- Email: [email protected]
Thickness optimization of auricular silicone scaffold based on finite element analysis
journal of the mechanical behavior of biomedical materials 53 (2016) 397–402
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Short Communication
Thickness optimization of auricular silicone scaffold based on finite element analysis Tao Jiangn, Jianzhong Shang, Li Tang, Zhuo Wang Department of Mechanical Engineering, National University of Defense Technology, No. 109 Deya Road, Kaifu District, Changsha 410073, Hunan province, PR China
art i cle i nfo
ab st rac t
Article history:
An optimized thickness of a transplantable auricular silicone scaffold was researched. The
Received 5 May 2015
original image data were acquired from CT scans, and reverse modeling technology was
Received in revised form
used to build a digital 3D model of an auricle. The transplant process was simulated in
17 August 2015
ANSYS Workbench by finite element analysis (FEA), solid scaffolds were manufactured
Accepted 24 August 2015
based on the FEA results, and the transplantable artificial auricle was finally obtained with
Available online 2 September 2015
an optimized thickness, as well as sufficient intensity and hardness. This paper provides a
Keywords:
reference for clinical transplant surgery.
Auricular silicone scaffold
& 2015 Elsevier Ltd. All rights reserved.
Reversed modeling Finite element analysis Thickness optimization
1.
Introduction
Auricular deformities are a common clinical condition. According to statistics, pediatric microtia occurs 0.83–4.34 times per 10,000 births (Reiffel et al., 2013) and auricular deformities account for 1 out of 2090 auricular issues, making it the second most common facial deformity after the cleft lip and palate (Yan, 2000). Autologous tissue transplant surgery is the most frequently used treatment for this disorder, in which a patient's costal cartilage is harvested, sculpted to a scaffold based on the shape of the normal ear, and implanted under the periauricular skin before the cavity is vacuumized to adhere the skin to the scaffold to form a normal auricle (Brent, 1999; Nagata, 1993; Yu et al., 2011; Chen, 2013). This surgery is technically demanding, and to make it even less appealing, the similarity of the reformed auricle to the normal n
Corresponding author. Tel.: þ1 5145183481. E-mail address: [email protected] (T. Jiang).
http://dx.doi.org/10.1016/j.jmbbm.2015.08.032 1751-6161/& 2015 Elsevier Ltd. All rights reserved.
auricle is relatively low (Bichara et al., 2012; Liu et al., 2010), and patients have to suffer postoperative pain in the chest wall. With the developments in clinical transplant technology in recent years, artificial bio-materials are increasingly used to fabricate the scaffold. Porous polyethylene (also known as Medpor) is one of these materials as a replacement of rib cartilage. This porous material features an average inner porous diameter of 240 μm, which provides spatial room for tissue ingrowth and vascularization (Shanbhag et al., 1990). Despite its short time in clinical field, reports already showed successful facial implantation surgeries with polyethylene as the scaffold material (Driscoll and Lee, 2010; Berghaus and Braun, 2010). However, because it enables tissue ingrowth, it is difficult to remove the implant in a later time. And according
398
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
to experiments in Fourth Military Medical University, the material has a high stiffness and implants tend to exposure. Silicone is also a widely used bioinert material (Morehead and Holt, 1994) that has been successfully transplanted into the human body, without immune rejection, in intrauterine devices, skin, and breasts (Bellamy and Waters, 2005; Xu and Tan, 2004; Chen et al., 2014). It features high cellular adhesiveness (Matsuoka et al., 2009), sufficient hardness, and low deformation over long periods of time (Leininger et al., 1964). The disadvantages include low vascular permeability and the risk of extrusion reported in some clinic experiments (Shanbhag et al., 1990). This paper aimed to replace costal cartilage with biomaterial and accurately build a scaffold by taking advantage of 3D printing technology. Because silicone is an already well developed material, tunable in mechanical properties by varying ratio of base silicone and curing agent, and very easy to get, this paper used silicone as experimental material. According to clinical experimental results from the Fourth Military Medical University, people of different ages and genders have varied skin thicknesses in the auricle area, ranging from less than 1 mm to approximately 3 mm. If the original auricular model is used as the scaffold for transplantation, the formed auricle will be larger than the normal ear as additional skin is wrapped outside the scaffold. Hence, a model needs to account for the skin thickness of the original one. To ensure the scaffold could be used for patients with relatively thick skin, the scaffold with the smallest thickness needed to be found, to be certain that no visible deformation occurred during or after the transplant process. In short, the scaffold needed to satisfy the following requirements:
(1) the appearance of the reformed auricle needed to be similar to that of the normal auricle; (2) the appropriate intensity and hardness was needed to resist deformation during and after the transplant process; (3) the scaffold had to be appropriately smaller than the original model to account for the skin that would be wrapped around it.
