Stearic acid-capped mesoporous silica microparticles as novel needle-like-structured drug delivery carriers

Mesoporous silica are widely utilised as drug carriers due to their large pore volume and surface area, which facilitate effective loading. Additionally, they can be used to enhance drugs stability and protect against enzymatic degradation due to their silica framework. However, without the addition of a capping material, the loaded cargo may be prematurely released before reaching the target site. This work reports the functionalisation of a commercially available silica microparticle (SYLOID XDP 3050) with stearic acid at various stearic acid loading concentrations (20-120 % w/w). Scanning electron microscopy (SEM) analysis revealed that the pores were capped with stearic acid, with the filling ratio increasing proportionally to the loading concentration. Notably, needle-like structures appeared when the stearic acid amount exceeded 80 % w/w, surpassing the calculated theoretical maximum pore filling ratio (64.32 %). The molecular interactions were highlighted using Fourier-transform infrared spectroscopy (FTIR), as the intensity of the CH₃ increased with increased stearic acid loading concentrations. The needle-structures phenomenon was corroborated by 3D confocal imaging. It utilised the autofluorescence properties of stearic acid to demonstrate its presence within the carrier, with fluorescence intensity increasing alongside the stearic acid concentration. Differential scanning calorimetry (DSC) indicated the crystalline nature of these needle structures, which was further confirmed by X-ray diffraction (XRD) analysis, validating the crystallisation of the stearic acid needles. Moreover, nitrogen porosimetry was employed to assess the pore volume and surface area, where the formulation containing 120 % stearic acid exhibited the lowest pore volume (0.59 cc). This value was smaller than unloaded SYLOID (2.1 cc), indicating near-complete filling of the carrier. This newly developed SYLOID-stearic acid carrier will now be used to enhance formulation development as a platform to enhance protein oral drug delivery.