Auxetic incisions with alternating slit shapes: a promising technique for enhancing synthetic skin grafts expansion

Split thickness skin grafts are widely used in treating burn injuries. To date, with limited availability of donor skin and minimal expansions offered by conventional skin grafting, it is challenging to cover large and severe burns. In this study, novel synthetic skin grafts with alternating slit (AS) shaped cut patterns were developed and tested to evaluate the expansion potentials offered by auxetic or negative Poisson’s ratio structures in skin grafting. A range of auxetic incision patterns were designed with varying unit cell dimensions, and these were projected onto skin using 3D printing. The mechanical properties and digital image correlation of the created synthetic skin grafts were used to determine stress, effective Poisson’s ratio, meshing ratio (MR), and generated strains for strain loadings of up to 150%. The AS graft simulant with equal slit lengths and low slit spacings exhibited the maximum negative Poisson’s effect, expansion, and Mr Expansions were inversely related with the spacing between slits. The lowest value for the MR and highest stress was observed with high spacing, high horizontal slit length, and low vertical slit length. The expansions were highly sensitive to the applied strain, with low strains exhibiting high auxeticity. Such an extensive experimental investigation of the expansion potentials and stress estimations of skin grafts with varying AS dimensional parameters have not been conducted previously. The findings would be crucial for advancing research on mitigation of large burn injuries using high expansion skin grafts.


Introduction
Every year, millions of people suffer from skin injuries and diseases [1]. The skin is the biggest organ in the body. It covers and protects our muscles, organs, and bones [2]. The skin is a two-layered structure, comprising the epidermis and dermis [3,4]. The epidermis, the outermost layer, consists of two sublayers: the stratum corneum and viable epidermis. Positioned between the epidermis and the subcutaneous tissue (hypodermis), the dermis provides structural support to the skin and contains various types of collagens oriented in different directions [5]. The hypodermis, which encompasses subcutaneous fat and covers the muscles, is a multi-layered and heterogeneous material. The thickness of the epidermis ranges from 0.07 mm to 0.12 mm, while the dermis can vary in thickness from 1 mm to 4 mm, depending on the body region [6][7][8][9]. Skin damage can be caused by injuries such as cuts or wounds. Minor injuries can heal with time, while serious injuries (such as burns) require skin grafts transplantation [10,11]. It plays an essential role in regulating body temperature, sensation, and protection against external factors such as bacteria, viruses, and UV radiation [11][12][13]. However, skin can be damaged due to various reasons, such as burns, wounds, infections, or genetic conditions, which can lead to severe health complications [14]. To procure the large burn region, skin grafting technology was used in the 1960 s [15][16][17]. Full-thickness, split-thickness, and Meek grafting were the majorly used method to recover large burn regions [18][19][20]. Skin grafting is a surgical procedure where skin is removed from one part of the body and transplanted onto another part of the body [21]. It is commonly used to treat burns, wounds, and other injuries Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. that have damaged the skin. While skin grafting has been successful in many cases, this procedure faces some global issues (i.e., limited donor skin, cost, lack of suitable donors, donor site morbidity) [22,23].
Mesh grafts refer to a specific category of skin grafts characterized by the cutting of the skin into a mesh-like pattern [24]. This technique, known as meshing, serves the purpose of augmenting the surface area of the skin graft, facilitating its coverage of larger wounds pattern [24,25]. Conversely, autologous skin grafts (autografts) entail a surgical procedure in which a segment of the individual's own skin is extracted from one region of their body (donor site) and transplanted to another area of the same individual's body (recipient site) for the purpose of wound or injury coverage [26,27]. The utilization of autologous skin grafts presents the advantage of minimal risk of rejection, as the body recognizes the transplanted tissue as self [28]. In addition, post-surgical complications such as pain, pigmentation disturbances, and impaired hair regeneration have been reported as concerns associated with the donor site [4]. In literature, autologous skin grafting is widely acknowledged as the most efficacious modality for treating extensive and profound burns [29,30]. Nevertheless, in instances where the wound exhibits insufficient vascularization to support an independent split-thickness skin transplant, allogeneic skin grafts offer a viable alternative for fostering wound healing [31].
