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Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns

Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 6696612, 7 pages https://doi.org/10.1155/2021/6696612 Research Article Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns 1 2 1 1 1 Tan Xiaohua , Xiao Xia, Li Qiu, Wang Lijie, and Li Baizhou Tianjin Key Lab of High Speed Cutting and Precision Machining, Tianjin University of Technology and Education, China 300222 School of Mechanical Engineering, Tianjin Polytechnic University, China 300387 Correspondence should be addressed to Tan Xiaohua; tanxiaohua268@163.com Received 3 December 2020; Revised 26 February 2021; Accepted 6 March 2021; Published 22 March 2021 Academic Editor: Donato Romano Copyright © 2021 Tan Xiaohua et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Artificial bionic skin material is playing an increasingly important role in the field of medicine and bionic engineering and becoming a research hotspot in many disciplines in recent years. In this work, the digital moiré method was used to measure the mechanical field of the bionic skin material under different suturing conditions. Through the digital image process, the deformation characteristics and the stress distribution near the contact area between the bionic skin material and the suture were obtained and discussed. The different healing effects caused by suturing mode were further explored, which can provide mechanical guidance for wound suturing in clinical medicine. the reflectivity of the polarized light before and after the large 1. Introduction deformation of the tissue and measured the stress changes of Artificial bionic skin materials are widely applied in the field skin tissue before and after the surgical suturing. Yang et al. of biology and clinical medicine. People can study the specific [6] evaluated the recovery of skin wounds after using three function of natural biological performance by simulating the different suture treatments, including N, V, and NV shapes, natural biological structure using biomimetic skin material, taking into account the mechanical tensile strength and the which is essential in tissue engineering and regenerative effect of surgical line thickness [7]. At present, the study of medicine [1]. In clinical medicine and orthopedic surgery, wound healing is mainly based on histological analysis, lacking the stress and deformation of skin tissue near the suture site of considering mechanical factors. are one of the direct factors impacting wound healing and Due to the characteristics of large deformation, Gao and Qian and Gao and Gao [8, 9] found that the deformation of scar formation; the surgeon in the operation needs to choose the way of suturing according to the shape and location of the soft materials can be divided into two configurations, that wound [2]. is, expansion and shrinking zone. This theory was validated Stress concentration usually occurs during the contact of by the experimental observations of Xia et al. [10]. Basing the bionic skin material with the surgical suture. Hoy and on the preliminary experimental work [10], this paper Gingras [3] compared the effects of smooth suture and barbed mainly focuses on the assessment of the displacement field, suture on wound healing using photoelastic technique. Bucalo strain field, and stress distribution of skin wounds after using and Iriondo [4] used a transparent soft material similar to the different suturing patterns. By studying the deformation law earlobe tissue to simulate the suture of two different shapes of of the large deformation material under concentrated force, wound and observed the distribution of the stripes near the the mechanical characteristics of the area around the suture suture line through photoelastic measurement technology. were obtained, and the influence of the suture on the wound John et al. [5] proposed a sensor for measuring the tensile recovery is discussed, which provides mechanical guidance deformation of a soft material, which utilizes the change in for clinical surgery. 2 Applied Bionics and Biomechanics and vertical gratings, and then, a denoising was performed to 1.1. Digital Moiré Experiment of Bionic Skin enhance the gate line information. The collected images may 1.1.1. Specimen Preparation and Experiment. Human skin be affected by the uneven illumination during the experi- tissue is nonuniform material, with nonlinearity, structure ment. Therefore, the grating image was divided into several particularity, anisotropy, and specificity of physical and subregions, and the average gray value was selected as the mechanical properties [11]. Mechanical properties of biolog- threshold in each subregion. The second step is image ical skin tissue are extremely complex, so the bionic material processing. The binary transform grid diagram was superim- for human skin should have similar mechanical properties. posed on the four phase shift reference grid graphs, and the The bionic skin material used in this paper was a kind of logical moiré pattern was obtained by logical operation. commercial soft matrix composite material (which was Wavelet decomposition was applied to the logical moiré applied as the limb skin, provided by Tianyan Medical image, and the high-frequency noise was filtered by selecting Science and Education Company). It is a special functional a proper threshold. And then, a clear, continuous digital moiré material with good flexibility. Before the experiment, the pattern was obtained after fringe reconstruction using low- material was made into a rectangular specimen with a size of frequency information. After the parcelled phase diagram 15 × 4 mm for uniaxial tensile test. The thickness of the speci- was calculated, the Macy algorithm was used to get the men was about 1 mm. The flow