, and scale dependent hydraulic behavior. Claims of narrower distributions or easier scale up should be confirmed for the formulation and commercial equipment under study.

High pressure homogenizers are widely used for emulsions and nanosuspensions. In the selected propofol examples, the supplied cavitation data show narrower particle or droplet size distributions with fewer passes or at lower pressure than the reported gap homogenizer conditions [1]. These comparisons are illustrative development data, not a universal ranking of technologies. Commercial performance depends on the complete formulation, equipment configuration, and manufacturing process [9-15].

Figure 9.18. Micrographs of commercial propofol and high pressure gap homogenizer lots. Source: [1].

Figure 9.19. Micrographs of commercial propofol and additional homogenizer lots. Source: [1].

Figure 9.20. Particle size and micrograph comparison between cavitation emulsion technology and a gap homogenizer. Source: [1].

Figure 9.21. Additional comparison of cavitation and gap homogenizer particle and emulsion processing. Source: [1].

9.4.9 Scale Up, Reproducibility, and Wear

Scale up should preserve the local mechanism that controls the product. For fixed geometry microchannel systems, scale up commonly uses parallel channels while maintaining channel dimensions, pressure, and formulation. For controlled hydrodynamic cavitation, scale up must preserve cavity inception, pressure recovery, active volume, and collapse field, not merely pump pressure. In both cases, flow distribution and heat removal can become limiting at higher throughput [1,17,18,33-35,39].

Equipment wear is a critical lifecycle variable. Chamber or orifice wear changes hydraulic resistance, flow rate, velocity, pressure drop, and possibly the product distribution. A validated process should include a wear indicator, flow acceptance range, pressure trace, preventive replacement criterion, and post maintenance equivalence check. For high value pharmaceutical products, metal or ceramic wear particles should be monitored using a risk based analytical strategy.

Modern hydrodynamic cavitation studies on pharmaceutical crystals have shown that crystal breakage can exhibit two regimes, with rapid reduction at low pass count and slow reduction after a threshold. This behavior supports the use of empirical limiting size models or population balance models during development rather than simple linear pass extrapolation [40,41].

9.5 Cavitation Assisted Crystallization and Bottom Up Particle Creation

9.5.1 Supersaturation, Nucleation, and Growth

Crystallization forms particles from dissolved molecules. It therefore controls attributes that cannot always be repaired by later milling, including polymorphic form, crystal habit, internal defects, residual solvent inclusion, surface condition, and primary crystal size. Conventional batch antisolvent crystallization can create local zones of high supersaturation near the feed point and low supersaturation elsewhere. This spatial nonuniformity can produce a broad distribution of nucleation and growth histories. Rapid micromixing reduces the lifetime of these gradients and can favor a large nucleation population before extensive growth occurs [26-30].

Supersaturation is the thermodynamic driving force for nucleation and growth. Concentration based supersaturation is commonly expressed as the ratio of actual solute concentration to equilibrium solubility at the same temperature and solvent composition:

$$S=\frac{C}{C_{eq}}$$

(9.14)

where S is the supersaturation ratio; C is actual solute concentration; and $C_{eq}$ is equilibrium solubility at the same temperature and solvent composition. A value of S greater than 1 denotes a supersaturated solution.

Classical nucleation theory represents the competition between favorable bulk free energy and unfavorable creation of a new interface. The homogeneous nucleation barrier decreases sharply as the magnitude of the volumetric free energy driving force increases. Heterogeneous surfaces, seed crystals, impurities, bubbles, and equipment walls can reduce the effective barrier.

$$\mathrm{\Delta }{G}^{\text{*}}=\frac{16\pi {\gamma }^3}{3{\left(\mathrm{\Delta }{G}_v\right)}^2}$$

(9.15)

where ΔG* is the critical nucleation free energy barrier; γ is crystal solution interfacial energy; and ΔGᵥ is the volumetric free energy change for formation of the solid phase.

$$J=J_0\exp (-\frac{\mathrm{\Delta }{G}^{\text{*}}}{k_{B}T})$$

(9.16)

where J is nucleation rate; J₀ is the kinetic preexponential factor; $k_B$ is the Boltzmann constant; and T is absolute temperature.

Figure 9.22. Conceptual free energy barrier for formation of a critical nucleus. Source: diagram based on classical nucleation theory and [26-30].

The equations clarify why mixing and surface effects matter. Fast solvent and antisolvent contact generates supersaturation. Cavities and interfaces can provide transient heterogeneous sites. Collapse induced pressure pulses and collisions can dislodge incipient crystals, create new fragments, and increase secondary nucleation. At the same time, excessive temperature rise or insufficient stabilization can accelerate growth, agglomeration, or polymorphic conversion. The net result depends on the full time history of supersaturation, temperature, and surface coverage [1,5,29,30].

9.5.2 How Hydrodynamic Cavitation Can Alter Crystallization

Hydrodynamic cavitation can influence crystallization through several coupled mechanisms. Rapid acceleration and pressure reduction can intensify macro mixing and micro mixing. Cavity interfaces can act as transient nucleation sites. Collapse can create localized high pressure, deformation, and secondary nucleation by crystal fragmentation. The circulating flow can return crystals to active zones, creating repeated opportunities for breakage and surface renewal. In a multistage device, nucleation and growth sections can be separated so that different energy levels are applied to different stages [1,29,30].

Figure 9.23. Conceptual sequence for cavitation influenced crystallization. Original schematic based on [1,29,30].

The patent literature describes several flow arrangements. One process mixes feed solution and antisolvent, passes the combined stream through a local constriction to create hydrodynamic cavitation and seed crystals, then uses an intermediate section and a second cavitation growth section. Other embodiments vary the sequence of nucleation, growth, recirculation, and pressure recovery. These patents provide useful engineering concepts, but product specific claims require experimental confirmation and freedom to operate analysis [29-32].