This paper utilized spiral CT scans to acquire the auricular data from which the digital model was built through reverse modeling technology, and several experimental scaffold models with different offset values smaller than the original
model were built by CAD software. ANSYS Workbench was utilized to perform a finite element analysis of the transplant process, through which the different deformations of respective offset scaffolds were obtained. Based on the analysis results, auricular molds were manufactured by 3D printing, and silicone was poured into the molds to make solid scaffolds for practical tests. Finally, an optimized scaffold with the minimal thickness, as well as sufficient intensity and hardness, was obtained.
2.
Modeling and analysis
2.1.
Reverse modeling of the auricle
A spiral CT scanner (Toshiba Aquilion/64) was utilized to collect the original data of a 25-year-old adult's head. The images were imported to Mimics 10.0 (Materialise Corp, Belgium), the auricular slices were selected, and a stereolithography (.STL) file was built. The file was then imported into Geomagic Studio 2012 (Geomagic, USA) to perform postprocessing such as denoising, smoothing, and solidifying. To find the maximal thickness which could be cut off from the original model, an offset based on the original exterior surfaces was required. The following principles were adhered to in the offsetting process: (1) the original features of the auricle were maintained; (2) sharp angles generated during the process were eliminated because such structures do not exist in real auricular cartilage; (3) the termination condition was when the thickness of the feature areas became less than zero; (4) the entire process was accompanied with error analysis.
Thickness analysis of the original auricular model was done by SolidWorks (Dassault System Corp, USA), and the results indicated that the thickness of over 90% of the auricle exceeded 4 mm (including feature areas such as the helix, antihelix, scapha, and earlobe), while the slightest area was the cymba conchae at 2.8 mm. According to the above principles, the mid-surfaces of the original model were maintained, and the offset was executed on both sides of the auricle with a decrement of 0.25 mm each time. Postprocessing, such as sharp angle elimination, hole filling, and
Fig. 1 – Candidate scaffolds: (A) original model; (B)–(I) thinned scaffolds with offsets of 0.25 mm to 2.0 mm.
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
mesh optimization, were performed on each offset model, and eight candidate scaffold models were built (see Fig. 1).
2.2.
Validation of the model
To ensure the validity of the research, the errors generated by post-processing needed to be examined. The examining process followed steps that firstly moved the geometric center of the offset scaffold to that of the original model, and then the actual removed solid was obtained by subtraction of boolean operation of the two models, after which the thickness analysis was executed to obtain the error between the theoretical and actual value of the offset distance. Analysis indicated that the error increased with the offset value (shown in Fig. 2), but no significant differences were found between the theoretical values and the actual values (P40.05), and the error was acceptable in morphology. Hence, the offset process was considered valid.
2.3.
Finite element analysis of transplant process
In order to ensure that the offset silicone scaffolds maintained their shape during and after the transplant process, finite element analysis was performed on each scaffold by ANSYS Workbench (ANSYS Inc., USA). A drum-shaped shell was built to act as the periauricular skin, standard atmospheric pressure was added to the outside surface of the skin, and then the deformation of the scaffold was monitored in the wrapping process. The constraint conditions included: fSear g \ fSskin g ¼ Φ
ð1Þ
max f gap ðPear ; Pskin Þoe
ð2Þ
PðiÞ ¼ 10; 135 Pa ¼ 1 atm; i ¼ 1; 2:::n
ð3Þ
In the equations above: fSear g and fSskin g represent the element entities of the scaffold and the skin. Eq. (1) ensures that the elements on the scaffold and the skin do not penetrate each other; and f gap ðPear ; Pskin Þ is a function describing the distance between the scaffold and the skin. Eq. (2) defines that the distance should not exceed the tolerance e; PðiÞ is the pressure added to the No. (i) surface of the skin. Eq. (3) shows 1 atm is added to the entire surface of the skin.
