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

9 Analytical Characterization and Data Interpretation

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].

9.2 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.

9.3 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].

9.4 Recommended Reporting Package

Table 9.1. Minimum recommended reporting package for a pharmaceutical particle engineering study.

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.