Porous silicon and silica carriers for delivery of peptide therapeutics

Peptides have gained tremendous popularity as biological therapeutic agents in recent years due to their favourable specificity, diversity of targets, well-established screening methods, ease of production, and lower cost. However, their poor physiological and storage stability, pharmacokinetics, and fast clearance have limited their clinical translation. Novel nanocarrier-based strategies have shown promise in overcoming these issues. In this direction, porous silicon (pSi) and mesoporous silica nanoparticles (MSNs) have been widely explored as potential carriers for the delivery of peptide therapeutics. These materials possess several advantages, including large surface areas, tunable pore sizes, and adjustable pore architectures, which make them attractive carriers for peptide delivery systems. In this review, we cover pSi and MSNs as drug carriers focusing on their use in peptide delivery. The review provides a brief overview of their fabrication, surface modification, and interesting properties that make them ideal peptide drug carriers. The review provides a systematic account of various studies that have utilised these unique porous carriers for peptide delivery describing significant in vitro and in vivo results. We have also provided a critical comparison of the two carriers in terms of their physicochemical properties and short-term and long-term biocompatibility. Lastly, we have concluded the review with our opinion of this field and identified key areas for future research for clinical translation of pSi and MSN-based peptide therapeutic formulations.

Controlling and Predicting the Dissolution Kinetics of Thermally Oxidised Mesoporous Silicon Particles: Towards Improved Drug Delivery

Porous silicon (pSi) continues to receive considerable interest for use in applications ranging from sensors, biological scaffolds, therapeutic delivery systems to theranostics. Critical to all of these applications is pSi degradation and stabilization in biological media. Here we report on progress towards the development of a mechanistic understanding for the dissolution behavior of native (unoxidized) and thermally oxidized (200-600 °C) pSi microparticles. Fourier transform infrared (FTIR) spectroscopy was used to characterize the pSi surface chemistry after thermal oxidation. PSi dissolution was assessed using a USP method II apparatus by monitoring the production of orthosilicic acid, and the influence of gastro-intestinal (GI) fluids were examined. Fitting pSi dissolution kinetics with a sum of the exponential model demonstrated that the dissolution process strongly correlates with the three surface hydride species and their relative reactivity, and was supported by the observed FTIR spectral changes of pSi during dissolution. Finally, the presence of GI proteins was shown to hamper pSi dissolution by adsorption to the pSi surface acting as a barrier preventing water attack. These findings are significant in the optimal design of pSi particles for oral delivery and other controlled drug delivery applications.

Harnessing the Aqueous Chemistry of Silicon: Self-Assembling Porous Silicon/Silica Microribbons

The synthesis of microribbons based on the assembly of porous silicon nanoparticles (pSiNPs) in a silica matrix is reported. The formation of these structures is driven by dissolution and reprecipitation of silica derived from the NPs upon drying of an aqueous colloidal dispersion. The process generates composite films that fracture into filaments due to geometric stresses associated with drying of the film on a curved surface. By controlling NP concentration, solvent, and temperature during the evaporation process, well-defined microribbons with a rectangular cross section of ∼25 × 100 microns and lengths on the order of 1 cm are formed. Partial thermal oxidation of the ribbons generates luminescent Si-SiO₂ core-shell composites, and complete oxidation generates porous SiO₂ ribbons with retention of the mesoporous nanostructure. The pores can be infiltrated with daunorubicin as a model drug, and the resulting material shows sustained release of the chemotherapeutic for more than 70 days.

Structure-Relaxivity Relationships of Magnetic Nanoparticles for Magnetic Resonance Imaging

Magnetic nanoparticles (MNPs) have been extensively explored as magnetic resonance imaging (MRI) contrast agents. With the increasing complexity in the structure of modern MNPs, the classical Solomon-Bloembergen-Morgan and the outer-sphere quantum mechanical theories established on simplistic models have encountered limitations for defining the emergent phenomena of relaxation enhancement in MRI. Recent progress in probing MRI relaxivity of MNPs based on structural features at the molecular and atomic scales is reviewed, namely, the structure-relaxivity relationships, including size, shape, crystal structure, surface modification, and assembled structure. A special emphasis is placed on bridging the gaps between classical simplistic models and modern MNPs with elegant structural complexity. In the pursuit of novel MRI contrast agents, it is hoped that this review will spur the critical thinking for design and engineering of novel MNPs for MRI applications across a broad spectrum of research fields.

Mesoporous Silica and Organosilica Nanoparticles: Physical Chemistry, Biosafety, Delivery Strategies, and Biomedical Applications

Predetermining the physico-chemical properties, biosafety, and stimuli-responsiveness of nanomaterials in biological environments is essential for safe and effective biomedical applications. At the forefront of biomedical research, mesoporous silica nanoparticles and mesoporous organosilica nanoparticles are increasingly investigated to predict their biological outcome by materials design. In this review, it is first chronicled that how the nanomaterial design of pure silica, partially hybridized organosilica, and fully hybridized organosilica (periodic mesoporous organosilicas) governs not only the physico-chemical properties but also the biosafety of the nanoparticles. The impact of the hybridization on the biocompatibility, protein corona, biodistribution, biodegradability, and clearance of the silica-based particles is described. Then, the influence of the surface engineering, the framework hybridization, as well as the morphology of the particles, on the ability to load and controllably deliver drugs under internal biological stimuli (e.g., pH, redox, enzymes) and external noninvasive stimuli (e.g., light, magnetic, ultrasound) are presented. To conclude, trends in the biomedical applications of silica and organosilica nanovectors are delineated, such as unconventional bioimaging techniques, large cargo delivery, combination therapy, gaseous molecule delivery, antimicrobial protection, and Alzheimer's disease therapy.

Degradability and Clearance of Silicon, Organosilica, Silsesquioxane, Silica Mixed Oxide, and Mesoporous Silica Nanoparticles

The biorelated degradability and clearance of siliceous nanomaterials have been questioned worldwide, since they are crucial prerequisites for the successful translation in clinics. Typically, the degradability and biocompatibility of mesoporous silica nanoparticles (MSNs) have been an ongoing discussion in research circles. The reason for such a concern is that approved pharmaceutical products must not accumulate in the human body, to prevent severe and unpredictable side-effects. Here, the biorelated degradability and clearance of silicon and silica nanoparticles (NPs) are comprehensively summarized. The influence of the size, morphology, surface area, pore size, and surface functional groups, to name a few, on the degradability of silicon and silica NPs is described. The noncovalent organic doping of silica and the covalent incorporation of either hydrolytically stable or redox- and enzymatically cleavable silsesquioxanes is then described for organosilica, bridged silsesquioxane (BS), and periodic mesoporous organosilica (PMO) NPs. Inorganically doped silica particles such as calcium-, iron-, manganese-, and zirconium-doped NPs, also have radically different hydrolytic stabilities. To conclude, the degradability and clearance timelines of various siliceous nanomaterials are compared and it is highlighted that researchers can select a specific nanomaterial in this large family according to the targeted applications and the required clearance kinetics.