Fig. 2 – Offset–error graph.
399
Several studies have laid the foundation for FEA of soft tissue materials (Zhang and Zheng, 1997; Xu et al., 2011). It has been discovered that for bio-materials, ultrathin examples possess a few folds of Young's modulus compared to thicker ones. The work done by Xu et al. (2011) further suggests that if the strain is larger than 45% of the original thickness, Young's modulus value will experience an abrupt increase in ultrathin polymer films. Hence, it becomes a typical non-linear problem. This study assumed that large deformations would occur in the process so Young's modulus increased with the deformations. The properties of the silicone materials were acquired from experimental data and clinical results from the Fourth Medical Military University (FMMU) and Xijing Hospital. The models were meshed to tetrahedrons (scaffolds) and quadrilateral dominant (skin) elements through ANSYS Workbench, and augmented Lagrange formulations were defined for the contact between the scaffold and skin. Hyper-elastic material was defined for the scaffold and the skin. As the skin would deform greatly in the vacuuming process, large deflection switch was turned on and deformable mesh was defined. The edges at the bottom of the skin and around the cochleae were fixed to simulate surgical sutures. Initially, the skin remained in its original drum-like shape and the scaffold was inside the “drum”, and from 0 to 0.5 s, pressure was linearly added to 1 atm onto the outer surface of the skin. The pressure was consistently maintained on the surface even when the surface suffered deformation. The variation of pressure and deformation on the scaffold was achieved after iterative computations (see Fig. 3). The procedure was repeated, and the relation between deformation and offset distance was obtained (see Fig. 4). Fig. 4 shows that deformations of the scaffolds were slight if the offset value did not exceed 1.75 mm. For example, when the offset value was 1.75 mm, the deformation was merely 1.8967 mm, which accounted for only 3.0% of the total dimension of the auricle in the deforming direction, and the shape of the helix basically remained unchanged. When the offset value reached 2 mm, however, the scaffold experienced an abrupt increase in deformation, reaching 15.986 mm. The minimal thickness of the auricular concha was only 0.16 mm when the offset value reached 2 mm, and the auricular concha, which is a supporting part of the auricle, became unable to hold the pressure. In this case, the cavum conchae bent significantly and the total depth of the scaffold decreased drastically. To further study the relation between the deformation and the minimal thickness of the supporting areas and feature areas, thickness analyses for the helix, auricular concha, cavum conchae, and cymba conchae were performed for each offset scaffold (results are shown in Fig. 5). A conclusion could be drawn based on the results that the auricle would experience large amounts of deformation if the thickness of the helix, auricular concha, and cavum conchae was less than 0.38 mm, 0.14 mm, and 0.17 mm respectively. Hence, it was reasonable to set 0.4 mm as the minimal required thickness criterion for these three areas. For the cymba conchae, however, its thickness did not have an obvious influence on the auricular deformation because it is a feature structure rather than a supporting structure.
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journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
Fig. 3 – The transplant process simulated by ANSYS Workbench: (A) geometrical modeling; (B) mesh tetrahedron elements to the auricle; (C) deformation results of scaffold with offset value of 1.75 mm.
Fig. 4 – Offset–maximum deformation graph.
Though non-linear models were introduced to improve simulation accuracy, the actual situation still required experimentation. Notably, some factors were ignored in the simulations, such as the frictional co-efficiency between the skin and the scaffold, because this was unable to be obtained and it varies depending on the specific case. The simulations illustrated that the offset value of 1.75 might be a transition value for this auricular scaffold, and hence not all scaffolds were to manufacture.
3.
Experiments and discussion
3.1.