In recent years, Meek grafting involves taking a small piece of skin from the donor site and dividing it into many smaller cuts, called micrografts [14,32,33]. These micrografts are then transplanted onto the recipient site, which allows for more coverage of the wound area and better healing [33]. It is beneficial for large or complex wounds, such as burns or extensive skin loss, where traditional skin grafting methods may not be as effective [18]. It can also be used to treat chronic wounds or non-healing ulcers [14,33]. The main drawback of meek grafting was availability of donor skin, risk of infection, and required spacilized training to cover the burn sites. Current skin grafting trends focus on improving the availability, effectiveness, expansions, and safety of the procedure [23]. Auxetic patterns were recently used in skin grafting to increase the expansion ratio and recover large burn areas effectively [34][35][36][37].
Several computational, experimental, and clinical studies reported that the claimed meshing ratio of the skin graft was less than the actual meshing ratio. Capek et al [38] used the finite element method (FEM) to study the importance of the direction of Langer's lines. They compared the deformations in perpendicular and parallel orientations of the graft and found that the expansions were up to 25% different. Gupta et al [21] fabricated ovalshaped synthetic skin graft patterns with varying slit sizes and spacing to understand the effect of skin graft curvature. They observed that oval-shaped traditional skin graft patterns showed significantly lower expansion, and there is a need to change the slit pattern for higher expansion. Koetsier et al [12] used digital image analysis to test the total body burn area and graft-induced wound healing in a group of people ranging in age from 20 to 84. They saw that photo assessment methods could be effective evaluation tools for clinical trials with more than one site. But in this study, the graft expansions that were seen were limited and much lower than what was expected.
Shirzed et al [39] performed a comprehensive investigation into the application of auxetic metamaterials in bone-implanted medical devices. Their study focused on addressing challenges and identifying future research directions in the field of biomedical devices. They concluded that auxetic structures exhibit a high rate of cell proliferation. Additionally, they emphasized that bone-implanted medical devices encompass various applications, including bone scaffolds, bone implants, bone screws, and intervertebral disk implants. In a study conducted by Zolfagharian et al [40], the utilization of 3D-printed mechanical metamaterials for vibration isolation and buckling control was examined. They focused on the exploration of the design, manufacturing, and programmability aspects of these materials in order to attain specific mechanical properties. Through their investigation, they successfully showcased the capability to modify the response of the metamaterials to vibrations, enabling efficient vibration isolation and control. The outcomes of this study underscore the significant potential of 3D-printed programmable mechanical metamaterials as adaptable and customizable solutions for applications involving vibration isolation and buckling control. Gharaie et al [41] developed a twopart silicone resin for fabricating highly stretchable structures using 3D printing (direct ink writing method). In their work, cost-effective extrusion system for a two-part high-viscous resin was developed to fabricate soft and immensely stretchable structures. In their findings, static mixer was capable of evenly mixing the two viscous resins at extremely low flow system. Zolfagharian et al [42] used programable 4D printing techniques to develop customize actuators such as lower inertia, higher payload, and high accuracy. They used these linier actuators in the field of soft robotics to generate high degree of freedom.