limit of the material was found unpacked phase diagram. The phase map without wrapping to be 0.96 MPa, the Young modulus was 1.9 MPa, the maxi- corresponds linearly to the fringe series distribution. Given mum strength was 2.9 MPa, and the elongation was 520%. the phase value of the fringe order zero, the phase value of each The deformation fields of the area around the pinhole point can be determined, and then, the displacement map of using I-shape and V-shape suturing pattern were measured the whole field was obtained. Finally, a data file was generated by the digital moiré experiment method. The specimen from the resulting deformation distribution information. shown in Figure 1(b) is an I-shape sutured specimen; the preparation process is as follows: firstly, the bionic skin mate- 1.2. Result Analysis rial was cut into two 60 mm × 30 mm × 1 mm pieces; then, the two pieces of material were docked and stitched using a 1.2.1. Displacement Pattern. After the digital image process- medical silk braided suture (Chinese traditional specification ing, the displacement field information in vicinity of the code: 1; diameter 0.2~0.249 mm), making sure the stitching suture area was directly obtained. By coordinate transforma- edge of the area was directly contacted without fold and the tion, the displacement fields are presented in radial and surgical lines were straight. According to the suture standard circumferential form for better illustrating, as shown in of clinical surgery [12], the distance from the pinhole to the Figure 3. In the figure, the pinhole of the suture was set as suture edge was 10 mm, the spacing between adjacent pin- the origin of the coordinate, where the nearby material defor- holes was 15 mm, and the suture length was 20 mm, which mation was complicated. The pinhole area is marked out by ensure the tensile direction was vertical to the wound. circular dotted lines in Figure 3. Then, the black and white orthogonal gratings with a grid It can be seen from the displacement field information frequency of 3 dots/mm were prepared by spraying around that the bionic skin material clearly exhibits the characteris- the sutured area. Finally, the stitched specimens were fixed tics of the section configuration near the area in contact with on the Instron 3343 electronic universal testing machine the suture. If set u ≈ 0 as characteristic lines (i.e., the black (load range 10 N~1000 N), applying a uniaxial tensile load dotted line in the figure), the observation area can be divided of 2 mm/min. The experimental images were recorded by a into two different regions. In Figure 3(a), the four character- BaslerA202K CCD camera (resolution 1003 pixel × 1004 istic lines divide the deformation field into four fan-shaped pixel) and the associated image acquisition system. The sec- regions; the displacement of the upper and lower sectors is ond specimen was sutured using a V-shape pattern as shown negative; that is, the particles were moving towards the in Figure 1(a) with a suture pitch of 7.5 mm and with the rest origin, and the left and right sectors are positive, suggesting of the parameters the same as the previous specimen. Two particles moving away from the origin. In Figure 3(b), the samples of the deformed grating images of the two specimens characteristic lines composed by u ≈ 0 (gray dashed lines) are shown in Figure 1(c), respectively. are approximately located near the center of the four sectors. The characteristics of the displacement field show that after 1.1.2. Digital Moiré Process. In this paper, the orthogonal loading, the upper and lower sectors are the expansion zone, grating images captured at the load of 28 N are analyzed. where the material approaches the concentrated force acting The horizontal and vertical grid lines were extracted after point in the radial direction, and become wide and occupy image preprocessing, and then, the images were processed the majority of the sharp angle area, marked as EX in using digital moiré image technology, mainly achieved Figure 3; the left and right sectors are shrinking zones in through GUI programming of Matlab software. The brief which the material is radially detached from the pinhole area processes are shown in Figure 2. and contracted in the circumferential direction. This section The whole process can be divided into three steps. The becomes narrow and occupies only the smaller corner area first step is image preparation, in which the images of the after deformation, marked as SH in Figure 3. deformed gratings were preprocessed and binarized, and It can be found that the partitioning features of the the reference gratings were generated by the software. In this expansion zone and shrinking zone can be found near the paper, the deformed grid lines were extracted into horizontal pinhole both in I-shape and V-shape sutured specimens. Applied Bionics and Biomechanics 3 Deformed grating 7.5 Region of interest Region of interest (a) (b) Figure 1: Experimental measurement of large deformation bionic skin using V-shape suturing (a) and I-shape suturing (b), respectively. Unit of lengths: mm. The difference is that the displacement field of I-shape sutur- table in this process, the noise of the displacement field was ing is composed of four sectors, and the fan-shape area of the filtered out by wavelet transform in this work, and then, the V-shape suturing is composed of three parts. The possible strain field was calculated based on the large deformation reason for the difference is that, under the same load, the theory. The radial strain field ε , the circumferential strain specimens were subjected to equivalent concentrate forces field ε , and the shear strain field ε of the pin tip region θ rθ in the vertical direction, the component force in the horizon- are shown in Figure 4. tal direction will