9.5.3 Naproxen Cavitational Crystallization Example

In the example of naproxen crystallization via cavitation, a solution of the substance in ethanol was introduced into an aqueous antisolvent phase containing polyvinylpyrrolidone and a Pluronic stabilizer. The cavitation device was equipped with a single 2 mm diameter orifice and operated at a pressure of 0.7 MPa and a flow rate of 12 L/min. Comparative data showed that the D4.3, D50, D80, and D90 values obtained via cavitation crystallization were significantly lower than those obtained via conventional crystallization with stirring; specifically, the D50 values were 0.35 µm and 2.91 µm, respectively [1].

Figure 9.24. Particle size metrics for cavitational and classical naproxen crystallization in the supplied study. Source: [1].

Table 9.4. Naproxen crystallization comparison. Source: [1].

Process	D[4,3], µm	D10, µm	D50, µm	D80, µm	D90, µm
Cavitational crystallization	0.72	0.12	0.35	1.13	1.87
Classical stirred crystallization	6.73	0.31	2.91	10.19	18.84

This result is consistent with fast supersaturation generation and a high nucleation to growth ratio. It does not establish that cavitation will always outperform a stirred crystallizer. Solvent system, stabilizer adsorption, temperature, antisolvent ratio, nucleation kinetics, orifice geometry, back pressure, and post process hold time can reverse the result. The product should be followed after processing because submicron particles may grow, transform, or agglomerate during solvent removal or storage.

9.5.4 Microfluidics Reaction Technology and the PureNano Crystallizer

Microfluidics Reaction Technology, or MRT, is a bottom up platform based on impinging cavitating jets. The supplied paper describes two opposing liquid jets that collide within a microliter volume. Reported channel dimensions were approximately 75 to 150 µm, average velocities could exceed 400 m/s, and jet Reynolds numbers exceeded 20,000. Solvent and antisolvent streams were proportioned to limit uncontrolled mixing before the chamber and were then exposed to intense turbulence and rapid micromixing in the collision region [26].

The MRT paper reports stable nanosuspensions with median sizes from 50 to 767 nm for several active pharmaceutical ingredients. Azithromycin produced a reported median size of 50 to 100 nm. Oxycarbazepine produced a median size of 767 nm by the bottom up route, while a control sample grown in a beaker and reduced by top down Microfluidizer processing reached 1.2 µm after 25 passes. API 2 produced reported median sizes from 166 to 312 nm. Loratadine formed submicron particles initially but showed growth and stability problems, demonstrating that rapid nucleation does not guarantee long term stability [26].

Figure 9.25. Simplified PureNano configuration without premixing for rapid crystallization. Source: based on [27].

The PureNano technical literature describes two configurations. In the arrangement without premixing, the streams remain separate until the reaction chamber to support very rapid crystallization. In the controlled premixing arrangement, a coaxial macromixing zone creates a selected residence time, often several milliseconds, before the chamber. The source reports feed ratio control from 1:1 to 1:40 and identifies solvent, antisolvent, surfactant, pressure, concentration, and residence time as development variables [27].

Figure 9.26. PureNano laboratory crystallizer shown in the supplied Microfluidics material. Source: [27].

9.5.5 Oncology Nanosuspension Case Study

The supplied PureNano case study describes a poorly water soluble oncology compound whose identity was withheld. The target median size was below 0.200 µm. Top down processing in an M-110EH-30 reduced the mean size from 1.401 µm to 0.410 µm after 40 passes at 23,000 psi, with D50 decreasing from 0.470 µm to 0.379 µm. Additional passes did not provide sufficient reduction and the resulting suspension did not meet the source stability requirement [27].

A bottom up process dissolved the compound in polyethylene glycol and mixed it with water as antisolvent. The source reports a stable nanosuspension with D50 of 0.102 µm and mean size of 0.126 µm at 50 mg/mL and a 1:10 solvent to antisolvent ratio after one pass. Higher concentration did not automatically improve the result. At 120 mg/mL and 1:10, the reported D50 was 0.529 µm. This illustrates the nonlinear interaction among concentration, supersaturation, ratio, viscosity, and stabilization [27].

Figure 9.27. Comparison of top down and bottom up processing in the supplied oncology case study. Source: [27].

Table 9.5. Selected MRT and PureNano crystallization results. Source: [26,27].

Material or case	Route and conditions	Reported product size	Key observation
Azithromycin	MRT solvent and antisolvent crystallization	50 to 100 nm median	Stable nanosuspension in the reported system.
Oxycarbazepine	MRT bottom up crystallization	767 nm median	Smaller than 1.2 µm obtained after 25 top down passes in the source comparison.
API 2	MRT crystallization with varying supersaturation	166 to 312 nm median	Size and process efficiency decreased as supersaturation decreased.
Loratadine	MRT with different solvents and surfactants	Initial 90 to 379 nm, followed by growth	Particle formation was rapid, but stability remained formulation dependent.
Oncology Compound V	PureNano, PEG and water, 50 mg/mL, 1:10 ratio	D50 0.102 µm; mean 0.126 µm	Single pass bottom up process met the source target region.

9.5.6 Population Balance Interpretation

A population balance model provides a systematic way to represent nucleation, growth, aggregation, and breakage. It is useful when a process contains repeated passage through a high energy zone or when crystallization and breakage occur simultaneously. A one dimensional number density balance can be written as:

$$\frac{\partial n(L,t)}{\partial t}+\frac{\partial \left[G(L,t)n(L,t)\right]}{\partial L}=B(L,t)-D(L,t)$$

(9.17)

where n(L,t) is the number density at characteristic size L and time t; G(L,t) is the size dependent growth rate; B(L,t) is the birth term from nucleation, aggregation, or breakage of larger particles; and D(L,t) is the disappearance term from aggregation, dissolution, or breakage.

For cavitation assisted crystallization, the birth term can include primary nucleation driven by supersaturation and secondary nucleation driven by collision or fragmentation. The disappearance term can include agglomeration, dissolution, or fracture into smaller classes. A practical model should be fitted to time resolved size data, not only final D50. Where shape is important, a two dimensional population balance using length and width is preferable [40,41].

