Hybrid Silk Fibroin/Poly(lactic acid) Macromer Nanogels for Advanced Drug Delivery: Influence of Organic and Silsesquioxane Crosslinker Topologies on Molecular Organization and Physicochemical Properties

Molecular orientation and structural topology play critical roles in determining the performance of biomacromolecular nanocarriers for controlled drug delivery. Herein, the influence of organic and silsesquioxane crosslinker topologies on hybrid nanogels composed of an allyl-functionalized silk fibroin macromer (SFM) and poly(lactic acid) macromer (PLAM) was systematically investigated. Three crosslinkers with distinct molecular architectures were employed: a linear organic framework (N,N'-methylenebisacrylamide, MBAm), a ladder-like silsesquioxane [LD-4Ph-2MeVi, (LDSQ)], and a well-defined double-decker silsesquioxane [(DDSQ-2MeVi, (DDSQ)]. Crosslinker topology markedly influenced nanogel morphology and surface characteristics, producing average particle diameters of approximately 292 nm [NG1(MBAm)], 688 nm [NG2(LDSQ)], and 315 nm [NG3(DDSQ)], while silsesquioxane-containing nanogels exhibited negative zeta potentials (-10 to -30 mV). High-resolution XPS, HRTEM, and XRD analyses revealed that the rigid-cage-like DDSQ topology promotes localized molecular ordering and more compact chain packing of β-sheet-rich SFM and PLAM chains within the hybrid nanogel network. These structural differences significantly affected pH-responsive biphasic Rhodamine B release. At physiological pH 7.4, electrostatic interactions between the nanogel matrix and cargo molecules minimized premature release, whereas acidic conditions (pH 5.5) induced protonation-mediated swelling and accelerated molecular diffusion, resulting in accelerated release. Among all formulations, NG3(DDSQ) exhibited the highest cumulative release efficiency, which was attributed to its ordered network structure and stable nanochannel-like free volume. Furthermore, all nanogels demonstrated excellent cytocompatibility (>98% viability) and significantly promoted fibroblast proliferation (p < 0.05). These findings establish a clear topology-structure-property relationship in silk fibroin/PLA hybrid nanogels and demonstrate that silsesquioxane topology is an effective molecular design parameter for developing pH-responsive biomacromolecular drug delivery systems.