Recent reports indicate that auxetic materials produce extremely negative Poisson's ratios (NPR) and strong polymer expansion [43][44][45]. Also, it can be used for the development of high expansion skin grafts [23]. Particularly, alternating slit (AS) shaped auxetic structures have demonstrated a high capacity for generating areal enlargements [46]. Grima et al [36] conducted an experimental study with rubber, a polymer of isoprene with random cuts, and studied their deformation. This study revealed that the randomly oriented cut auxetic structure exhibited a maximum negative Poisson's ratio value of −0.8 under maximum deformation. In literature, several studies were performed to understand the effect of structural parameters such as unit cell length, spacing and orientation in Poisson's ratio [22,[47][48][49]. Based upon the previous studies, on increasing the axial length of the slits, the effective Poisson's ratio decreased [23,50,51]. In previous studies, Singh et al [35] performed the experiments on polymer based rotating triangle auxetic patterns with varying angles from 0°to 135°and tested uniaxially. They observed that on increasing the rotating triangle angle, the Poisson's ratio decreased. Lakes et al [52] conducted a comprehensive material analysis to investigate the impact of different solid auxetic structures on effective Poisson's ratio. The study demonstrated noticeable changes in effective Poisson's ratio when both the strain rates and auxetic designs were varied within the experimental framework. In another study, Gupta et al [22] conducted computational study on different auxetic skin graft patterns and observed the different effective Poisson's ratios changing with the auxetic designs. Cabras et al [53] performed the experiments on thermoplastic lattice structures and tested uniaxially. They conducted a theoretical analysis of the effective properties, and the expression of the macroscopic constitutive properties was given in full analytical form as a function of the constitutive properties of the elements of the lattice and on the geometry of the microstructure. They observed that at microstructural level and low strain, effective Poisson's ratio values were same. In a recent study by Gupta et al [46], the mechanical properties of alternating slit structures applied to skin were studied computationally. The variations in the ratio of slit lengths and spacings were observed to lead to significant changes in the effective Poisson's ratios. To better understand such trends and quantify the design strategies which may lead to higher skin expansions with alternating slit patterns, the current study was conducted on a biofidelic skin simulant.
This study aimed to design, fabricate and test AS-shaped auxetic synthetic skin grafts with variable slit lengths and spacing for expansion. The fabrication process involved projecting the auxetic patterns onto a synthetic skin using additive manufacturing techniques. Mechanical characteristics and digital image correlation (DIC) tests were used to characterise the mechanical responses as well as the expansion potential of the synthetic skin grafts. For 50%, 100%, and 150% applied strains under uniaxial tensile loading, the stresses caused by the grafts, as well as the effective Poisson's ratios and meshing ratios, were determined. In section 2, we cover the graft's modelling and testing procedures; in section 3, the data is presented; and in section 4, we draw our conclusions.

AS-shaped synthetic auxetic skin graft models
The unit cell (lattice structures) shown in figure 1(a) is an extension of a 2D unit cell based on the previous studies [46,54,55]. In literature, Grima et al [36] developed the unit cell of an auxetic perforated system based on perpendicularly arranged alternating slits which mimics the behaviour of rotating square system. They orientated the slits with equal length and maximum angle of 30°in their study. In their study, they calculated the young's modulus and poisson's ratio with varying angles. Li et al [47] reported that enhancing the stiffness of joints can efficiently improve the negative Poisson's ratio effect of lattice structures. Based upon their study, a chiral lattice structure with symmetrically alternating slits based auxetic patterns were designed. The 3D unit cell was obtained by symmetrically patterning the unit cell along the x and y axis as shown in figure 1(b) and 2 mm extruded along the z direction. 3D modelling software (i.e., SolidWorks 2020) was utilised to design the ASshaped auxetic skin graft patterns with varying dimensional parameters. Figure 1 displays the complete design, including the unit cell and front view, with scale bar. The skin graft simulant patterns have fixed dimensions of 50 millimetre × 50 millimetre × 2 millimetres, with L1 and L2 representing the vertical slit length and horizontal slit lengths, respectively. S indicates the distance between the slits, while T denoted the thickness of the slits. In figure 1, the dimensions (L1, L2, S, and T) were all given in mm and varied further as per table 1. The original dimensions L1 and L2 are equal to exhibit symmetry.