9.5.7 Downstream Stabilization and Isolation

Crystallization does not end at the device outlet. The product can continue to grow, ripen, agglomerate, dissolve, or transform while it is held, concentrated, washed, filtered, dried, or redispersed. Stabilizer selection should consider adsorption kinetics, toxicological acceptability, route of administration, solvent exchange, and compatibility with drying. Rapid dilution can reduce supersaturation and suppress further growth. Crossflow filtration, centrifugation, diafiltration, solvent removal, or lyophilization may be needed, but each downstream operation can change the particle distribution [26,27].

The hold vessel should be treated as part of the crystallizer. Mixing intensity, wall material, temperature profile, gas exposure, and residence time distribution can all change the product. For continuous manufacturing, diversion logic and traceability should define which material belongs to the accepted state and how startup and shutdown material is handled [12-14].

9.6 Emulsification, Liposome Production, and Lipid Based Drug Delivery

9.6.1 Nanoemulsions and Liposomes Are Different Structures

A nanoemulsion is a kinetically stabilized dispersion of droplets, commonly oil in water or water in oil. A liposome is a vesicle composed of one or more phospholipid bilayers surrounding an aqueous space. Both systems can be processed by high pressure equipment, but their failure modes differ. Emulsion droplets are governed by interfacial tension, surfactant coverage, coalescence, and Ostwald ripening. Liposomes add bilayer phase behavior, lamellarity, membrane permeability, free drug, encapsulation efficiency, leakage, lipid oxidation, hydrolysis, and payload partitioning [7,15,16].

The FDA liposome guidance emphasizes chemistry, manufacturing, and controls, pharmacokinetics and bioavailability, and labeling information specific to liposome drug products. Product characterization should address the lipid components, drug to lipid ratio, vesicle size distribution, morphology, surface characteristics, encapsulated and free drug, release, leakage, impurities, and stability. Nanomaterial guidance further supports a risk based development approach for products whose performance depends on nanoscale attributes [7,8].

Figure 9.28. Liposome structure and representative payload locations. Original schematic based on [7,15,16].

9.6.2 Conventional and High Energy Preparation Routes

Common liposome formation routes include thin film hydration, ethanol or solvent injection, reverse phase evaporation, detergent removal, dehydration and rehydration, and microfluidic mixing. These methods generally create a coarse vesicle population that may require extrusion, sonication, high pressure homogenization, Microfluidizer processing, or another controlled energy step to reduce size and lamellarity. Scalable preparation requires attention to solvent handling, lipid concentration, membrane transition temperature, drug partitioning, and the sequence of hydration and size reduction [15].

Figure 9.29. Representative liposome manufacturing sequence from lipid selection through final filling or drying. Original schematic based on [7,15,16].

For pressure based processing, the coarse dispersion should be free of particles that can obstruct the selected channel. Feed temperature may need to remain above or below a lipid phase transition depending on the desired remodeling mechanism. The cooling system must remove pressure work rapidly enough to prevent payload degradation, oxidation, hydrolysis, or uncontrolled bilayer permeability. Oxygen exposure, light, and trace metals can accelerate lipid oxidation and should be controlled as appropriate.

9.6.3 Microfluidizer Impinging Cavitating Jet Examples for Liposome Production

A topical drug liposome formulation was processed in a Microfluidizer M-110S at 20,000 psi for seven passes. The reported median size decreased from 4.523 µm to 0.097 µm, or 97 nm [22]. The supplied vitamin E example used an M-110EH-30 at 12,000 psi for two passes and reported a decrease from 5,281 nm to 72 nm while incorporating vitamin E into the liposomal system [23]. The BioPharmaceutical Formulations brochure reports a liposomal phospholipid reduction from 10.188 µm to 0.254 µm after two passes at 15,000 psi and a liposomal insect repellent reduction from 10.155 µm to 0.079 µm after one pass [24].

Figure 9.30. Reconstructed size distributions for the supplied topical liposome example. Source: [22].

Figure 9.31. Reported mean size reduction for the supplied vitamin E liposome example. Source: [23].

The Nanoencapsulation presentation reports a drug loaded liposome example processed for five passes at 18,000 psi with an F20Y chamber. The reported unloaded vesicle size was 58 nm and the loaded vesicle size was 84 nm. The same source presents a 0.1% plasmid DNA liposome example in which the reported particle size decreased more rapidly at 18,000 psi than at 5,000 psi and states that the DNA remained intact after processing [18].

Figure 9.32. Reported plasmid DNA liposome size response to pressure and pass count. Source: [18].

Table 9.6. Selected supplied liposome processing cases. Source: [18,22-24].

Formulation	Processor and conditions	Reported starting size	Reported final size	Additional source observation
Topical drug liposome	M-110S; 20,000 psi; 7 passes	4.523 µm median	0.097 µm median	Final median below 100 nm.
Vitamin E liposome	M-110EH-30; 12,000 psi; 2 passes	5,281 nm mean	72 nm mean	Source reports successful vitamin E deposition.
Liposomal phospholipid	15,000 psi; 2 passes	10.188 µm	0.254 µm	Large early reduction in the corporate brochure.
Liposomal insect repellent	15,000 psi; 1 pass	10.155 µm	0.079 µm	Sub 100 nm result in one reported pass.
Drug loaded liposome	F20Y; 18,000 psi; 5 passes	Not stated	58 nm unloaded; 84 nm loaded	Loaded vesicles were larger than unloaded vesicles.
Plasmid DNA liposome	5,000 or 18,000 psi; 2 passes	Approximately 400 nm	Approximately 190 or 110 nm	Source states DNA remained intact.

