In situ hydrogel capturing nitric oxide microbubbles accelerates the healing of diabetic foot

Diabetic foot ulcer (DFU) is a devastating complication in diabetes patients, imposing a high risk of amputation and economic burden on patients. Sustained inflammation and angiogenesis hindrance are thought to be two key drivers of the pathogenesis of such ulcers. Nitric oxide (NO) has been proven to accelerate the healing of acute or chronic wounds by modulating inflammation and angiogenesis. However, the use of gas-based therapeutics is difficult for skin wounds. Herein, therapeutic NO gas was first prepared as stable microbubbles, followed by incorporation into a cold Poloxamer-407 (P₄₀₇) solution. Exposed to the DFU wound, the cold P₄₀₇ solution would rapidly be transformed into a semisolid hydrogel under body temperature and accordingly capture NO microbubbles. The NO microbubble-captured hydrogel (PNO) was expected to accelerate wound healing in diabetic feet. The NO microbubbles had an average diameter of 0.8 ± 0.4 μm, and most of which were captured by the in situ P₄₀₇ hydrogel. Moreover, the NO microbubbles were evenly distributed inside the hydrogel and kept for a longer time. In addition, the gelling temperature of 30% (w/v) P₄₀₇ polymer (21 °C) was adjusted to 31 °C for the PNO gel, which was near the temperature of the skin surface. Rheologic studies showed that the PNO gel had mechanical strength comparable with that of the P₄₀₇ hydrogel. The cold PNO solution was conveniently sprayed or smeared on the wound of DFU and rapidly gelled. In vivo studies showed that PNO remarkably accelerated wound healing in rats with DFU. Moreover, the sustained inflammation at the DFU wound was largely reversed by PNO, as reflected by the decreased levels of proinflammatory cytokines (IL-1β, IL-6 and TNF-α) and the increased levels of anti-inflammatory cytokines (IL-10, IL-22 and IL-13). Meanwhile, angiogenesis was significantly promoted by PNO, resulting in rich blood perfusion at the DFU wounds. The therapeutic mechanism of PNO was highly associated with polarizing macrophages and maintaining the homeostasis of the extracellular matrix. Collectively, PNO gel may be a promising vehicle of therapeutic NO gas for DFU treatment.