Manufacture of scaffolds
Based on the analysis results, solid silicone auricular scaffolds were manufactured. Given the complexity of the auricle, conventional cutting methods were not the best options; 3D printing was more suitable in this case. Poly-Lactic Acid was used as the printing material for the construction of the auricular molds. SY-1 Type medical silicone was poured into the mold and cured, the mechanical properties of which are shown in Table 1.
Fig. 5 – Relation between offset and minimal thickness of helix, auricular concha, cavum conchae, and cymba conchae. Table 1 – Mechanical properties of the SY-1 silicone (Shao, 2000). Mark of product Mass ratio of curing (base silicone:curing agent) Curing time – curing temperature After curing Shore A hardness Elasticity modulus Density Tearing strength Breaking strength
SY-1 100:1 60 min – 80 1C 27 2.14 MPa 1.17 g/cm3 18.0 kN/m 4.2 MPa
The mold was generated by a subtraction of boolean operation of a cube and the scaffold, and optimized according to 3D printing principles by SolidWorks. The 3D printer CubeX (3D Systems, USA) was utilized for the printing process. A thin silicone film was used as the skin in the experiments which wrapped the auricular scaffold and then was vacuumized. According to the results of finite element analysis, only those with offset values of approximately 1.75 mm needed to be studied; hence, three auricular silicone scaffolds with offset values of 1.5 mm, 1.75 mm,
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
401
Table 2 – Deformation results of the scaffolds with different offset values Area
Offset value
Helix Cymba conchae Auricular concha Cavum conchae
1.5 mm
1.75 mm
2.0 mm
No obvious deformation
Slight bend No obvious deformation
Slight bend Slight deformation Severe deformation Severe deformation
Fig. 6 – Deformation of scaffolds with different offset values after wrapping in thin film and vacuumizing: (A), (B), and (C) show the scaffold with offset value of 2 mm, large deformations occur in the areas circled; (D), (E), and (F) show the scaffold with offset value of 1.75 mm, no large deformations occur. and 2 mm were manufactured according to the method described above.
3.2.
Results and discussion
In the process of vacuumizing, the scaffold with the offset value of 2 mm folded quickly at the back side of the auricular concha, and the cavum conchae suffered great deformation. The scaffold with the offset of 1.75 mm saw a slight bend at the top side of the helix, but the entire structure was mostly maintained. The scaffold with the offset of 1.5 mm had no obvious deformation during the process. The experimental results are presented in Table 2, and the deformations can be more clearly seen in Fig. 6. To summarize, the experiments demonstrated that the maximum offset value of the auricle should be 1.75 mm so that no large deformations occur, meaning the scaffold possesses an appropriate intensity and hardness for the transplant process. The scaffold with an offset value of 2 mm, however, was not capable of resisting the pressure of the process. Although it is rather difficult to measure the precise value of deformation of the solid scaffold, the severe deformation of the 2 mm scaffold in the area of auricular concha and cavum conchae matched the FEA results in its trend to some extent. A more exact optimized point between 1.75 mm and 2 mm could be found, but the step accuracy in this paper (0.25 mm) is sufficient for medical transplant usage; hence, no further study will be performed. It is of importance to note that because different individuals have different auricle features, the maximum offset
value will differ slightly, and therefore individual experimentation is the best method to find the appropriate maximum offset value for each case. However, for the thickness of supporting structures such as the helix, auricular concha, and cavum conchae, this paper suggests that the value should not be less than 0.4 mm, as was discovered in the above analyses and experiments. It is also noteworthy that silicone may bring in side effects like exposure or extrusion. Hence for further clinic usages, animal experiments and long term bio-stability experiments are necessary. This publication mainly focused on providing a method of reversed modeling and reconstruction of auricular, and thickness optimization of the scaffold, so no further experiments will be executed here.
4.
Conclusion
With the developments of 3D printing and bio-engineering technology, the replacement of autologous tissue transplants with manufactured bio-materials will soon be the norm in the medical field. Through simulation, analyses and experimentation, this paper accomplished the following: (1) an auricular model with high accuracy was built using CT scans and reverse modeling. The error of the thinned scaffold increased with the offset value, but within an acceptable range; (2) finite element analysis was successfully used to simulate the transplant process, which provided a tangible
402
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
reference for the offset value; (3) the auricular silicone scaffold was found to deform very easily in the transplant process if the thickness of the supporting areas was less than 0.4 mm; (4) for the auricle described in this paper, the maximum offset value was 1.75 mm, in which case the auricle maintained the basic features of a normal auricle, and had sufficient intensity and hardness to resist deformation.
