Injectable Polyurethane Composite Scaffolds Delay Wound Closure and Support Cellular Infiltration and Remodeling In Rat Excisional Wounds

Injectable scaffolds present compelling opportunities for wound repair and regeneration due to their abilities to fill irregularly shaped defects and deliver biologics such as growth factors. However, there are several challenges that must be overcome, including toxicity of reactants or intermediates, curing in a reasonable amount of time, forming a sufficient pore structure, and achieving robust mechanical properties.  In this study, we investigated the properties of injectable polyurethane (PUR) biocomposite scaffolds and their application in cutaneous wound repair using a rat excisional model. PUR scaffolds were synthesized by reacting a lysine triisocyanate-poly(ethylene glycol) prepolymer with a polysaccharide filler (hyaluronic acid [HA] or carboxymethyl cellulose [CMC]) and a polyester triol with a backbone comprising 60% caprolactone, 30% glycolide, and 10% lactide.  Cure profiles of the scaffolds were measured using a rheometer, thermal transitions were evaluated by differential scanning calorimetry, and mechanical properties were measured using a dynamic mechanical analyzer. The reactivities of polyester triol, HA, CMC, and water with LTI-PEG were determined using attenuated total reflectance-fourier transform infrared spectroscopy. The capacity of the scaffolds to facilitate dermal wound healing was evaluated in an excisional wound model in Sprague-Dawley rats. The groups investigated were blank wounds, PUR scaffolds with 15 wt% HA, and PUR scaffolds with 15% CMC.  Wounds were harvested at days 7, 17, 26, and 35 (n = 4) and processed for histological evaluation.  Hematoxylin & eosin, Gomori's trichrome, picrosirius red, TUNEL, Ki67, a-SMA, and procollagen I immunostaining were performed on the tissue sections. The scaffolds have a minimal reaction exotherm (<10°C), a working time of 6-7 min, and a setting time of 16-19 min.  Moreover, the compressive Young's modulus of the scaffolds under physiologic conditions ranges from 30-60 kPa, which is comparable to that of human skin.  In the rat excisional wound model, injection of settable biocomposite scaffolds stented the wounds at early time points, resulting in a regenerative rather than a scarring phenotype at later time points. Measurements of wound length and thickness revealed that the treated wounds were less contracted at day 7 compared to blank wounds. Analysis of cell proliferation and apoptosis showed that the scaffolds were biocompatible and supported tissue ingrowth. Furthermore, the number of Ki67+ cells in the PUR scaffolds was significantly higher than in the blank wounds from days 17 – 26.  Myofibroblast formation and collagen organization provided evidence that the scaffolds have a positive effect on extracellular matrix remodeling by disrupting the formation of an aligned matrix under elevated tension. In summary, we have developed an injectable biodegradable polyurethane biocomposite scaffold that enhances cutaneous wound healing in a rat model.