9.6.4 Encapsulation Efficiency, Drug Loading, and Size Distribution

Liposome size cannot be optimized independently of encapsulation and retention. A small vesicle has a high surface to volume ratio and less aqueous internal volume per unit lipid than a larger vesicle. Hydrophilic payload encapsulation may therefore decrease during aggressive size reduction unless the process includes remote loading, concentration control, or another compensating mechanism. Lipophilic payloads can be lost if bilayer disruption or lipid phase changes promote leakage.

$$EE=\frac{m_{encapsulated}}{m_{totaldrug}}\times 100\%$$

(9.18)

where EE is encapsulation efficiency; $m_{encapsulated}$ is the mass of drug retained in the carrier fraction; and $m_{total}$ is the total drug mass in the formulation.

$$DL=\frac{m_{encapsulated}}{m_{lipid}+m_{encapsulated}}\times 100\%$$

(9.19)

where DL is drug loading relative to the combined lipid and encapsulated drug mass; $m_{lipid}$ is lipid mass; and $m_{encapsulated}$ is encapsulated drug mass.

Size distribution should be reported with more than one statistic. D10, D50, D90, and span are useful for volume based laser diffraction data. For DLS, hydrodynamic diameter and polydispersity index should be reported together with the analysis model, viscosity, temperature, and dilution method. A low D50 can conceal a small but important coarse fraction that affects sterile filtration or injectability.

$$Span=\frac{D_{90}-D_{10}}{D_{50}}$$

(9.20)

where D10, D50, and D90 are the diameters below which 10%, 50%, and 90% of the selected distribution basis are found.

Zeta potential can indicate electrostatic stabilization in suitable dilute systems, although it is not a universal predictor for sterically stabilized or high ionic strength formulations. Under the Smoluchowski approximation:

$$\zeta =\frac{{\eta }_e{u}_e}{{\epsilon }_r{\epsilon }_0}$$

(9.21)

where ζ is zeta potential; ${\eta }_e$ is the dynamic viscosity of the electrophoresis medium; $u_e$ is electrophoretic mobility; ${\epsilon }_r$ is relative permittivity; and ε₀ is vacuum permittivity.

9.6.5 Cavitation Assisted Production and Lipid Coating

Controlled hydrodynamic cavitation can couple particle size reduction with lipid coating or encapsulation. Figure 9.33 shows a processed drug and lipid system with a corresponding particle size distribution. Figure 9.34 compares cefazolin and oxytetracycline controls with samples processed in the presence of lipid at working pressures below 1500 psi [1].

The supplied examples suggest that lipid can adsorb to newly created surfaces during particle size reduction. Because intensive processing can also generate heat and shear, solid state and morphology should be evaluated to exclude process induced transformation and to confirm formulation stability [1].

These examples support the concept that newly created particle surfaces can be coated during the same operation that reduces particle size [1].

Figure 9.33. Micrograph and particle size distribution for a processed drug and egg yolk lipid system at a 3:1 ratio and 400× magnification. Source: [1].

3% Cefazolin (400×)

3% Oxytetracycline (400×)

Figure 9.34. Cefazolin and oxytetracycline controls and lipid processed samples at 400× magnification. Source: [1].

A plausible process sequence is as follows. First, the drug particles or lipid aggregates encounter a high velocity cavitating or high shear field. Second, fracture and deagglomeration create new surface. Third, lipid or surfactant molecules adsorb onto the surface before reagglomeration. Fourth, additional passes refine the coating and distribution. This mechanism can produce a lipid stabilized particle dispersion even when a classical closed bilayer vesicle is not formed. Therefore, microscopy, cryogenic electron microscopy, free drug separation, lamellarity analysis, and release testing are needed to distinguish true liposomes from lipid coated particles, mixed micelles, emulsions, or other colloidal structures.

9.6.6 Processing Window for Liposomes

Liposome processing requires a defined intensity window. At low exposure, coarse multilamellar vesicles and a broad distribution may remain. At moderate exposure, vesicle size and lamellarity can decrease while encapsulation and bilayer integrity are preserved. At excessive exposure, further size reduction may be small while leakage, free drug, lipid oxidation, payload degradation, or equipment wear increases. Pressure and pass count should therefore be optimized against a multivariate objective, not against size alone [7,15,16].

Figure 9.35. Conceptual liposome processing window balancing size reduction against leakage and bilayer damage risk. Original schematic based on [7,15,16].

9.6.7 Release Models and Interpretation

Drug release from particles and liposomes can be governed by diffusion, dissolution, membrane permeation, erosion, degradation, desorption, pore formation, or a combination of mechanisms. Empirical models are useful for comparison but should not be mistaken for proof of mechanism. The Higuchi square root relation is commonly used when diffusion through a matrix is approximately controlling:

$$M_t=k_H\sqrt{t}$$

(9.22)

where $M_t$ is cumulative released mass at time t; and $k_H$ is the Higuchi release constant for the selected test conditions.

A first order expression can be used for a system in which the unreleased fraction decreases exponentially:

$$\frac{M_t}{M_{\infty }}=1-exp(-k_{1}t)$$

(9.23)

where $M_t$ is released mass at time t; M∞ is asymptotic released mass; and k₁ is the apparent first order release rate constant.

Release testing should be discriminatory with respect to process variability. The test medium, sink conditions, membrane or separation method, agitation, temperature, analytical recovery, and prevention of particle carryover should be justified. For liposome products, a method should distinguish released drug from intact carrier and should be sensitive to leakage or bilayer changes caused by manufacturing and storage [7].

9.7 Particle Design, Encapsulation, and Surface Stabilization

9.7.1 Surface Creation Must Be Coupled with Stabilization

Every reduction in particle or droplet size creates new interface. The formulation must stabilize that interface on the same time scale as it is created. Electrostatic stabilization uses surface charge and the resulting repulsive interaction. Steric stabilization uses adsorbed polymers or surfactants that create an excluded volume barrier. Electrosteric systems combine both. Lipid coating can alter wetting, protect a drug surface, reduce direct contact with tissue, or provide a controlled release barrier. The correct mechanism depends on the continuous phase, ionic strength, route of administration, excipient acceptability, and intended downstream operation [1,7,15,16].

