Modeling of metamaterial based incision patterns for generating high expansions in skin grafts

BACKGROUND

Split-thickness skin grafting is a widely used treatment for burn patients. It requires removing a small portion of healthy skin, making parallel incisions on it, stretching it, and surgically implanting it into the burn site. Although skin graft mesher companies claim to significantly expand the size of the skin graft, in reality, the amount of expansion achieved is much smaller.

METHODS

This study aimed to improve the expansion potential of skin grafts by designing new incision patterns with auxetic properties, using a skin simulant material and additive manufacturing. The mechanical properties and digital image correlation was used to analyze the strain, effective Poisson's ratio, local strains, void area, and meshing ratio of the auxetic skin graft simulants.

FINDINGS

The results showed that the Y-shaped skin graft simulant had the highest effective negative Poisson's ratio, largest areal expansions, lowest maximum induced stresses and strains, and uniform strain distribution properties, making it the best choice for generating high expansions in skin grafts.

INTREPRETATION

The study found that the expansions were highly strain sensitive, with higher auxeticity observed at lower strains. The novel findings with auxetic skin graft simulants are expected to provide valuable insights for developing skin grafts with higher expansion potential in the future.

Development of Novel Hierarchical Designs for Skin Graft Simulants with High Expansion Potential

Skin graft designs play an essential role in healing severe burn injuries. Split-thickness skin grafting (STSG) is one of the commonly used techniques for treating large burn injuries. In STSG, parallel cuts are projected onto a small portion of excised healthy skin for expansion and covering a larger burn area. To date, expansions reported for STSG are very low and insufficient to cover large burn areas. In this work, novel traditional and auxetic hierarchical cuts were designed on skin graft simulants, which were mechanically tested to study their expansion potential. Additive manufacturing and a two-part polymeric material were employed to fabricate the skin graft simulants. The nonlinear mechanical behavior of the hierarchical skin graft simulants was characterized using hyperelastic models. The effective Poisson's ratio, meshing ratios, and induced stresses in first and second-order hierarchical cut patterns were estimated across all skin graft simulants for up to 300% strain. Also, Statistical analysis was performed to calculate the significance among the groups. From the analysis, the skin graft simulants with second-order auxetic incision patterns were found to exhibit the lowest induced stresses and maximum expansion of approximately four times, at 300% strain. To date, traditional skin grafts have only been able to achieve up to three times expansion. Therefore, the expansions realized with the novel hierarchical skin graft simulants is unprecedented, with the potential to generate ground-breaking advances in burn injury treatment.

Mechanisms of fatigue crack initiation and propagation in auxetic meta-biomaterials

The fatigue performance of additively manufactured auxetic meta-biomaterials made from commercially pure titanium has been studied only recently. While certain assumptions have been made regarding the mechanisms underlying their fatigue failure, the exact mechanisms are not researched yet. Here, we studied the mechanisms of crack formation and propagation in cyclically loaded auxetic meta-biomaterials. Twelve different designs were subjected to compression-compression fatigue testing while performing full-field strain measurement using digital image correlation (DIC). The fatigue tests were stopped at different points before complete specimen failure to study the evolution of damage in the micro-architecture of the specimens using micro-computed tomography (micro-CT). Furthermore, finite element models were made to study the presence of stress concentrations. Structural weak spots were found in the inverted nodes and the vertical struts located along the outer rim of the specimens, matching the maximum principal strain concentrations and fracture sites in the DIC and micro-CT data. Cracks were often found to originate from internal void spaces or from sites susceptible to mode-I cracking. Many specimens maintained their structural integrity and exhibited no signs of rapid strain accumulation despite the presence of substantial crack growth. This observation underlines the importance of such microscale studies to identify accumulated damage that otherwise goes unnoticed. The potential release of powder particles from damaged lattices could elicit a foreign body response, adversely affecting the implant success. Finding the right failure criterion, therefore, requires more data than only those pertaining to macroscopic measurements and should always include damage assessment at the microscale. STATEMENT OF SIGNIFICANCE: The negative Poisson's ratio of auxetic meta-biomaterials makes them expand laterally in response to axial tension. This extraordinary property has great potential in the field of orthopedics, where it could enhance bone-implant contact. The fatigue performance of additively manufactured auxetic meta-biomaterials has only recently been studied and was found to be superior to many other bending- and stretch-dominated micro-architectures. In this study, we go beyond these macroscopic measurements and focus on the crack initiation and propagation. Full-field strain measurements and 3D imaging are used to paint a detailed picture of the mechanisms underlying fatigue. Using these data, specific aspects of the design and/or printing process can be targeted to improve the performance of auxetic meta-biomaterials in load-bearing applications.

Fatigue performance of auxetic meta-biomaterials

Meta-biomaterials offer a promising route towards the development of life-lasting implants. The concept aims to achieve solutions that are ordinarily impossible, by offering a unique combination of mechanical, mass transport, and biological properties through the optimization of their small-scale geometrical and topological designs. In this study, we primarily focus on auxetic meta-biomaterials that have the extraordinary ability to expand in response to axial tension. This could potentially improve the longstanding problem of implant loosening, if their performance can be guaranteed in cyclically loaded conditions. The high-cycle fatigue performance of additively manufactured (AM) auxetic meta-biomaterials made from commercially pure titanium (CP-Ti) was therefore studied. Small variations in the geometry of the re-entrant hexagonal honeycomb unit cell and its relative density resulted in twelve different designs (relative density: ~5-45%, re-entrant angle = 10-25°, Poisson's ratio = -0.076 to -0.504). Micro-computed tomography, scanning electron microscopy and mechanical testing were used to respectively measure the morphological and quasi-static properties of the specimens before proceeding with compression-compression fatigue testing. These auxetic meta-biomaterials exhibited morphological and mechanical properties that are deemed appropriate for bone implant applications (elastic modulus = 66.3-5648 MPa, yield strength = 1.4-46.7 MPa, pore size = 1.3-2.7 mm). With an average maximum stress level of 0.47 σ_y at 10⁶ cycles (range: 0.35 σ_yσ_y- 0.82 σ_yσ_y), the auxetic structures characterized here are superior to many other non-auxetic meta-biomaterials made from the same material. The optimization of the printing process and the potential application of post-processing treatments could improve their performance in cyclically loaded settings even further. STATEMENT OF SIGNIFICANCE: Auxetic meta-biomaterials have a negative Poisson's ratio and, therefore, expand laterally in response to axial tension. Recently, they have been found to restore bone-implant contact along the lateral side of a hip stem. As a result, the bone will be compressed along both of the implant's contact lines, thereby actively reducing the risk of implant failure. In this case the material will be subjected to cyclic loading, for which no experimental data has been reported yet. Here, we present the first ever study of the fatigue performance of additively manufactured auxetic meta-biomaterials based on the re-entrant hexagonal honeycomb. These results will advance the adoption of auxetic meta-biomaterials in load-bearing applications, such as the hip stem, to potentially improve implant longevity.