The AS-shaped auxetics were chosen as they have been shown to produce negative Poisson's ratios, which is an essential property for skin grafts to exhibit. A thorough review of the relevant literature [46,54,55] was conducted to select the appropriate auxetic models. Nine different designed with varying lengths (L1 and L2), spacing (S), and thickness (T). These models were labelled as D1-D3, D4-D6, and D7-D9, and each design had different slit lengths and spacings. The designs with varying slit lengths were presented in table 1, where the spacing was either 0.5 mm, 1 mm, or 2 mm. All designs had a fixed thickness of 1mm. The selected designs were then used to fabricate the synthetic skin grafts for further analysis and testing.

Fabrication of synthetic skin grafts
The AS-shaped auxetic geometrical models were utilized to design 3D graft molds with uniform dimensions of 50 mm × 50 mm × 2 mm, but with changing incision parameters, as illustrated in figure 2. The graft mold designs were saved as STL files and subsequently converted into g-code before printing. The 3D printers (Shenzhen Creality 3D Technology Co., China), were used to print the molds with 45 mm min −1 printing speed, 210°C nozzle speed and 60°C bad temperature. These molds were then utilized to cast the synthetic auxetic skin grafts.
The study involved the fabrication and testing of numerous skin simulant compositions under uniaxial stress, with a displacement rate of 24 millimetre min −1 , based on previous research [21]. A suitable synthetic skin composition was selected based on the previous work [21]. A two-part silicone with a shore hardness of 15A mixed in a 1:1 weight ratio to develop the selected composition. After 8 h, the material was placed into moulds 3D printed with auxetic patterns. As seen in figure 3, the final shore hardness of the synthetic skin grafts was 15A ± 2A, making them mechanically comparable to cadaver skin.

Mechanical characteristics and digital image correlation (DIC)
Using a universal testing machine (UTM), the mechanical characteristics of synthetic skin grafts of varying lengths and distances between fix points were determined. Uniaxial loading was applied to the opposite end of the clamped samples ( figure 4(A)). Engineering stress-strain curves were generated from the force-displacement data for stresses of up to 150%. Synthetic skin grafts effective Poisson's ratio was calculated by dividing the total lateral strain by the strain in the longitudinal direction. Maximum graft motion in the direction of uniaxial loading was divided by graft length at the outset to determine the longitudinal strain. By dividing the largest orthogonal displacement by the graft's width at the outset of the test, we may determine the lateral strain. During uniaxial testing, the grafts growth was evaluated using the meshing ratio, which is the ratio of the distorted area to the area of the original graft model.
Spray paint was used to create thick speckle patterns on the surface of synthetic skin grafts ( figure 4(B)) for digital image correlation (DIC) analysis during testing. A high-resolution camera, secured to a tripod (as illustrated in figure 4(C)), was used to record the tensile tests. In order to gather 15-20 photos for each tensile test, we first recorded them and then ran them through an open-source media player. SC, USA-based Correlated Solutions Inc.'s VIC-2D. Images were processed in software to derive strain fields on the grafts of synthetic skin. Following the motion of spray particles through various stages of deformation during UTM tensile testing allowed for an examination of the strain fields.
Each of the simulant were tested five times to validate the repeatability. ANOVA (Analysis of Variance) is a statistical method employed to assess whether there exist disparities among the averages of three or more sets. ANOVA examines the diversity observed between sets against the diversity found within sets to ascertain if the means of the sets exhibit significant dissimilarity. Within ANOVA, the overall diversity in the data is partitioned into two constituent parts: the diversity between sets and the diversity within sets. If the diversity between sets surpasses the diversity within sets, it implies that the means of the sets are indeed significantly distinct. This analysis utilizes a one-way ANOVA. The outcome of ANOVA is an F-statistic, which quantifies the ratio of the diversity between sets to the diversity within sets. By comparing the F-statistic to a critical value derived from the

Results and discussions
Experimental analysis was performed to investigate the effect of cell type, dimensions, and orientation on the elastic moduli of the lattice structures, giving rise to a valuable set of numerical parameters which were used to calculate the Stress, strain, Poisson's ratio and meshing ratio of the lattice designs.

