Systematic Development and Characterization of Enzyme-Free, Borax-Crosslinked Microneedles for Glucose-Responsive Insulin Delivery and In Vivo Glycemic Mitigation

<b>Background:</b> Conventional insulin injections cannot mimic physiological pancreatic function and often lead to dangerous hypoglycemic events that glucose-responsive systems aim to prevent. Glucose-responsive microneedles (MNs) offer a promising closed-loop alternative. We developed an enzyme-free, glucose-responsive MN patch composed of a PVA/Dextran hydrogel dynamically crosslinked with borax, and evaluated its performance, biosafety, and in vivo efficacy. <b>Methods:</b> MNs were fabricated from PVA/Dextran via micromolding and crosslinked with borax. The formulation was systematically optimized based on mechanical properties and glucose-responsive release kinetics. Physicochemical properties, biosafety (cytotoxicity, skin barrier recovery, boron leaching), and in vivo efficacy in a type 1 diabetic mouse model were evaluated in comparison to a subcutaneous (SC) insulin injection. <b>Results:</b> The optimized MNs showed robust mechanics (per-needle fracture force approximately 1.0 N) for reliable skin penetration. The system demonstrated clear glucose sensitivity, with a release flux ratio ≥1.5 between hyperglycemic (e.g., 400 mg·dL^-1) and normoglycemic (100 mg·dL^-1) conditions and exhibited excellent reversibility under alternating glucose levels. The patch was highly biocompatible, with >95% cell viability, the only transient skin barrier disruption that fully recovered within 24 h, and had low boron release from patches in vitro. In vivo, the optimized sI-MN patch demonstrated a sustained, glucose-responsive release profile, maintaining blood glucose in diabetic mice near 100 mg·dL^-1 for approximately 8 h. This pharmacokinetic profile contrasts markedly with the rapid hypoglycemic nadir and rebound hyperglycemia observed with a standard subcutaneous insulin bolus, highlighting the patch's potential for mitigating hypoglycemia. <b>Conclusions:</b> The enzyme-free PVA/Dextran/borax MN patch enables autonomous, glucose-responsive insulin delivery. It provides more stable and safer glycemic control than conventional injections by mitigating the risk of hypoglycemia. By mitigating the hypoglycemic risk associated with bolus injections, this systematically optimized platform represents a potential step toward a safer, patient-friendly diabetes therapy, though significant challenges in duration and dose scaling remain.