Demonstrating translational relevance of organoid and multicellular tissues for preclinical models of human disease
Full-thickness burns where both the dermal and epidermal layer of the skin are destroyed result in high patient mortality due to infection, dehydration, and shock. The current standard of care involves the direct application of an acellular crosslinked protein scaffold which forms a temporary physical barrier and promotes host cell migration into the wound area; however, this is problematic in severe burns where little healthy skin is available for repair. Delivery of patient-derived autologous or immunoprivileged allogeneic cells are emerging as potential treatment options due to continuous extracellular matrix remodeling and persistent cell signaling, but challenges include homogenous delivery of cells onto a large, non-flat wound topography. Although approaches such as cell spraying and microparticle injecting have been explored in the field, the continuous formation of three-dimensional, hydrogel-based tissue constructs uniformly on a physiological wound surface remains unsolved. Here, we report the development of a handheld bioprinter which delivers wound-conformal dermal tissue constructs to improve wound healing in full-thickness burns. Mesenchymal stromal cell (MSC)-containing fibrinogen bioink and thrombin crosslinker solutions were delivered through on-board syringe pumps to a microfluidic printhead with internal bifurcated channels. Dermal tissue constructs of consistent thickness covered with the crosslinker were obtained at the exit. Wound-conformal delivery of these MSC-laden dermal tissue constructs was achieved by translating the printhead along the wound surface by a soft silicone wheel, while a two-axis gimbal design allowed it to adapt to the wound topology. We observed that the addition of 1% hyaluronic acid (HA) provided desirable shear-thinning behavior of the bioink (1.2 Pa·s at shear rate 1/s; 0.35 Pa·s at shear rate 100/s), resulting in 83% of the starting thickness to be maintained for deposition surfaces with inclination angles of 45 degrees. Furthermore, these fibrin-HA hydrogels maintained high biocompatibility with the co-delivered MSCs ( >94%), in addition to long-term preservation of 3D morphology and cell proliferation as shown with Hoechst/Phalloidin+ immunostaining over one week. To demonstrate the clinical utility of this approach, we uniformly distributed 1x10^6 MSCs/ml of the fibrin-HA hydrogel on a porcine 5cm x 5cm full-thickness burn wound model and quantified a 1.4-fold improvement of macroscopic re-epithelialization speed, a 1.3-fold increase in collagen density in the dermal layer, and a 2.5-fold reduction in CD11b+ inflammatory cell activity after 28 days compared to burn controls, as observed via microscopic analysis of H&E histological stains. Taken together, we have shown that the handheld bioprinter can conformally deliver MSC-containing dermal tissue constructs directly on wound substrates with physiological topographies, leading to full thickness burn wound repair as shown in porcine pre-clinical case studies.