Induced stress in AS-shaped synthetic skin grafts
The maximum stresses produced on the synthetic skin grafts during uniaxial expansion of up to 150% are presented in figure 5. The maximum stress values were calculated for the grafts subjected to uniaxial straining along the Y-direction, with D6 exhibiting the lowest stress (10.53 KPa) and D7 exhibiting the highest stress (122.11 KPa). The second-highest and second-lowest stress values were 96.84 KPa in D1 and 17.89 KPa in D3, respectively. The other grafts values fell between those of D6 and D7, as depicted in figure 4. The skin grafts with L1>L2, namely D1, D4, and D7, exhibited the highest induced stress, while the skin grafts with L1<L2, namely D3, D6, and D9, showed the lowest induced stress. Based on these findings, D6 was identified as the most suitable skin graft simulant, while D7 was identified as the least suitable, as it allows for uniaxial expansions of up to 150% without over-stressing the skin material. Furthermore, it was observed that for smaller spacing, L1>L2 induced more stress, whereas L1<L2 induced less stress.

Effective poisson's ratio of as-shaped synthetic skin grafts
The synthetic skin grafts' effective Poisson's ratio was estimated for different dimensions and under various strains. The estimated Poisson's ratios ranged from −0.6 to −0.04, −0.15 to 0.05, and −0.06 to 0.08 for 50%, 100%, and 150% strains, respectively, as shown in figure 6. The lowest spacing and unit cell dimensions of the synthetic skin grafts produced the most negative Poisson's ratio at 50% strain. As the strain increased, the Poisson's ratio decreased, which may be due to changes in the synthetic skin grafts' internal structure during straining. This finding may be attributed to the change in internal structure of synthetic skin grafts with straining, which is in line with literature reporting [59].

Expansion potential of AS-shaped synthetic skin grafts
The deformation and cover area estimations of AS-shaped synthetic skin grafts, under 50%, 100%, and 150% strains, are shown in table 2. The 50%, 100%, and 150% strains corresponded to Y deformation of 75 mm, 100 mm, and 125 mm respectively. From table 2, the covered area was observed to be maximum in case of unit cell with equal length and lowest spacing (i.e., D2: L1 = 8, L2 = 8, and S = 0.5) across the three values of strains. With increased unit cell spacing and same lengths, the covered area decreased. In the case of L1>L2, high unit cell spacing (i.e., D7: L1 = 8, L2 = 4, and S = 2) lowest cover area was generated across all strains. Areas covered by other skin graft simulant variants had values in between D2 and D7. For 50% strain, as the spacing between the slits of synthetic skin grafts was increased, auxeticity was found to decrease. In the case of L1<L2, with the smallest spacing (i.e., D3: L1 = 4, L2 = 8, and S = 0.5), the concave or non-auxetic behaviour increased with an increase in the strain. For the design D2 (L1 = L2, S = 0.5), the observed size of the incision or void was maximum with increasing strain. This is highly undesirable for skin grafting, as with high void area, cell proliferation would be challenging, leading to poor wound healing.
Results showed that synthetic skin grafts with alternating slits of equal length and small spacing generated the highest expansions at 50% strain. Similarly, the optimal relative dimensions of alternating slits for maximum expansions were found to be the same at 100% strain. Considering the applied strains, the skin graft simulant with alternating slits of length ratios of two and low spacings was identified as the most stable design for expansion without the risk of skin rupture.