cause the shrinkage zone of the second spec- The cool color in the figure represents a negative value, imen under the action of the V-concentrated force, and the and the warm color represents a positive value. It can be seen range of the shrinkage zone is related to the angle of the that the radial strain is negative and the circumferential suture lines. Due to this reason, the expansion deformation strain is positive in the upper region of the suture point, on top of the suture point was weakened or offset. and the shear strain on the left and right sides of the surgical line is opposite. These features indicate that the material in 1.2.2. Strain Fields. To understand the rule of sector division this region produces radial compression and annular expan- more intuitively, the experimental results were transformed sion under the extrusion of the surgical line. The material into the strain field under the polar coordinate system, using deforms towards the suture point in the radial direction the displacement-strain relation shown in Equation (1). and extends around the suture point in the circumferential direction. The radial strain is positive, and the circumferen- "# 2 2 tial strain is negative in the lower position of the concentrated ∂u 1 ∂u ∂u r r θ ε = + + , r > > force point. The shear strains on the left and right sides of the ∂r 2 ∂r ∂r surgical line are also opposite, indicating radial stretching "# > 2 2 1 ∂u u 1 1 ∂u u 1 ∂u u θ r θ r r θ and circumferential shrinkage in this area, which is just ε = + + + + − , r ∂θ r 2 r ∂θ r r ∂θ r > opposite to the deformation in the upper area of the suture point. In conjunction with the displacement field results, it ∂u 1 ∂u u ∂u 1 ∂u u ∂u 1 ∂u u > θ r θ r r θ θ θ r > 2ε = + − + − + + ,; rθ can be seen that the displacement and strain values are almost ∂r r ∂θ r ∂r r ∂θ r ∂r r ∂θ r zero in the area surrounded by the two intersecting sutures, ð1Þ indicating that the deformation of the region is relatively small and remains relatively stable under external loads. where ∂u /∂r, ∂u /∂θ, ∂u /∂r, and ∂u /∂θ are the partial It can be seen that the bionic skin exhibits a sector divi- r r θ θ sion configuration of expanding sector and shrinking sector derivatives of the displacement components in the polar coordinate system. The strain field at each point can be under the tension of I-shape and V-shape sutures. However, directly calculated from the displacement by interpolation. differences can be seen in the two cases after comparison. Since the strain is a secondary information, errors are inevi- There are four fan-shape zones under the action of I-shape 4 Applied Bionics and Biomechanics Step 1 Step 2 Step 3 Deformed gratings Four-step Unwrapping Logical Filter Displacement phase procedures operations operations field output Reference shifting gratings Figure 2: Flow chart of digital image processing. EX EX 6 200 SH SH SH SH 4 4 400 u ≈0 2 𝜃 u ≈0 r 0 –2 –4 –2 EX –6 –4 –8 800 EX –10 –6 200 400 600 800 200 400 600 800 (a) (b) SH SH 4 8 SH SH 400 400 –2 u ≈0 u ≈0 r r u ≈0 u ≈0 –2 –4 600 600 –4 EX –6 –6 EX –8 –8 800 800 u ≈0 –10 –10 200 400 600 800 200 400 600 800 (c) (d) Figure 3: Displacement fields near the pinhole area of tensile bionic skin: (a) radial displacement of I-shape suturing; (b) circumferential displacement of I-shape suturing; (c) radial displacement of V-shape suturing; (d) circumferential displacement of V-shape suturing. Units: mm. suturing, and only three sectors can be found in the V-shape It can be seen from the figure that the differences of the suturing. In the latter case, there is a triangular region distribution of the principal stress between the two cases within the range of the intersection of the surgical lines, are not very evident. The values are apparently different. In where the strain is gentler and the circumferential strain is the relevant medical analysis, the “average stress index” slightly contracted. The difference in the configuration of (ASI), i.e., the ratio of the principal stress integral to the area the two suture patterns will affect the healing of the wound in the calculated area, is commonly used to assess the stress to a certain extent. level near the suture [13]. In this paper, the principal stress values near the suture area are extracted from the finite 1.3. FEM Analysis. By inputting the experimentally mea- element analysis results and the ASI near the pinhole was sured mechanical properties of bionic skin materials into calculated. The results are shown in Table 1. It can be seen ABAQUS software, finite element analysis was performed that the ASI produced by the V-shape suturing is larger than to further explore the effect of suturing on wound healing. the I-shape suturing. This can provide useful clinical infor- The constitutive model and its parameters were optimized mation for the surgeon since stress is an important factor in the healing of the wound. by a variety of fitting methods. And then, numerical models were established on this basis. The geometrical and material parameters of the numerical models were the same as those 2. Discussion and Conclusions of the experiment. The image with the external load of 28 N was analyzed. The distributions of the principal stress are In the above two experiments, the incision of bionic skin can shown in Figure 5. remain fixed under the constraints of the surgical suture and Applied Bionics and Biomechanics 5 Shear strain Radial strain Circumferential strain 0.2 0.15 200 200 200 0.1 400 400 400 0.05 600 600 600 –0.05 –0.1 800 800 800 –0.15 200 400 600 800 200 400 600 800 200 400 600 800 (c) (a) (b) Shear strain Radial strain Circumferential strain 0.2 200 200 200 0.15 0.1 400 400 400 0.05 600 600 600 –0.05 –0.1 800 800 800 –0.15 200 400 600 800 200 400 600 800 200 400 600 800 (f) (d) (e) Figure 4: Strain fields of bionic