The expansion of conventional prosthetic skin grafts has been quantified to be very low and insufficient for treating large and severe burn injuries with small amounts of available donor skin. Auxetic metamaterials and structures are the new class of materials which shows expansion in both directions while stretching in one direction (i.e., negative Poisson’s effect). This property of auxetic materials offers unprecedented potential for expansions in prosthetic skin grafts, which is crucial for covering large burn injury sites. This review article focuses on summarizing the up-to-date literature on design and analysis of auxetic skin grafts, and their expansion potential. This knowledge is anticipated to advance burn mitigation research and allow for development of prosthetic skin grafts for severe burn skin replacements.

Severe burn injures lead to millions of fatalities every year due to lack of skin replacements. While skin is a very limited and expensive entity, split thickness skin grafting, which involves the projection of a parallel incision pattern on a small section of healthy excised skin, is typically employed to increase the expansion and cover a larger burn site. To date, the real expansion capacity of such grafts are low (<3 times) and insufficient for treatment of severe burn injuries. In this study, novel I-shaped auxetic incision patterns, which are known to exhibit high negative Poisson’s ratios, have been tested on the skin to investigate their expansion potential. Fourteen two-layer skin graft models with varying incision pattern parameters (i.e., length, spacing, and orientation) were developed using finite element modelling and tested under uniaxial and biaxial tensile loads. The Poisson’s ratio, meshing ratios, and induced stresses were quantified across all models. Graft models tested uniaxially along the orthogonal directions indicated opposite trends in generated Poisson’s ratios, as the length of the I-shape incisions were increased. Biaxially, with a symmetric and closely spaced I-shape pattern, graft meshing ratios up to 15.65 were achieved without overstressing the skin. Overall, the findings from the study indicated that expansion potentials much higher than that of traditional skin grafts can be achieved with novel I-shaped auxetic skin grafts, which would be indispensable for covering large wounds in severe burn injuries.

Skin graft expansion is the key to the treatment of severe burn injuries requiring skin transplantation. While high expansions have been claimed by a majority of graft manufacturers, the realistic expansions reported to date with skin grafts are much lower. To clarify this discrepancy, we attempted to understand the biomechanics of skin grafts through the study of common graft pattern sizes, spacing, and orientation, and their influence on mesh expansion and induced stress. A novel skin simulant material and additive manufacturing were employed to develop the skin graft models. Tensile testing experiments were conducted to study expansion and overall stresses, and a finite element model (FEM) was used to characterize the local trends. At low strains (i.e., <1), the mesh expansion ratio was reported to be below 1, which increased up to 1.93 at a high strain of 2. The pattern size and spacing were not observed to affect the expansion much (i.e., <10%). With a change in orientation, the expansion decreased across all graft models and strains. High localized induced stresses were reported for high strains, which varied with graft orientation. The novel observations highlight the achievable expansions without overstressing, with standard slit patterning in skin grafts. These findings will not only help achieve better mesh expansion outcomes in burn surgeries but also guide the development of novel graft patterns for enhanced expansion in the future.