Acknowledgment The authors thank academics and staff from mechanical design laboratory in department of Mechanical engineering for guidance on reversed modeling and 3D printing. Tao Jiang is also grateful for the medical experimental support from Prof. Shu-zhong Guo from the Fourth Military Medical University and Xijing Hospital.
references
Bellamy, K.E., Waters, M.G., 2005. Designing a prosthesis to simulate the elastic properties of skin. Bio-Med. Mater. Eng. 14, 21–27. Berghaus, A., Braun, T., 2010. Porous polyethylene for the reconstruction of severe Cosman cleft ear deformities. Eur. J. Plast. Surg. 35 (7), 547–550. Bichara, DA, O’Sullivan, NA, Pomerantseva, I, Zhao, X, Sundback, CA, et al., 2012. The tissue-engineered auricle: past, present, and future. Tissue Eng. Part B Rev. 18, 51–61. Brent, B., 1999. Technical advances in ear reconstruction with autogenous rib cartilage grafts: personal experience with 1200 cases. Plast. Reconstr. Surg. 104 (2), 319–334. Chen, Xin, 2013. Observation and nursing care of the complication of pediatric auricular reconstruction. Chin. Clin. Nurs. 5 (4), 330–331. Chen, Yong-jun, Luo, Yuan-fang, et al., 2014. Preparation and characterization of silicone rubber/nano-copper nano composites for use in intrauterine devices. Bio-Med. Mater. Eng. 24 (1), 1269–1274.
Driscoll, D.N., Lee, J.H., 2010. Combining scalp tissue expansion with porous polyethylene total ear reconstruction in burned patients. Ann. Plast. Surg. 64 (2), 183–186. Leininger, RI, Mirkovitch, V, Peters, A, et al., 1964. Change in properties of plastics during implantation. Trans. Am. Soc. Artif. Intern. Organs 10, 320–322. Liu, Y, Zhang, L, et al., 2010. In vitro engineering of human earshaped cartilage assisted with CAD/CAM technology. Biomaterials 31, 2176–2183. Matsuoka, M, Akasaka, T, et al., 2009. Improvement in cell proliferation on silicone rubber by carbon nanotube coating. Bio-Med. Mater. Eng. 19 (2), 155–162. Morehead, JM, Holt, GR, 1994. Soft-tissue response to synthetic biomaterials. Otolaryngol. Clin. N. Am. 27, 195–202. http:// europepmc.org/abstract/med/8159421. Nagata, S., 1993. A new method of total reconstruction of the auricle for microtia. Plast. Reconstr. Surg. 92 (2), 187–201. Reiffel, AJ, et al., 2013. High-fidelity tissue engineering of patientspecific auricles for reconstruction of pediatric microtia and other auricular deformities. PLoS One 8 (2), e56506. Shanbhag, A., Friedman, H.I., et al., 1990. Evaluation of porous polyethylene for external ear reconstruction. Ann. Plast. Surg. 24 (1), 32–39. Shao, Long-quan, 2000. Study on the properties and biosafety of SY-1 silicone elastomer and facial skin chromatic value distribution of Xi’an residents (Masters Degree Dissertation). Fourth Military Medical University. http://d.wanfangdata.com. cn/Thesis/Y353727 (in Chinese; Chinese database: Wangfangdata). Xu, Hong-xia, Tan, Yun-hua, 2004. Different incision augmentation mammoplasty with silica gel-cyst prosthesis and treatment of the complications. Chin. J. Aesthet. Med. 13 (2), 185–187. Xu, Wenwei, Chahine, Nadeen, Sulchek, Todd, 2011. Extreme hardening of PDMS thin films due to high compressive strain and confined thickness. Langmuir 27, 8470–8477. Yan, Chen-Xian, 2000. Pediatric Otorhinolarynogology. Tianjin Science and Technology Press, Tianjin, 127–131. Yu, Guo-jun, Feng, Bi-zhao, et al., 2011. Expanded mastoid area flap and rib cartilage framework for auricle reconstruction in 46 cases. J. Clin. Rehabil. Tissue Eng. Res. 15 (38), 7217–7220. Zhang, Ming, Zheng, Y.P., 1997. Estimating the effective Young’s modulus of soft tissues from indentation tests—nonlinear finite element analysis of effects of friction and large deformation. Med. Eng. Phys. 19 (6), 512–517.