skin under polar coordinate system. Units: %. S, max, in-plane principal S, max, in-plane principal SNEG, (fraction = –1.0) SNEG, (fraction = –1.0) (Avg: 75%) (Avg: 75%) +2.658e+00 +2.299e+00 +2.299e+00 +2.054e+00 +2.054e+00 +1.810e+00 +1.810e+00 +1.565e+00 +1.565e+00 T +1.321e+00 T +1.321e+00 +1.076e+00 +1.076e+00 +8.313e–01 R Z +8.313e–01 +5.867e–01 +5.867e–01 +3.421e–01 +3.421e–01 +9.746e–02 +9.745e–02 –1.472e–01 –1.472e–01 –3.918e–01 –3.918e–01 –6.364e–01 –6.364e–01 (a) (b) Figure 5: Distribution of principal stress near the pinhole of bionic skin, I-shape (a) and V-shape (b) suturing. Units: MPa. Table 1: The average stress index (ASI) of bionic skin in an area of showed a similar deformation law. Deformation differences 19 × 19 mm . can also be found according to the displacement and strain field, which will have some effect on the recovery of the Principal stress Principal stress wound. In the case of I-shape suturing, the expanding sector σ σ 1(GPa) 3(GPa) appears on both sides of the surgical line, and the material in I-shape suturing 0.067 -0.262 this region is moving towards the contact point and extends V-shape suturing 0.081 -0.282 towards both sides with the suture as the center. In the V- shape suturing case, little movement can be seen in the 6 Applied Bionics and Biomechanics an animal skin suture experiment to conduct a comprehen- triangular region surrounded by the suture intersects due to the restriction of the suture. sive consideration. When held under tension, an expansion sector that devi- ates from the edge of the wound and caused by the vertical Data Availability concentrate force (I-shape suturing) appeared near the con- tact area. This is not conducive to the recovery of the wound All the underlying data can be found through the authors. and may lead to scars after cure. The deformation of the Anybody who is interested in the data can email the authors wound is relatively much smaller in the triangular area sur- directly. rounded by surgical sutures under the action of crossconcen- trated force (V-shape suturing), indicating that the V-shape Conflicts of Interest suturing can play a more stable role in the wound healing under the same external force. Moreover, the concentrated The authors declare that they have no conflicts of interest. stress can be dispersed by V-shape sutures, resulting in a wider area of expanding sector with smaller strain gradient, Acknowledgments which is a positive factor for wound recovery. Selecting an appropriate suture density can further reduce the strain This work was financially supported by the National gradient, hence promoting wound recovery. Natural Science Foundation of China (No. 11702193, No. In addition, the value of ASI is greater in V-shape sutur- 11772227), Scientific Research Project of Tianjin Education ing than I-shape suturing. Sutures can provide appropriate Commission (No. 2017KJ108, No. 2017KJ113), the National mechanical support during the wound healing period, but Key Technology R&D Program (2015BAK06B04), the key the relevant local stress caused by the sutures also can change technologies R&D program of Tianjin (14ZCZDSF00022, the wound repair environment to a certain extent. Wound 15ZXZNGX00260), and the research project of Tianjin Uni- healing is a complex process where moderate stress can stim- versity of Technology and Education (KYQD1702, KJ1705). ulate growth factors conducive to wound healing. However, excessive stress can cause wound inflammation, cracking, References ischemia, and even gangrene phenomenon, because transi- tional restrictions on the deformation of the wound, espe- [1] H. Zhou, C. You, X. Wang et al., “The progress and challenges cially in the triangle area surrounded by crossed sutures, for dermal regeneration in tissue engineering,” Journal of Bio- will lead to local poor blood circulation and subcutaneous medical Materials Research Part A, vol. 105, no. 4, pp. 1208– nervous system obstruction, leaving the drug and nutrition 1218, 2017. cannot be normally delivered to the wound, affecting tissue [2] D. A. Lott-Crumpler and H. R. Chaudhry, “Optimal pat- regeneration and repair. In addition, material instability terns for suturing wounds of complex shapes to foster heal- phenomenon may happen when the stress level reaches or ing,” Journal of Biomechanics, vol. 34, no. 1, pp. 51–58, exceeds a certain threshold, resulting in surface wrinkles 2001. affecting the appearance. [3] D. Hoy and K. Gingras, “Photoelastic study of wound clo- Based on the above analysis, it can be seen that the sure stresses in barbed versus traditional smooth-surface sutures,” Experimental Techniques., vol. 39, no. 2, pp. 31– suturing pattern can affect the level and mode of constraint 37, 2015. near the wound area even under the same external force. [4] B. D. Bucalo and M. Iriondo, “Photoelastic models of wound Therefore, the appropriate suturing pattern should be cho- closure stress,” Dermatologic Surgery., vol. 21, no. 3, pp. 210– sen according to the following situations: V-shape suture 212, 1995. can effectively promote wound healing in common cases; [5] J. F. Federici, N. Guzelsu, H. C. Lim et al., “Noninvasive light- but for joints with more frequent movement or patients reflection technique for measuring soft-tissue stretch,” Applied with skin microcirculation disorders, I-shape suturing is Optics, vol. 38, no. 31, pp. 6653–6660, 1999. more suitable. In addition, skin wrinkles may easily caused [6] C. S. Yang, C. H. Yeh, M. Y. Chen, C. H. Jiang, F. C. Su, and by large stress concentration, which will affect the appear- M. L. Yeh, “Mechanical evaluation of the influence of different ance of the human face skin. However, it can also promote suture methods on temporal skin healing,” Dermatologic Sur- nutrient absorption in the esophagus, intestine, and other gery, vol. 35, no. 12, pp. 1880–1885, 2009. parts of the wall mucosa, playing an important physiological [7] C.