Meshing ratio of AS-shaped synthetic skin grafts
The meshing ratios of all AS synthetic skin grafts under different levels of applied strain were presented in figure 7, with MR values of 1.5, 2, and 2.5 claimed for 50%, 100%, and 150% strains, respectively. The MR was calculated as the ratio of the final covered area to the starting covered area. The results showed that for 50% strain, D2 (L1 = L2 = 8 and S = 0.5) had the maximum MR value of 1.95, while D7 (L1 = 8, L2 = 4, and S = 2) had the minimum MR value of 1.53, and all other MR values were between those of D2 and D7. At 100% strain, D2 had the highest MR value of 2.28, while D7 and D9 had the lowest MR values of 1.89. For 150% strain, D2 had the maximum MR value of 2.71, while D7 had the minimum MR value of 2.21, and all other MR values were between those of D2 and D7. Furthermore, the MR values were found to increase with an increase in the applied strain. Across all unit cell spacings, the MR values were observed to be the highest for L1 = L2, while no specific trends were observed across slit spacings. Overall, the MRs of the AS synthetic skin grafts were found to be superior to those of conventional skin grafts.

Strain analysis of AS-shaped synthetic skin grafts using DIC
The auxetic graft were subjected to tensile strains of up to 100%, and the induced localized strains were measured using Digital Image Correlation (DIC). At the selected loading rate of 0.4 mm s −1 , 50 % and 100% straining were attained in 62.5 s and 125 s respectively, across all the models. Significant strain build-up was observed near the UTM clamps in all the synthetic skin grafts, indicating the possibility of skin rupture (see figure 8). At 50% strain, the D1 skin graft simulant showed the maximum strain value of 0.19, with stretched unit   cells of diamond and oval-shaped geometries. The difference in the strain distribution across the unit cells was small, indicating low chance of skin rupture at the unit cells. The maximum strain estimated for the D2 skin graft simulant was 0.34, with diamond shaped stretched unit cells.
The maximum strain region was near to the upper clamp and minimum strain region was around the lower clamp. The wide differences in induced strains across the model's area indicated possibilities of skin rupture. For the D3 skin graft simulant, the maximum induced strain was 0.15, with the unit cells showing a uniform concertina pattern. This model generated the lowest strains across all synthetic skin grafts. In the D4 skin graft simulant, the maximum strain was recorded as 0.18, and the unit cells were in form of a stretched concertina pattern. The strain distribution across the unit cells was not uniform, indicating possibilities of skin rupture. The maximum strain induced at the D5 skin graft simulant was 0.35 and stretched unit cells were oval in shape. The strain distribution was uniform, indicating low chances of skin rupture. In the D6 skin graft simulant, the maximum induced strain was 0.31, with the stretched unit cells in concertina pattern. Highly non-uniform strain distribution was observed, which presents high possibilities of skin rupture. For the D7 skin graft simulant, the maximum strain recorded was 0.14, with observations of high stretching at the unit cells. Localized strain build-ups were observed, which may lead to possible skin rupture. For the D8 skin graft simulant, the maximum strain induced was 0.27, and the stretched unit cells were found to be of oval shape. Strain distribution across the unit cells was non-uniform, indicating possibilities of skin rupture. For the D9 skin graft simulant, the maximum strain was 0.48, and stretched unit cells were in concertina pattern with non-uniform strain distribution. This indicated some possibilities of skin rupture. Overall, the maximum and minimum induced strains were reported in D9 and D7 models respectively. D3 was concluded to be the most desirable skin graft simulant with a low maximum strain value and uniform strain distribution. This graft model is anticipated to lead to low straining while expanding, with low possibility of skin rupture.