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Short Communication
Thickness optimization of auricular silicone scaffold based on finite element analysis Tao Jiangn, Jianzhong Shang, Li Tang, Zhuo Wang Department of Mechanical Engineering, National University of Defense Technology, No. 109 Deya Road, Kaifu District, Changsha 410073, Hunan province, PR China
art i cle i nfo
ab st rac t
Article history:
An optimized thickness of a transplantable auricular silicone scaffold was researched. The
Received 5 May 2015
original image data were acquired from CT scans, and reverse modeling technology was
Received in revised form
used to build a digital 3D model of an auricle. The transplant process was simulated in
17 August 2015
ANSYS Workbench by finite element analysis (FEA), solid scaffolds were manufactured
Accepted 24 August 2015
based on the FEA results, and the transplantable artificial auricle was finally obtained with
Available online 2 September 2015
an optimized thickness, as well as sufficient intensity and hardness. This paper provides a
Keywords:
reference for clinical transplant surgery.
Auricular silicone scaffold
& 2015 Elsevier Ltd. All rights reserved.
Reversed modeling Finite element analysis Thickness optimization
1.
Introduction
Auricular deformities are a common clinical condition. According to statistics, pediatric microtia occurs 0.83–4.34 times per 10,000 births (Reiffel et al., 2013) and auricular deformities account for 1 out of 2090 auricular issues, making it the second most common facial deformity after the cleft lip and palate (Yan, 2000). Autologous tissue transplant surgery is the most frequently used treatment for this disorder, in which a patient's costal cartilage is harvested, sculpted to a scaffold based on the shape of the normal ear, and implanted under the periauricular skin before the cavity is vacuumized to adhere the skin to the scaffold to form a normal auricle (Brent, 1999; Nagata, 1993; Yu et al., 2011; Chen, 2013). This surgery is technically demanding, and to make it even less appealing, the similarity of the reformed auricle to the normal n
Corresponding author. Tel.: þ1 5145183481. E-mail address: [email protected] (T. Jiang).
http://dx.doi.org/10.1016/j.jmbbm.2015.08.032 1751-6161/& 2015 Elsevier Ltd. All rights reserved.
auricle is relatively low (Bichara et al., 2012; Liu et al., 2010), and patients have to suffer postoperative pain in the chest wall. With the developments in clinical transplant technology in recent years, artificial bio-materials are increasingly used to fabricate the scaffold. Porous polyethylene (also known as Medpor) is one of these materials as a replacement of rib cartilage. This porous material features an average inner porous diameter of 240 μm, which provides spatial room for tissue ingrowth and vascularization (Shanbhag et al., 1990). Despite its short time in clinical field, reports already showed successful facial implantation surgeries with polyethylene as the scaffold material (Driscoll and Lee, 2010; Berghaus and Braun, 2010). However, because it enables tissue ingrowth, it is difficult to remove the implant in a later time. And according
398
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
to experiments in Fourth Military Medical University, the material has a high stiffness and implants tend to exposure. Silicone is also a widely used bioinert material (Morehead and Holt, 1994) that has been successfully transplanted into the human body, without immune rejection, in intrauterine devices, skin, and breasts (Bellamy and Waters, 2005; Xu and Tan, 2004; Chen et al., 2014). It features high cellular adhesiveness (Matsuoka et al., 2009), sufficient hardness, and low deformation over long periods of time (Leininger et al., 1964). The disadvantages include low vascular permeability and the risk of extrusion reported in some clinic experiments (Shanbhag et al., 1990). This paper aimed to replace costal cartilage with biomaterial and accurately build a scaffold by taking advantage of 3D printing technology. Because silicone is an already well developed material, tunable in mechanical properties by varying ratio of base silicone and curing agent, and very easy to get, this paper used silicone as experimental material. According to clinical experimental results from the Fourth Military Medical University, people of different ages and genders have varied skin thicknesses in the auricle area, ranging from less than 1 mm to approximately 3 mm. If the original auricular model is used as the scaffold for transplantation, the formed auricle will be larger than the normal ear as additional skin is wrapped outside the scaffold. Hence, a model needs to account for the skin thickness of the original one. To ensure the scaffold could be used for patients with relatively thick skin, the scaffold with the smallest thickness needed to be found, to be certain that no visible deformation occurred during or after the transplant process. In short, the scaffold needed to satisfy the following requirements:
(1) the appearance of the reformed auricle needed to be similar to that of the normal auricle; (2) the appropriate intensity and hardness was needed to resist deformation during and after the transplant process; (3) the scaffold had to be appropriately smaller than the original model to account for the skin that would be wrapped around it.