-S. Yang, C.-Y. Chen, C.-H. Chiang et al., “The effect of role. Hence, this factor also needs to be considered in the suture size on skin wound healing strength in rats,” Journal choice of suture pattern. V-shape suturing should be recom- of Medical and Biological Engineering, vol. 31, no. 5, pp. 339– mended for the skin, and the I-shape suturing is recom- 343, 2011. mended for inner lumen structures. [8] Y. C. Gao and H. S. Qian, “Analysis of the contact of a rubber It is worth mentioning that this article mainly focuses on notch with a rigid wedge,” Mechanics Research Communica- the effects of stress and deformation on wound healing, but tions., vol. 29, no. 2-3, pp. 165–176, 2002. the wound healing of real skin is a complex process. In addi- [9] Y. C. Gao and T. J. Gao, “Large deformation contact of a rub- tion to mechanical stimulation, the motion state, nutrition ber notch with a rigid wedge,” International Journal of Solids intake, drug delivery, surgical technique, radiation, cytokines, and Structures., vol. 37, no. 32, pp. 4319–4334, 2000. and other complex physical and chemical factors can also [10] K. Y. Xiaoxia, D. Weilin, L. Xiaolei, Q. Wei, and T. Xiaohua, have great influence. The authors are currently preparing “Digital moiré measurement of large deformation field for Applied Bionics and Biomechanics 7 bionic skin soft material,” Journal of Experimental Mechanics, vol. 28, no. 1, pp. 1–9, 2013, in Chinese. [11] T. Lu and F. Xu, “Mechanical properties of skin: a review,” Advances in Mechanics, vol. 38, no. 4, pp. 393–426, 2008, (in Chinese). [12] D. L. Dunn, Wound Closure Manual, Ethicon Inc, 2009. [13] L. Capek, E. Jacquet, L. Dzan, and A. Simunek, “The analysis of forces needed for the suturing of elliptical skin wounds,” Med- ical & Biological Engineering & Computing., vol. 50, no. 2, pp. 193–198, 2012. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Bionics and Biomechanics Hindawi Publishing Corporation

Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns

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Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 6696612, 7 pages https://doi.org/10.1155/2021/6696612 Research Article Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns 1 2 1 1 1 Tan Xiaohua , Xiao Xia, Li Qiu, Wang Lijie, and Li Baizhou Tianjin Key Lab of High Speed Cutting and Precision Machining, Tianjin University of Technology and Education, China 300222 School of Mechanical Engineering, Tianjin Polytechnic University, China 300387 Correspondence should be addressed to Tan Xiaohua; tanxiaohua268@163.com Received 3 December 2020; Revised 26 February 2021; Accepted 6 March 2021; Published 22 March 2021 Academic Editor: Donato Romano Copyright © 2021 Tan Xiaohua et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Artificial bionic skin material is playing an increasingly important role in the field of medicine and bionic engineering and becoming a research hotspot in many disciplines in recent years. In this work, the digital moiré method was used to measure the mechanical field of the bionic skin material under different suturing conditions. Through the digital image process, the deformation characteristics and the stress distribution near the contact area between the bionic skin material and the suture were obtained and discussed. The different healing effects caused by suturing mode were further explored, which can provide mechanical guidance for wound suturing in clinical medicine. the reflectivity of the polarized light before and after the large 1. Introduction deformation of the tissue and measured the stress changes of Artificial bionic skin materials are widely applied in the field skin tissue before and after the surgical suturing. Yang et al. of biology and clinical medicine. People can study the specific [6] evaluated the recovery of skin wounds after using three function of natural biological performance by simulating the different suture treatments, including N, V, and NV shapes, natural biological structure using biomimetic skin material, taking into account the mechanical tensile strength and the which is essential in tissue engineering and regenerative effect of surgical line thickness [7]. At present, the study of medicine [1]. In clinical medicine and orthopedic surgery, wound healing is mainly based on histological analysis, lacking the stress and deformation of skin tissue near the suture site of considering mechanical factors. are one of the direct factors impacting wound healing and Due to the characteristics of large deformation, Gao and Qian and Gao and Gao [8, 9] found that the deformation of scar formation; the surgeon in the operation needs to choose the way of suturing according to the shape and location of the soft materials can be divided into two configurations, that wound [2]. is, expansion and shrinking zone. This theory was validated Stress concentration usually occurs during the contact of by the experimental observations of Xia et al. [10]. Basing the bionic skin material with the surgical suture. Hoy and on the preliminary experimental work [10], this paper Gingras [3] compared the effects of smooth suture and barbed mainly focuses on the assessment of the displacement field, suture on wound healing using photoelastic technique. Bucalo strain field, and stress distribution of skin wounds after using and Iriondo [4] used a transparent soft material similar to the different suturing patterns. By studying the deformation law earlobe tissue to simulate the suture of two different shapes of of the large deformation material under concentrated force, wound and observed the distribution of the stripes near the the mechanical characteristics of the area around the suture suture line through photoelastic measurement technology. were obtained, and the influence of the suture on the wound John et al. [5] proposed a sensor for measuring the tensile recovery is discussed, which provides mechanical guidance deformation of a soft material, which utilizes the change in for clinical surgery. 