Pore size is a critical parameter for obtaining sustained protein release from electrochemically synthesized mesoporous silicon microparticles.

Mesoporous silicon has become a material of high interest for drug delivery due to its outstanding internal surface area and inherent biodegradability. We have previously reported the preparation of mesoporous silicon microparticles (MS-MPs) synthesized by an advantageous electrochemical method, and showed that due to their inner structure they can adsorb proteins in amounts exceeding the mass of the carrier itself. Protein release from these MS-MPs showed low burst effect and fast delivery kinetics with complete release in a few hours. In this work, we explored if tailoring the size of the inner pores of the particles would retard the protein release process. To address this hypothesis, three new MS-MPs prototypes were prepared by electrochemical synthesis, and the resulting carriers were characterized for morphology, particle size, and pore structure. All MS-MP prototypes had 90 µm mean particle size, but depending on the current density applied for synthesis, pore size changed between 5 and 13 nm. The model protein α-chymotrypsinogen was loaded into MS-MPs by adsorption and solvent evaporation. In the subsequent release experiments, no burst release of the protein was detected for any prototype. However, prototypes with larger pores (>10 nm) reached 100% release in 24-48 h, whereas prototypes with small mesopores (<6 nm) still retained most of their cargo after 96 h. MS-MPs with ∼6 nm pores were loaded with the osteogenic factor BMP7, and sustained release of this protein for up to two weeks was achieved. In conclusion, our results confirm that tailoring pore size can modify protein release from MS-MPs, and that prototypes with potential therapeutic utility for regional delivery of osteogenic factors can be prepared by convenient techniques.

Protein delivery based on uncoated and chitosan-coated mesoporous silicon microparticles.

Mesoporous silicon is a biocompatible, biodegradable material that is receiving increased attention for pharmaceutical applications due to its extensive specific surface. This feature enables to load a variety of drugs in mesoporous silicon devices by simple adsorption-based procedures. In this work, we have addressed the fabrication and characterization of two new mesoporous silicon devices prepared by electrochemistry and intended for protein delivery, namely: (i) mesoporous silicon microparticles and (ii) chitosan-coated mesoporous silicon microparticles. Both carriers were investigated for their capacity to load a therapeutic protein (insulin) and a model antigen (bovine serum albumin) by adsorption. Our results show that mesoporous silicon microparticles prepared by electrochemical methods present moderate affinity for insulin and high affinity for albumin. However, mesoporous silicon presents an extensive capacity to load both proteins, leading to systems were protein could represent the major mass fraction of the formulation. The possibility to form a chitosan coating on the microparticles surface was confirmed both qualitatively by atomic force microscopy and quantitatively by a colorimetric method. Mesoporous silicon microparticles with mean pore size of 35 nm released the loaded insulin quickly, but not instantaneously. This profile could be slowed to a certain extent by the chitosan coating modification. With their high protein loading, their capacity to provide a controlled release of insulin over a period of 60-90 min, and the potential mucoadhesive effect of the chitosan coating, these composite devices comprise several features that render them interesting candidates as transmucosal protein delivery systems.

Porous silicon for photosensitized formation of singlet oxygen in water and in simulated body fluid: two methods of modification by undecylenic acid.

Initially H-terminated and therefore hydrophobic surface of electrochemically prepared luminescent porous silicon (PSi) powder was transformed to the hydrophilic one by means of surface modification by undecylenic acid. Physical adsorption of undecylenic acid as a non-ionic surfactant and its chemical binding through C[triple bond]C bond opening and Si-C bond formation were applied as two different methods of PSi surface modification, physical and chemical modification, respectively. Luminescence of aqueous suspensions of the both types of modified PSi powders in merely water and in simulated body fluid physiological electrolyte was measured as a function of time. Many-fold (up to 20 times) building-up of the luminescence intensity was observed for PSi aqueous suspensions during the first few days, the growth was followed by a slower (a week and more) luminescence intensity decay. As it is evidenced by FTIR spectra and SEM images, the effect of the luminescence growth and decay of PSi suspension in water can be in part attributed to the PSi surface oxidation accompanied by its dissolution and de-aggregation of large PSi particles. It is concluded also from the experiments on PSi luminescence reversible quenching by O2 that SiO-related surface states with the excitation energy about 2.2 eV are formed during water-assisted oxidation of Si nanocrystal surface. An appearance of a large number of such surface states can be also responsible for the observed PSi luminescence building-up.

Intragenomic heterogeneity of the 16S rRNA-23S rRNA internal transcribed spacer among Pseudomonas syringae and Pseudomonas fluorescens strains.

The 16S-23S rRNA internal transcribed spacer regions (ITS1) from 14 strains of Pseudomonas syringae and P. fluorescens were sequenced. ITS1 exhibited significant sequence variability among different operons within a single genome. From 1 to 4 types of ITS1 were found in individual genomes of the P. syringae and P. fluorescens strains. A total of eight ITS1 types were identified among strains studied. The ITS1 nucleotide sequences consisted of conserved blocks including, among others, a stem-forming region of box B, tRNAIle and tRNAAla genes and several variable blocks. The differences in the variable regions were mostly due to insertions and/or deletions of nucleotide blocks. The intragenomic heterogeneity of ITS1 was brought about by different combinations of variable blocks, which possibly have resulted from recombination and horizontal transfer.