Stabilizer underdosing leaves unprotected surface and promotes aggregation. Excess stabilizer can create micelles, change osmolality, alter drug partitioning, increase free excipient, or complicate purification. A rational approach estimates newly created surface area, measures adsorption where possible, and uses a design of experiments to evaluate particle size, free stabilizer, viscosity, zeta potential, dissolution, and storage stability together.

9.7.2 Lipid Coated Particles, Polymer Nanoparticles, and Hybrid Carriers

The supplied Microfluidics nanoencapsulation materials describe three broad carrier platforms: nanoemulsions, liposomes, and polymer particles. The bottom up MRT examples include polycaprolactone and poly(lactide co glycolide) particles formed by solvent and antisolvent precipitation. The supplied materials report a polycaprolactone median size of 239 nm and a poly(lactide co glycolide) example with average sizes of 141 nm by solvent and antisolvent precipitation and 108 nm by nanoemulsion evaporation [18]. These data illustrate how polymer precipitation and emulsion evaporation can be integrated with intensive micromixing.

Hybrid structures can combine a crystalline or amorphous drug core with a lipid or polymer shell. Such systems may improve wetting and retention while preserving high drug loading. However, morphology must be demonstrated. A small DLS diameter alone cannot distinguish a solid core lipid shell particle from a liposome, micelle, nanoemulsion, or polymer aggregate. Orthogonal imaging and compositional separation are necessary.

9.7.3 Coating and Encapsulation Performance Measures

A development report should separate three questions. First, how much drug is associated with the carrier fraction? Second, where is the drug located within the structure? Third, how stable is that association during processing, storage, dilution, and administration? Encapsulation efficiency addresses the first question, while microscopy, spectroscopy, partition measurements, and release studies address the second and third. For a lipid coated crystal, drug loading can be high because the drug is the core. For a conventional liposome, loading is constrained by aqueous volume, bilayer capacity, remote loading gradients, and the drug to lipid ratio.

Surface treatment performed in the active processing zone can be advantageous because stabilizer encounters newly created surface before substantial reagglomeration. The process can be staged by using an upstream module for deagglomeration, a high energy zone for size reduction, and a lower energy downstream stage for coating or equilibration. The optimum order depends on adsorption kinetics and whether the stabilizer itself is shear sensitive.

9.7.4 Storage Stability and Redispersion

The final particle size at the processor outlet is only an initial condition. Stability studies should evaluate sedimentation, creaming, aggregation, crystal growth, polymorphic conversion, leakage, oxidation, hydrolysis, and container interaction. Accelerated stress can be useful, but the stress mechanism must remain relevant to normal storage. Freeze thaw studies are important when frozen storage is possible. Lyophilized systems require evaluation of cake structure, residual moisture, reconstitution time, and post reconstitution particle distribution.

A redispersible sediment can be acceptable for some oral or topical suspensions but may be unacceptable for parenteral or ophthalmic use. Redispersion testing should be quantitative where possible, using a defined inversion, shaking, or mixing protocol followed by dose uniformity and particle size testing. For a continuous process, hold time and agitation in intermediate vessels should be included in the stability design.

9.8 Quality by Design, Process Analytical Technology, and Manufacturing Control

9.8.1 Quality Target Product Profile and Risk Analysis

A Quality by Design approach begins with the quality target product profile and the intended route of administration. The development team identifies critical quality attributes, evaluates material attributes and process parameters that can affect them, and builds knowledge through designed experiments and mechanistic understanding. The design space is a multidimensional region in which the process has been shown to deliver acceptable quality. It is not simply a pressure range [9-12,16].

For particle engineering, a cause and effect analysis should include feed particle size, crystal form, lipid or polymer grade, stabilizer molecular weight, solvent composition, dissolved gas, viscosity, temperature, pressure, back pressure, flow rate, chamber geometry, pass count, loop volume, hold time, oxygen exposure, filter history, equipment wear, and cleaning state. The priority of each factor depends on the dosage form and mechanism.

Figure 9.36. Conceptual risk screening matrix connecting critical process parameters with critical quality attributes.

Table 9.7. Examples of material attributes, process parameters, and linked quality attributes. Source: compiled from [7-16,33-39].

Category	Variable	Potentially affected attributes	Typical controls
Material attribute	API crystal form and mechanical strength	Breakage rate, limiting size, dissolution, polymorphic conversion	Incoming PXRD, particle imaging, mechanical screening
Material attribute	Lipid composition and transition temperature	Vesicle size, leakage, encapsulation, release, oxidation	Supplier qualification, DSC, peroxide value, composition testing
Formulation	Stabilizer type and concentration	Aggregation, span, zeta potential, free drug, viscosity	Design of experiments, adsorption or surface coverage study
Hydraulic parameter	Pressure and back pressure	Velocity, cavitation number, shear, impact, temperature rise	Calibrated pressure sensors, alarm limits, trace review
Hydraulic parameter	Flow rate and chamber geometry	Residence time, dissipation, mixing, pass efficiency	Flow meter, chamber identification, wear monitoring
Exposure history	Pass count and recirculation ratio	Size, span, leakage, impurities, temperature history	Batch mass balance, loop volume, controlled sampling
Thermal parameter	Inlet and outlet temperature	Viscosity, vapor pressure, lipid phase state, degradation	Heat exchanger control, continuous temperature recording
Downstream hold	Time, agitation, gas exposure	Growth, aggregation, oxidation, leakage, bioburden	Validated hold time, inerting, agitation and temperature limits

9.8.2 Design of Experiments for Pressure, Passes, and Formulation

A useful screening design includes both formulation and process factors. For a nanosuspension, factors may include stabilizer type, stabilizer to drug ratio, solids concentration, pressure, pass count, chamber, temperature, and ionic strength. Responses should include D10, D50, D90, span, viscosity, dissolution, sedimentation, assay, impurities, and crystal form. For liposomes, responses should add PDI, encapsulation efficiency, free drug, leakage, lipid degradation, lamellarity, and release.