2 Applied Bionics and Biomechanics and vertical gratings, and then, a denoising was performed to 1.1. Digital Moiré Experiment of Bionic Skin enhance the gate line information. The collected images may 1.1.1. Specimen Preparation and Experiment. Human skin be affected by the uneven illumination during the experi- tissue is nonuniform material, with nonlinearity, structure ment. Therefore, the grating image was divided into several particularity, anisotropy, and specificity of physical and subregions, and the average gray value was selected as the mechanical properties [11]. Mechanical properties of biolog- threshold in each subregion. The second step is image ical skin tissue are extremely complex, so the bionic material processing. The binary transform grid diagram was superim- for human skin should have similar mechanical properties. posed on the four phase shift reference grid graphs, and the The bionic skin material used in this paper was a kind of logical moiré pattern was obtained by logical operation. commercial soft matrix composite material (which was Wavelet decomposition was applied to the logical moiré applied as the limb skin, provided by Tianyan Medical image, and the high-frequency noise was filtered by selecting Science and Education Company). It is a special functional a proper threshold. And then, a clear, continuous digital moiré material with good flexibility. Before the experiment, the pattern was obtained after fringe reconstruction using low- material was made into a rectangular specimen with a size of frequency information. After the parcelled phase diagram 15 × 4 mm for uniaxial tensile test. The thickness of the speci- was calculated, the Macy algorithm was used to get the men was about 1 mm. The flow limit of the material was found unpacked phase diagram. The phase map without wrapping to be 0.96 MPa, the Young modulus was 1.9 MPa, the maxi- corresponds linearly to the fringe series distribution. Given mum strength was 2.9 MPa, and the elongation was 520%. the phase value of the fringe order zero, the phase value of each The deformation fields of the area around the pinhole point can be determined, and then, the displacement map of using I-shape and V-shape suturing pattern were measured the whole field was obtained. Finally, a data file was generated by the digital moiré experiment method. The specimen from the resulting deformation distribution information. shown in Figure 1(b) is an I-shape sutured specimen; the preparation process is as follows: firstly, the bionic skin mate- 1.2. Result Analysis rial was cut into two 60 mm × 30 mm × 1 mm pieces; then, the two pieces of material were docked and stitched using a 1.2.1. Displacement Pattern. After the digital image process- medical silk braided suture (Chinese traditional specification ing, the displacement field information in vicinity of the code: 1; diameter 0.2~0.249 mm), making sure the stitching suture area was directly obtained. By coordinate transforma- edge of the area was directly contacted without fold and the tion, the displacement fields are presented in radial and surgical lines were straight. According to the suture standard circumferential form for better illustrating, as shown in of clinical surgery [12], the distance from the pinhole to the Figure 3. In the figure, the pinhole of the suture was set as suture edge was 10 mm, the spacing between adjacent pin- the origin of the coordinate, where the nearby material defor- holes was 15 mm, and the suture length was 20 mm, which mation was complicated. The pinhole area is marked out by ensure the tensile direction was vertical to the wound. circular dotted lines in Figure 3. Then, the black and white orthogonal gratings with a grid It can be seen from the displacement field information frequency of 3 dots/mm were prepared by spraying around that the bionic skin material clearly exhibits the characteris- the sutured area. Finally, the stitched specimens were fixed tics of the section configuration near the area in contact with on the Instron 3343 electronic universal testing machine the suture. If set u ≈ 0 as characteristic lines (i.e., the black (load range 10 N~1000 N), applying a uniaxial tensile load dotted line in the figure), the observation area can be divided of 2 mm/min. The experimental images were recorded by a into two different regions. In Figure 3(a), the four character- BaslerA202K CCD camera (resolution 1003 pixel × 1004 istic lines divide the deformation field into four fan-shaped pixel) and the associated image acquisition system. The sec- regions; the displacement of the upper and lower sectors is ond specimen was sutured using a V-shape pattern as shown negative; that is, the particles were moving towards the in Figure 1(a) with a suture pitch of 7.5 mm and with the rest origin, and the left and right sectors are positive, suggesting of the parameters the same as the previous specimen. Two particles moving away from the origin. In Figure 3(b), the samples of the deformed grating images of the two specimens characteristic lines composed by u ≈ 0 (gray dashed lines) are shown in Figure 1(c), respectively. are approximately located near the center of the four sectors. The characteristics of the displacement field show that after 1.1.2. Digital Moiré Process. In this paper, the orthogonal loading, the upper and lower sectors are the expansion zone, grating images captured at the load of 28 N are analyzed. where the material approaches the concentrated force acting The horizontal and vertical grid lines were