# Microspheres and Nanoparticles > Microspheres and nanoparticles in Pharmacy: PLGA depots, solvent evaporation, biphasic release, nanocrystals, Noyes-Whitney and size-dependent targeting. - Canonical URL: https://prepelephant.com/topics/allied/pharmacy/microspheres-nanoparticles - Exam / course: Allied Health · Subject: Pharmacy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Microspheres and Nanoparticles", PrepElephant, https://prepelephant.com/topics/allied/pharmacy/microspheres-nanoparticles ## Direct answer Microspheres are matrix-type spherical particles of 1-1,000 micrometres; nanoparticles are submicron carriers below about 1,000 nanometres, both built to control where a drug goes and how fast it leaves. Microspheres dominate as biodegradable polymeric depots: poly(lactic-co-glycolic acid) (PLGA) and polylactide microspheres release drugs for weeks to months — risperidone two-weekly, octreotide monthly, leuprolide one- to four-monthly, naltrexone monthly — made by solvent evaporation from oil-in-water emulsions for hydrophobic drugs, double water-in-oil-in-water emulsions for peptides, or spray drying. Their release is characteristically biphasic: an initial burst from surface drug, then a slower phase of diffusion plus polymer erosion, tuned by the lactide:glycolide ratio (50:50 degrades fastest). Nanoparticles cover polymeric particles, solid lipid nanoparticles, nanostructured lipid carriers and drug nanocrystals — pure drug milled to hundreds of nanometres whose dissolution rate rises with surface area per the Noyes-Whitney equation, rescuing poorly soluble drugs like aprepitant and sirolimus; size also steers biodistribution, from renal clearance below about 10 nanometres to tumour extravasation around 100-200 nanometres. ## What you must remember - **Size definitions:** microparticles 1-1,000 micrometres; nanoparticles 10-1,000 nanometres; nanosuspensions typically 100-500 nanometres of pure drug with stabiliser. - **Matrix versus reservoir:** microspheres disperse drug through a polymer matrix; microcapsules surround a core with a coat — an exam distinction that decides whole MCQs. - **Polymer logic:** PLGA/PLA are biodegradable, hydrolyse to lactic and glycolic acids, and release faster at 50:50 lactide:glycolide than glycolide-poor grades; release spans days to months. - **Biphasic release:** initial burst (surface drug) then diffusion-and-erosion plateau — the shape examiners ask candidates to sketch and label. - **Preparation map:** o/w solvent evaporation (hydrophobic drugs), w/o/w double emulsion (proteins and peptides), spray drying, coacervation; nanoparticles by nanoprecipitation, emulsification, or wet milling for nanocrystals. - **Noyes-Whitney:** dissolution rate = D × A × (Cs − C) / h — nanonisation raises surface area A and shrinks diffusion layer thickness h, the quantitative argument for nanocrystals. - **Size-biodistribution rules of thumb:** below about 10 nm renal clearance; around 100-200 nm leaky-tumour extravasation; larger particles land in liver and spleen; opsonisation drives RES uptake unless PEGylated. - **Characterisation:** size and distribution (laser diffraction, dynamic light scattering), zeta potential, morphology (SEM/TEM), entrapment efficiency, in vitro release; a zeta magnitude near ±30 mV is the classic colloidal-stability heuristic. ## Two patients, two depots A man with schizophrenia on risperidone microspheres arrives for his two-weekly injection. Why a depot at all: adherence in psychosis is fragile, and converting a daily tablet into a fortnightly intramuscular dose moves compliance from the patient's memory to the clinic calendar; the label's fine print even recommends oral coverage during the first weeks while the initial release profile establishes. The microspheres were made by double emulsion — peptide-class sensitivity demands gentle processing — and inside the muscle they release with the textbook biphasic curve: a small early burst, then steady diffusion-and-degradation output that pharmacokinetics flattens into a plateau. The detail examiners reward: changing the lactide:glycolide ratio or polymer molecular weight moves the plateau by weeks — one platform serves one-month and three-month leuprolide products. Contrast an oral nanocrystal. Aprepitant, poorly soluble and slow to dissolve as a conventional tablet, is wet-milled with stabilisers into submicron crystals; by the Noyes-Whitney relation the dissolution rate climbs with surface area and the shortened diffusion path, so bioavailability no longer depends on a heavy meal. No polymer, no targeting — pure physical chemistry. The two technologies bracket the chapter's idea: size and architecture are formulation variables as real as excipients, with depot duration bought with microns and dissolution rate bought with nanometres. ## Where students slip Microsphere versus microcapsule is the first separator: matrix dispersion versus core-shell reservoir, and mixing them collapses the classification question. Second, the burst release is often misread as a defect; it is intrinsic to surface-localised drug and is managed, not eliminated — indeed, depot antipsychotics plan oral supplementation around it. Third, nanosuspensions are confused with polymeric nanoparticles: a nanosuspension is essentially pure drug plus stabiliser, nothing polymeric at all, and its entire rationale is dissolution-rate physics. Fourth, the claim that nanoparticles "target tumours" needs the honest mechanism — passive extravasation through leaky endothelium with poor lymphatic drainage, effective for a size window near 100-200 nanometres — rather than a homing fiction. Finally, sterilisation: most depots are manufactured aseptically or gamma-treated with validated stability, because you cannot autoclave a polymer matrix that is designed to hydrolyse. ## Frequently asked questions ### How do microspheres differ from microcapsules? Microspheres are homogeneous polymer matrices with drug dispersed throughout; microcapsules are reservoir systems with a distinct core surrounded by a coating that controls release. ### Why is PLGA the standard depot polymer? It biodegrades to endogenous acids, its rate is tunable through lactide:glycolide ratio and molecular weight, and its safety record supports weeks-to-months release. ### What causes the initial burst in depot release? Drug sitting at or near the particle surface dissolves immediately upon hydration, producing a rapid early release before diffusion and polymer erosion establish the slower phase. ### How do drug nanocrystals improve bioavailability? Submicron size massively increases dissolution rate through surface area and a thinner diffusion layer (Noyes-Whitney), overcoming the dissolution-limited absorption of poorly soluble drugs. ### Which marketed products illustrate each class? Risperidone, octreotide, leuprolide and naltrexone PLGA depots; aprepitant and sirolimus nanocrystal products — names worth quoting.