For 100% strain, the D1 skin graft simulant showed a maximum strain value of 0.78 with stretched diamond and oval-shaped unit cells. The strain distribution was uniform across unit cells, indicating a low risk of skin rupture. With diamond-shaped stretched unit cells, the highest strain estimated for the D2 skin graft simulant was 1.5. A large strain zone was found near the upper clamp, whereas a low strain zone was observed near the bottom clamp. Large variations in induced strains over the model's surface area suggested the possibility of skin rupture. The maximum strain for the D3 skin graft simulant was 0.58, with the unit cells forming a concertina pattern. Across all synthetic skin grafts, this model produced the lowest strains. The maximal strain in the D4 skin graft simulant was 0.7, and the unit cells were arranged in a stretched concertina configuration. The strain distribution was highly nonuniform, with the presence of a low strain zone on the top and high strain zone at the bottom and indicating the possibility of skin rupture. At the D5 skin graft simulant, the highest strain was 3.17, and the stretched unit cells were in oval shape. The distribution of strain was uniform, except in the lower region, indicating slight risk of skin rupture. The maximum induced strain in the D6 skin graft simulant was 0.9, with the stretched unit cells forming a concertina pattern. The strain distribution was found to be highly nonuniform, indicating a high risk of skin rupture. The maximum strain obtained for the D7 skin graft simulant was 0.54, with considerable stretching observed at the unit cells. Localized strain increases were estimated at the lower region, which could lead to skin rupture. The maximum strain in the D8 skin graft simulant was 0.79, and the stretched unit cells were found to be oval in shape. The strain distribution was not homogeneous across the unit cells, indicating the possibility of skin rupture. The maximum strain in the D9 skin graft simulant was 3.64, and stretched unit cells were arranged in a concertina pattern with non-uniform strain distribution. High strains across most of the model and a small region of low strains at the bottom, indicated that this graft also pose some risks of skin rupture. In D9 and D7 models, the maximum and minimum induced strains were reported, respectively. Overall, with a low maximum strain value and homogenous strain distribution, D3 was the most desirable skin graft simulant.
3.6. Statistical analysis of AS-shaped synthetic skin grafts A thorough statistical analysis was conducted to assess the significance of the Stress values, poison's ratio, and meshing ratio. The statistical tests were performed with a confidence level of 95% (p < 0.05). In our study, a parametric ANOVA test was utilized to determine significant differences among the tested combinations. Table 3 displays the groupings based on transition types. For the stress values, the f-ratio value is 11.17, and the p-value is 0.000371, indicating a no significant difference in stress values at p < 0.05. Regarding the Poisson's ratio, the f-ratio value is 16.80, and the p-value is 0.000027, confirming a no significant difference in result at p < 0.05. Similarly, for the meshing ratio, the f-ratio value is 59.81, and the p-value is 0.00001, suggesting a no significant difference at p < 0.05.

Conclusions
This study investigated the biomechanical behavior of skin graft models with alternating slit-based auxetic patterns through an experimental approach. To simulate the mechanical properties of skin grafts, a siliconebased polymer was used, and the cast of skin graft models was fabricated using additive manufacturing techniques. Tensile testing was conducted to assess the stress and structural deformation of the synthetic skin grafts. Digital image correlation analysis was carried out to examine the local strain responses of the auxetic synthetic skin grafts. Results showed that synthetic skin grafts with alternating slits of equal length and small spacing generated the highest expansions at 50% strain. Similarly, the optimal relative dimensions of alternating slits for maximum expansions were found to be the same at 100% strain. Considering the applied strains, the skin graft simulant with alternating slits of length ratios of two and low spacings was identified as the most stable design for expansion without the risk of skin rupture.
There are few limitations of this work, polymeric material was used and tested under uniaxial loading conditions. However, in skin grafting experiments, either human or cadaveric skin samples are used and stretched in all the directions. In future studies, clinically relevant skin grafts will be developed to calculate the expansion ratio of the animal skin and human cadaver skins. We believe that an in-vivo test will provide valuable insights into the clinical applicability of the pattern and its potential to enhance wound healing outcomes. Additionally, it will allow us to better understand the underlying mechanisms and optimize the design parameters of the pattern for future applications.
Overall, the experimental study indicates that the AS-shaped skin graft models with varying dimensions of auxetic patterns can generate significant expansions in comparison to conventional split-thickness skin grafts. This suggests that the use of auxetic skin grafts can be a promising approach for burn surgery, potentially leading to more efficient utilization of donor skin and improved clinical outcomes. However, further research is needed to optimize the design and biomechanical properties of auxetic skin grafts, as well as to assess their long-term clinical performance.