extracted after point in the radial direction, and become wide and occupy image preprocessing, and then, the images were processed the majority of the sharp angle area, marked as EX in using digital moiré image technology, mainly achieved Figure 3; the left and right sectors are shrinking zones in through GUI programming of Matlab software. The brief which the material is radially detached from the pinhole area processes are shown in Figure 2. and contracted in the circumferential direction. This section The whole process can be divided into three steps. The becomes narrow and occupies only the smaller corner area first step is image preparation, in which the images of the after deformation, marked as SH in Figure 3. deformed gratings were preprocessed and binarized, and It can be found that the partitioning features of the the reference gratings were generated by the software. In this expansion zone and shrinking zone can be found near the paper, the deformed grid lines were extracted into horizontal pinhole both in I-shape and V-shape sutured specimens. Applied Bionics and Biomechanics 3 Deformed grating 7.5 Region of interest Region of interest (a) (b) Figure 1: Experimental measurement of large deformation bionic skin using V-shape suturing (a) and I-shape suturing (b), respectively. Unit of lengths: mm. The difference is that the displacement field of I-shape sutur- table in this process, the noise of the displacement field was ing is composed of four sectors, and the fan-shape area of the filtered out by wavelet transform in this work, and then, the V-shape suturing is composed of three parts. The possible strain field was calculated based on the large deformation reason for the difference is that, under the same load, the theory. The radial strain field ε , the circumferential strain specimens were subjected to equivalent concentrate forces field ε , and the shear strain field ε of the pin tip region θ rθ in the vertical direction, the component force in the horizon- are shown in Figure 4. tal direction will cause the shrinkage zone of the second spec- The cool color in the figure represents a negative value, imen under the action of the V-concentrated force, and the and the warm color represents a positive value. It can be seen range of the shrinkage zone is related to the angle of the that the radial strain is negative and the circumferential suture lines. Due to this reason, the expansion deformation strain is positive in the upper region of the suture point, on top of the suture point was weakened or offset. and the shear strain on the left and right sides of the surgical line is opposite. These features indicate that the material in 1.2.2. Strain Fields. To understand the rule of sector division this region produces radial compression and annular expan- more intuitively, the experimental results were transformed sion under the extrusion of the surgical line. The material into the strain field under the polar coordinate system, using deforms towards the suture point in the radial direction the displacement-strain relation shown in Equation (1). and extends around the suture point in the circumferential direction. The radial strain is positive, and the circumferen- "# 2 2 tial strain is negative in the lower position of the concentrated ∂u 1 ∂u ∂u r r θ ε = + + , r > > force point. The shear strains on the left and right sides of the ∂r 2 ∂r ∂r surgical line are also opposite, indicating radial stretching "# > 2 2 1 ∂u u 1 1 ∂u u 1 ∂u u θ r θ r r θ and circumferential shrinkage in this area, which is just ε = + + + + − , r ∂θ r 2 r ∂θ r r ∂θ r > opposite to the deformation in the upper area of the suture point. In conjunction with the displacement field results, it ∂u 1 ∂u u ∂u 1 ∂u u ∂u 1 ∂u u > θ r θ r r θ θ θ r > 2ε = + − + − + + ,; rθ can be seen that the displacement and strain values are almost ∂r r ∂θ r ∂r r ∂θ r ∂r r ∂θ r zero in the area surrounded by the two intersecting sutures, ð1Þ indicating that the deformation of the region is relatively small and remains relatively stable under external loads. where ∂u /∂r, ∂u /∂θ, ∂u /∂r, and ∂u /∂θ are the partial It can be seen that the bionic skin exhibits a sector divi- r r θ θ sion configuration of expanding sector and shrinking sector derivatives of the displacement components in the polar coordinate system. The strain field at each point can be under the tension of I-shape and V-shape sutures. However, directly calculated from the displacement by interpolation. differences can be seen in the two cases after comparison. Since the strain is a secondary information, errors are inevi- There are four fan-shape zones under the action of I-shape 4 Applied Bionics and Biomechanics Step 1 Step 2 Step 3 Deformed gratings Four-step Unwrapping Logical Filter Displacement phase procedures operations operations field output Reference shifting gratings Figure 2: Flow chart of digital image processing. EX EX 6 200 SH SH SH SH 4 4 400 u ≈0 2 𝜃 u ≈0 r 0 –2 –4 –2 EX –6 –4 –8 800 EX –10 –6 200 400 600 800 200 400 600 800 (a) (b) SH SH 4 8 SH SH 400 400 –2 u ≈0 u ≈0 r r u ≈0 u ≈0 –2 –4 600 600 –4 EX –6 –6 EX –8 –8 800 800 u ≈0 –10 –10 200 400 600 800 200 400 600 800 (c) (d) Figure 3: Displacement fields near the pinhole area of tensile bionic skin: (a) radial displacement of I-shape suturing; (b) circumferential displacement of I-shape suturing; (c) radial displacement of V-shape suturing; (d) circumferential displacement of V-shape suturing. Units: mm. suturing, and only three sectors can be found in the V-shape It can be seen from the figure that the differences of the suturing. In the latter case, there is a triangular region distribution of the principal stress between the two cases within the range of the intersection of the surgical lines, are not very evident. The values are apparently different. In