Nanostructuring materials can radically change their properties. Two interesting examples highlighted here are nanoscale porosity inducing biodegradability, and nanoscale confinement affecting the physical form of an entrapped drug. Mesoporous silicon is under increasing study for drug-delivery applications, and is the topic of this review. The authors focus on those properties of most relevance to this application, as well as those recent studies published on small molecule and peptide/protein delivery.

ζ potential measurements were used to determine the adsorbed layer thickness of poly(ethylene oxide)polypropylene oxide-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers at the surfaces of poly-(dimethylsiloxane) emulsion droplets, silica particles, and polystyrene latex particles. The adsorbed layer thickness (δ) was shown to depend on the copolymer structure, the surface chemistry of the adsorbent, and the level of cross-linking within droplets. For a wide range of copolymers, δ values are in the range 2-20 nm and directly proportional to the PEO block length. For copolymers with PEO segment lengths > PPO segment lengths, the plateau δ values for PDMS, latex, and silica correspond to ∼35%, 20%, and 7% of the fully extended PEO segment length. Equivalent values for copolymers with PEO segment lengths < PPO segment lengths are significantly greater but follow the same dependence on the type of adsorbent. As the level of internal cross-linking within the PDMS droplets was increased, the interface became less deformable and the adsorbed copolymer layer thickness significantly decreased. Findings are discussed with respect to the adsorbed copolymer conformations, which result from different adsorption mechanisms and the influence of interfacial penetrability.

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Surface functionalized mesoporous silicon (pSi) microparticles are reported as a solid dispersion carrier for improving dissolution and enhancing the orally administered pharmacokinetics (fasted rat model) of indomethacin (IMC), employed as a model poorly soluble BCS type II drug. IMC was loaded via immersion/solvent evaporation onto the thermally oxidized pSi particles, which provide a stable hydrophilic matrix with a nanoporous structure. The solid state properties of IMC loaded pSi were characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, differential scanning calorimetry and thermogravimetric analysis. IMC molecules are encapsulated in a noncrystalline state due to geometric confinement in the nanopores; stability of the noncrystalline state has been demonstrated for several months under accelerated storage conditions. The pSi carrier facilitates accelerated immediate release of IMC and enhanced oral delivery performance in comparison with crystalline indomethacin and Indocid i.e. a 4-times reduction on T(max), a 200% increase on C(max) and a significant increase in bioavailability. The in vitro-in vivo correlation is discussed based on the noncompartment model and gives insight into the delivery mechanism for the pSi carrier.