Sequential experimentation is more efficient than a single very large design. A first stage identifies dominant factors and unsafe regions. A second response surface design defines curvature and interactions. A third stage confirms robustness near the proposed set point and at material attribute extremes. Mechanistic modeling can reduce the number of experiments when it is anchored to measurements. Pressure, pass count, and temperature should be treated as interacting variables rather than optimized one at a time.

9.8.3 Process Analytical Technology

Process analytical technology can shorten feedback time and protect against drift. Candidate measurements include pressure and temperature traces, flow, density, viscosity, conductivity, turbidity, near infrared or Raman spectra, focused beam reflectance measurement, particle vision microscopy, inline imaging, and at line laser diffraction or DLS. No single method is sufficient for all products. Measurement location, dilution, sampling shear, and time delay must be included in the method design [13,37,38].

Figure 9.37. Representative process analytical technology and feedback control architecture for a continuous or semicontinuous particle process.

A PAT signal should be linked to a material decision. For example, a pressure trace can detect pump or chamber drift, but it does not directly prove particle size. An inline optical signal may correlate with size, but the correlation must be validated across formulation and temperature. The control strategy can use direct measurement, a soft sensor, or a combination. Startup, shutdown, and disturbance material should be tracked and diverted when the state is outside the validated acceptance region.

9.8.4 Residence Time Distribution and Traceability

Continuous and recirculating systems contain a distribution of residence times and pass histories. The nominal residence time in Equation (9.11) is only a mean. A residence time distribution, E(t), describes the probability density for material leaving at time t after a tracer input. It is normalized as:

$${\int }_0^{\infty }E(t)dt=1$$

(9.24)

where E(t) is the exit age distribution. The integral equals 1 when all tracer is recovered and the system mass balance is complete.

Residence time distribution testing can reveal bypass, dead volume, broad mixing, or long tails. These features are important when product quality changes rapidly with exposure or when startup and shutdown material must be separated. For recirculating systems, a pass distribution model may be more informative than a conventional single pass residence time distribution. The model should be checked by tracer experiments at representative viscosity and flow.

9.8.5 Cleaning, Sterile Processing, and Material Compatibility

The manufacturing system should be designed for cleanability, drainability, inspection, and material compatibility. Small channels can retain product or cleaning solution. Cleaning validation should address the hardest to clean formulation and the most difficult location. Where clean in place or steam in place is used, gasket materials, ceramics, seals, pressure sensors, and surface finishes must tolerate the cycle without altering hydraulic performance.

For sterile drug products, the process sequence must integrate bioburden control, sterilization or aseptic processing, sterile filtration where feasible, sterile connections, environmental control, and validated hold times. A nominal particle size below 200 nm may support filtration through a 0.22 µm membrane, but the full distribution, aggregation, viscosity, filter loading, drug adsorption, and vesicle deformation determine actual filterability. Sterile filtration must be demonstrated with product specific bacterial retention and integrity testing [7,36].

Materials of construction can contribute extractables, leachables, metal ions, or wear particles. This is particularly important for oxidative lipids, proteins, nucleic acids, and long campaigns. A risk assessment should consider contact time, pressure, temperature, solvent, pH, cleaning chemicals, and component replacement history. Baseline and aged equipment studies can help establish preventive maintenance limits.

9.8.6 Scale Up and Lifecycle Validation

Scale up should maintain the local processing environment that controls quality. For parallel microchannel systems, local channel geometry can remain constant while throughput increases through additional channels. Flow balance and pressure consistency across channels must be verified. For cavitation devices, geometric similarity alone may not preserve cavity inception, cloud structure, and collapse location. The scale up plan should identify the governing dimensionless groups, pressure recovery pattern, residence time, and heat removal requirement [1,3-5,39].

Process validation should demonstrate that the commercial process can reproducibly deliver the required attributes within the defined control strategy. Continued process verification should trend size distributions, pressure and flow, temperature, pass count, yield, impurities, filter performance, and equipment wear. Changes to chamber, pump, scale, material supplier, lipid grade, stabilizer, or analytical method should be evaluated through the pharmaceutical quality system [11,14].

Table 9.8. Scale up and validation checklist. Source: [9-14,33-39].

Area	Questions to answer before commercial transfer
Mechanism	Is the process controlled by fracture, deagglomeration, droplet breakup, cavitation collapse, micromixing, nucleation, growth, or a combination?
Hydraulics	Are pressure, back pressure, flow, channel dimensions, active volume, and residence time representative?
Thermal control	Is temperature rise per pass and cumulative temperature history controlled at the commercial scale?
Pass history	Does every portion of material receive the intended number and severity of passes?
Formulation	Are viscosity, solids loading, lipid phase behavior, dissolved gas, and stabilizer coverage equivalent?
Equipment condition	Are chamber wear, pump pulsation, seals, surfaces, and sensor calibration within acceptance limits?
Sampling and PAT	Do sampling methods preserve the product state, and are models valid at commercial scale?
Cleaning and sterility	Can the equipment be cleaned, sterilized, drained, inspected, and reassembled without hydraulic drift?
Lifecycle management	Are replacement criteria, continued verification, deviation handling, and change control defined?

9.9 Analytical Characterization and Data Interpretation

9.9.1 Particle Size Methods Are Not Interchangeable

Laser diffraction, dynamic light scattering, nanoparticle tracking analysis, microscopy, and focused beam reflectance measurement report different physical quantities. Laser diffraction commonly produces a volume based equivalent spherical diameter. DLS produces an intensity weighted hydrodynamic diameter and is highly sensitive to a small number of large particles. Nanoparticle tracking analysis tracks individual scattering objects within a suitable concentration range. Microscopy provides morphology but may examine a limited number of objects. FBRM reports chord lengths rather than direct particle diameters. Method selection and sample preparation must match the decision being made.

A robust report states the distribution basis, optical model, refractive index, viscosity, temperature, dilution medium, sonication or mixing used during sample preparation, number of measurements, and data treatment. Apparent size changes caused by dilution, salt concentration, or refractive index should not be interpreted as process effects. Orthogonal measurements are particularly important near a sterile filtration cutoff or when a coarse tail is safety relevant [7,8,37,38].