where the strain is gentler and the circumferential strain is the relevant medical analysis, the “average stress index” slightly contracted. The difference in the configuration of (ASI), i.e., the ratio of the principal stress integral to the area the two suture patterns will affect the healing of the wound in the calculated area, is commonly used to assess the stress to a certain extent. level near the suture [13]. In this paper, the principal stress values near the suture area are extracted from the finite 1.3. FEM Analysis. By inputting the experimentally mea- element analysis results and the ASI near the pinhole was sured mechanical properties of bionic skin materials into calculated. The results are shown in Table 1. It can be seen ABAQUS software, finite element analysis was performed that the ASI produced by the V-shape suturing is larger than to further explore the effect of suturing on wound healing. the I-shape suturing. This can provide useful clinical infor- The constitutive model and its parameters were optimized mation for the surgeon since stress is an important factor in the healing of the wound. by a variety of fitting methods. And then, numerical models were established on this basis. The geometrical and material parameters of the numerical models were the same as those 2. Discussion and Conclusions of the experiment. The image with the external load of 28 N was analyzed. The distributions of the principal stress are In the above two experiments, the incision of bionic skin can shown in Figure 5. remain fixed under the constraints of the surgical suture and Applied Bionics and Biomechanics 5 Shear strain Radial strain Circumferential strain 0.2 0.15 200 200 200 0.1 400 400 400 0.05 600 600 600 –0.05 –0.1 800 800 800 –0.15 200 400 600 800 200 400 600 800 200 400 600 800 (c) (a) (b) Shear strain Radial strain Circumferential strain 0.2 200 200 200 0.15 0.1 400 400 400 0.05 600 600 600 –0.05 –0.1 800 800 800 –0.15 200 400 600 800 200 400 600 800 200 400 600 800 (f) (d) (e) Figure 4: Strain fields of bionic skin under polar coordinate system. Units: %. S, max, in-plane principal S, max, in-plane principal SNEG, (fraction = –1.0) SNEG, (fraction = –1.0) (Avg: 75%) (Avg: 75%) +2.658e+00 +2.299e+00 +2.299e+00 +2.054e+00 +2.054e+00 +1.810e+00 +1.810e+00 +1.565e+00 +1.565e+00 T +1.321e+00 T +1.321e+00 +1.076e+00 +1.076e+00 +8.313e–01 R Z +8.313e–01 +5.867e–01 +5.867e–01 +3.421e–01 +3.421e–01 +9.746e–02 +9.745e–02 –1.472e–01 –1.472e–01 –3.918e–01 –3.918e–01 –6.364e–01 –6.364e–01 (a) (b) Figure 5: Distribution of principal stress near the pinhole of bionic skin, I-shape (a) and V-shape (b) suturing. Units: MPa. Table 1: The average stress index (ASI) of bionic skin in an area of showed a similar deformation law. Deformation differences 19 × 19 mm . can also be found according to the displacement and strain field, which will have some effect on the recovery of the Principal stress Principal stress wound. In the case of I-shape suturing, the expanding sector σ σ 1(GPa) 3(GPa) appears on both sides of the surgical line, and the material in I-shape suturing 0.067 -0.262 this region is moving towards the contact point and extends V-shape suturing 0.081 -0.282 towards both sides with the suture as the center. In the V- shape suturing case, little movement can be seen in the 6 Applied Bionics and Biomechanics an animal skin suture experiment to conduct a comprehen- triangular region surrounded by the suture intersects due to the restriction of the suture. sive consideration. When held under tension, an expansion sector that devi- ates from the edge of the wound and caused by the vertical Data Availability concentrate force (I-shape suturing) appeared near the con- tact area. This is not conducive to the recovery of the wound All the underlying data can be found through the authors. and may lead to scars after cure. The deformation of the Anybody who is interested in the data can email the authors wound is relatively much smaller in the triangular area sur- directly. rounded by surgical sutures under the action of crossconcen- trated force (V-shape suturing), indicating that the V-shape Conflicts of Interest suturing can play a more stable role in the wound healing under the same external force. Moreover, the concentrated The authors declare that they have no conflicts of interest. stress can be dispersed by V-shape sutures, resulting in a wider area of expanding sector with smaller strain gradient, Acknowledgments which is a positive factor for wound recovery. Selecting an appropriate suture density can further reduce the strain This work was financially supported by the National gradient, hence promoting wound recovery. Natural Science Foundation of China (No. 11702193, No. In addition, the value of ASI is greater in V-shape sutur- 11772227), Scientific Research Project of Tianjin Education ing than I-shape suturing. Sutures can provide appropriate Commission (No. 2017KJ108, No. 2017KJ113), the National mechanical support during the wound healing period, but Key Technology R&D Program (2015BAK06B04), the key the relevant local stress caused by the sutures also can change technologies R&D program of Tianjin (14ZCZDSF00022, the wound repair environment to a certain extent. Wound 15ZXZNGX00260), and the research project of Tianjin Uni- healing is a complex process where moderate stress can stim- versity of Technology and Education (KYQD1702, KJ1705). ulate growth factors conducive to wound healing. However, excessive stress can cause wound inflammation, cracking, References ischemia, and even gangrene phenomenon, because transi- tional restrictions on the deformation of the wound, espe- [1] H. Zhou, C. 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Journal

Applied Bionics and BiomechanicsHindawi Publishing Corporation

Published: Mar 22, 2021

References