9.9.2 Morphology and Solid State

Particle shape influences drag, settling, filtration, packing, and breakage. An equivalent spherical diameter can conceal long needles or thin plates. Automated image analysis should report length, width, aspect ratio, circularity, and particle count in addition to size. For submicron particles, SEM or TEM sample preparation can alter the structure, while cryogenic imaging can better preserve hydrated vesicles and emulsions.

Powder X ray diffraction, differential scanning calorimetry, Raman spectroscopy, infrared spectroscopy, and solid state NMR provide complementary information on polymorph, crystallinity, hydration, and amorphous content. Processing can induce transformation through pressure, heat, solvent exchange, surface creation, or prolonged recirculation. Solid state testing should be performed immediately after processing and after relevant holds or drying steps.

9.9.3 Liposome Specific Characterization

Liposome characterization should include vesicle size and distribution, morphology, lamellarity when relevant, lipid composition, drug to lipid ratio, encapsulated and free drug, release, leakage, surface charge, pH, osmolality, viscosity, residual solvent, lipid oxidation and hydrolysis products, sterility related attributes, and stability. Cryogenic TEM can visualize vesicle structure, but image selection and freezing artifacts must be considered. Size exclusion chromatography, ultracentrifugation, dialysis, or ultrafiltration can separate free and associated drug, but recovery and carrier disruption must be demonstrated [7,15,16].

A release method should distinguish true release from vesicle transport through the sampling membrane or separation device. Mass balance is essential. The method should detect process changes such as pressure, pass count, lipid ratio, free drug, or storage leakage. An apparently slow method may be controlled by the test apparatus rather than the product. Method development should therefore evaluate membrane resistance, sink conditions, agitation, sampling, and analytical recovery [7,37,38].

9.9.4 Recommended Reporting Package

Table 9.9. Minimum recommended reporting package for a pharmaceutical particle engineering study. Source: [7-16,37,38].

Topic	Information to report
Materials	API form, lot, particle size, purity, solvent, antisolvent, lipid, polymer, stabilizer, concentration, pH, ionic strength, dissolved gas treatment.
Equipment	Manufacturer, model, chamber or orifice, channel dimensions where available, materials of construction, active volume, cooling configuration.
Hydraulics	Inlet pressure, back pressure, flow, pressure drop, temperature profile, pass count, recirculation volume, residence time, sampling point.
Energy	Specific pressure energy or measured power, basis of energy calculation, local versus system averaged dissipation, heat balance.
Particle data	D10, D50, D90, span or PDI, distribution basis, method settings, sample preparation, replicate number, morphology.
Solid state	Polymorph, crystallinity, thermal behavior, hydration, amorphous content before and after processing and after hold.
Carrier data	Encapsulation efficiency, drug loading, free drug, leakage, lipid or polymer integrity, release, surface properties.
Stability	Immediate, hold time, accelerated, freeze thaw, dilution, redispersion, filtration, and storage results.
Quality and safety	Assay, related substances, residual solvent, elemental or wear contaminants, bioburden, endotoxin, sterility related controls.

9.10 Technology Comparison and Practical Decision Framework

9.10.1 Comparative Strengths and Limitations

No single technology is best for every active ingredient or dosage form. Media milling can provide robust nanosuspension production but requires media control and may involve long processing times. Jet milling is dry and scalable but may be inefficient for ductile crystals or very small targets. High pressure homogenization is well suited to liquid dispersions, emulsions, and liposomes, but it requires pressure equipment, cooling, and careful control of pass history. Controlled hydrodynamic cavitation can combine breakage, mixing, nucleation, and surface treatment, but the cavitation field must be characterized and equipment wear managed. Impinging jet crystallization provides rapid bottom up formation when solvent and antisolvent mixing controls nucleation, but long term stabilization and downstream solvent removal remain essential [1,17-35,40,41].

Table 9.10. Practical comparison of pharmaceutical particle engineering technologies. Source: [1,17-35,40,41].

Technology	Primary mechanism	Typical strengths	Principal risks or limitations
Rotor stator wet milling	Shear and circulation	Simple equipment, easy premixing, useful for coarse dispersion.	Limited submicron efficiency, broad stress field, heat at high tip speed.
Wet media milling	Attrition and collision with media	Established nanosuspension capability, high solids possible.	Media wear, separation, cleaning, long time, temperature control.
Jet milling	High velocity gas collision	Dry process, no liquid solvent, established powder handling.	Broad fines, electrostatics, low efficiency for some crystals, dust containment.
High pressure valve homogenization	Pressure drop, shear, turbulence, impact	Emulsions and suspensions, continuous operation.	Valve wear, heat, pass variability, possible broad distribution.
Microfluidizer processing	Fixed geometry microchannel shear, impact, turbulence	Tight distributions, pressure and pass control, liposome and emulsion use, parallel channel scale up.	Small channel plugging, wear, pressure equipment, formulation specific leakage or degradation.
Controlled hydrodynamic cavitation	Intentional cavity formation and collapse	Breakage, mixing, nucleation, surface treatment, continuous operation.	Cavitation field characterization, erosion, dissolved gas and back pressure sensitivity.
MRT or impinging jet crystallization	Rapid solvent and antisolvent micromixing	Bottom up particles, narrow nucleation history, continuous formation.	Solvent removal, stability, feed ratio control, possible premature mixing or fouling.

9.10.2 Decision Sequence

• Define the dosage form, route of administration, target dose, acceptable excipients, and quality target product profile.

• Identify whether the critical limitation is dissolution, coarse tail, morphology, polymorph, droplet size, vesicle size, encapsulation, leakage, or release.

• Choose top down, bottom up, cavitation assisted, high pressure microchannel, or hybrid processing based on the controlling mechanism.

• Screen material and formulation variables before maximizing equipment intensity.

• Measure orthogonal quality attributes, not only D50, and define a multivariate design space.

• Establish heat removal, pass history, hold time, cleaning, wear, and sterile processing controls early in development.

• Demonstrate scale up by preserving the local mechanism and hydraulic history, then confirm through continued process verification.

9.11 Conclusions

Pharmaceutical particle engineering links molecular and interfacial properties with manufacturing performance and drug delivery. Particle size reduction can increase accessible surface area and dissolution, but it also creates interfacial energy and stability risk. Crystallization can create the required primary particle directly, but supersaturation, nucleation, growth, and post process aging must be controlled. Liposome production adds bilayer integrity, drug partitioning, encapsulation, leakage, and release to the particle size problem.

Controlled hydrodynamic cavitation provides a versatile mechanism for particle fracture, deagglomeration, micromixing, crystallization, and surface treatment. Microfluidizer processing is a related high pressure platform that uses fixed geometry microchannels, high shear, impact, cavitation, turbulence, and controlled pass exposure. The technologies should be described accurately and selected according to the mechanism that controls product quality. The supplied Microfluidics examples show substantial particle and vesicle size reductions in suspensions, emulsions, liposomes, and crystallization systems, while also showing that pass efficiency, pressure response, and stability are formulation dependent [1,17-27].

A successful commercial process is defined by a design space and control strategy, not by a single pressure setting. The process must control the full size distribution, morphology, solid state, surface properties, encapsulation, leakage, impurities, temperature, residence time, and equipment condition. Quality by Design, process analytical technology, orthogonal characterization, risk based validation, and lifecycle monitoring convert an intensive cavitation or mixing event into a reproducible pharmaceutical manufacturing operation [7-16,36-39].

9.12 Nomenclature

Table 9.11. Roman symbols and abbreviations used in Equations (9.1) to (9.24).

Symbol	Definition	Typical unit
A	Accessible particle surface area	m²
B(L,t)	Particle birth term in the population balance	number m⁻¹ s⁻¹
C	Bulk or actual solute concentration	kg m⁻³ or mol m⁻³
$C_{eq}$	Equilibrium solubility concentration	kg m⁻³ or mol m⁻³
Cₛ	Concentration at the particle surface	kg m⁻³ or mol m⁻³
Cₛ(r)	Saturation concentration for a particle of radius r	kg m⁻³ or mol m⁻³
Cₛ,∞	Saturation concentration at a planar interface	kg m⁻³ or mol m⁻³
d	Particle or droplet diameter used in the Weber number	m
dₕ	Hydrodynamic diameter	m
dₚ	Diameter of a spherical particle	m
D	Translational diffusion coefficient	m² s⁻¹
D10, D50, D90	Percentile diameters of the selected particle size distribution	m, µm, or nm
D(L,t)	Particle disappearance term in the population balance	number m⁻¹ s⁻¹
DL	Drug loading	%
eₚ	Ideal specific pressure energy	J kg⁻¹
EE	Encapsulation efficiency	%
E(t)	Exit age or residence time distribution density	s⁻¹
G(L,t)	Particle growth rate	m s⁻¹
J	Nucleation rate	m⁻³ s⁻¹
J₀	Nucleation preexponential factor	m⁻³ s⁻¹
k	Overall dissolution mass transfer coefficient	m s⁻¹
k₁	Apparent first order release rate constant	s⁻¹
$k_B$	Boltzmann constant	J K⁻¹
$k_H$	Higuchi release constant	mass time⁻¹ᐟ²
L	Characteristic particle length or size coordinate	m
m	Particle or formulation mass	kg
$m_{encapsulated}$	Mass of drug retained in the carrier fraction	kg or mass unit
$m_{lipid}$	Lipid mass	kg or mass unit
$m_{total}$	Total drug mass in the formulation	kg or mass unit
M	Dissolved drug mass	kg or mass unit
$M_t$	Cumulative released mass at time t	kg or mass unit
M∞	Asymptotic released mass	kg or mass unit
n(L,t)	Particle number density distribution	number m⁻¹
P	Power dissipated in the selected volume	W
p₁	Upstream pressure	Pa
p₂	Downstream or recovery pressure	Pa
$p_B$	Pressure inside a bubble	Pa
p∞	Far field liquid pressure	Pa
pᵥ	Liquid vapor pressure	Pa
Q	Volumetric flow rate	m³ s⁻¹
r	Particle radius	m
R	Gas constant or bubble radius, as defined locally	J mol⁻¹ K⁻¹ or m
R₀	Initial bubble or cavity radius	m
dR/dt	Bubble wall velocity	m s⁻¹
d²R/dt²	Bubble wall acceleration	m s⁻²
Re	Reynolds number	dimensionless
S	Supersaturation ratio	dimensionless
span	Particle size distribution span	dimensionless
t	Time	s
$t_c$	Ideal bubble collapse time	s
T	Absolute temperature	K
u	Characteristic fluid velocity	m s⁻¹
$u_e$	Electrophoretic mobility	m² V⁻¹ s⁻¹
V	Process or active volume	m³
Vₘ	Molar volume	m³ mol⁻¹
We	Weber number	dimensionless

Table 9.12. Greek symbols used in the chapter equations.

Symbol	Definition	Typical unit
γ	Interfacial energy or surface tension, as defined locally	J m⁻² or N m⁻¹
ΔG*	Critical nucleation free energy barrier	J
ΔGᵥ	Volumetric free energy change for solid formation	J m⁻³
Δp	Pressure drop	Pa
ε	Energy dissipation rate	W kg⁻¹
ε₀	Vacuum permittivity	F m⁻¹
${\epsilon }_r$	Relative permittivity	dimensionless
${\eta }_e$	Dynamic viscosity used in the electrophoretic calculation	Pa s
${\eta }_K$	Kolmogorov length scale	m
μ	Dynamic viscosity	Pa s
ν	Kinematic viscosity	m² s⁻¹
ρ	Liquid density	kg m⁻³
ρₚ	Particle density	kg m⁻³
σ	Cavitation number	dimensionless
τ	Nominal residence time	s
ζ	Zeta potential	V or mV

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