This paper utilized spiral CT scans to acquire the auricular data from which the digital model was built through reverse modeling technology, and several experimental scaffold models with different offset values smaller than the original
model were built by CAD software. ANSYS Workbench was utilized to perform a finite element analysis of the transplant process, through which the different deformations of respective offset scaffolds were obtained. Based on the analysis results, auricular molds were manufactured by 3D printing, and silicone was poured into the molds to make solid scaffolds for practical tests. Finally, an optimized scaffold with the minimal thickness, as well as sufficient intensity and hardness, was obtained.
2.
Modeling and analysis
2.1.
Reverse modeling of the auricle
A spiral CT scanner (Toshiba Aquilion/64) was utilized to collect the original data of a 25-year-old adult's head. The images were imported to Mimics 10.0 (Materialise Corp, Belgium), the auricular slices were selected, and a stereolithography (.STL) file was built. The file was then imported into Geomagic Studio 2012 (Geomagic, USA) to perform postprocessing such as denoising, smoothing, and solidifying. To find the maximal thickness which could be cut off from the original model, an offset based on the original exterior surfaces was required. The following principles were adhered to in the offsetting process: (1) the original features of the auricle were maintained; (2) sharp angles generated during the process were eliminated because such structures do not exist in real auricular cartilage; (3) the termination condition was when the thickness of the feature areas became less than zero; (4) the entire process was accompanied with error analysis.
Thickness analysis of the original auricular model was done by SolidWorks (Dassault System Corp, USA), and the results indicated that the thickness of over 90% of the auricle exceeded 4 mm (including feature areas such as the helix, antihelix, scapha, and earlobe), while the slightest area was the cymba conchae at 2.8 mm. According to the above principles, the mid-surfaces of the original model were maintained, and the offset was executed on both sides of the auricle with a decrement of 0.25 mm each time. Postprocessing, such as sharp angle elimination, hole filling, and
Fig. 1 – Candidate scaffolds: (A) original model; (B)–(I) thinned scaffolds with offsets of 0.25 mm to 2.0 mm.
journal of the mechanical behavior of biomedical materials 53 (2016) 397 –402
mesh optimization, were performed on each offset model, and eight candidate scaffold models were built (see Fig. 1).
2.2.
Validation of the model
To ensure the validity of the research, the errors generated by post-processing needed to be examined. The examining process followed steps that firstly moved the geometric center of the offset scaffold to that of the original model, and then the actual removed solid was obtained by subtraction of boolean operation of the two models, after which the thickness analysis was executed to obtain the error between the theoretical and actual value of the offset distance. Analysis indicated that the error increased with the offset value (shown in Fig. 2), but no significant differences were found between the theoretical values and the actual values (P40.05), and the error was acceptable in morphology. Hence, the offset process was considered valid.
2.3.
Finite element analysis of transplant process
In order to ensure that the offset silicone scaffolds maintained their shape during and after the transplant process, finite element analysis was performed on each scaffold by ANSYS Workbench (ANSYS Inc., USA). A drum-shaped shell was built to act as the periauricular skin, standard atmospheric pressure was added to the outside surface of the skin, and then the deformation of the scaffold was monitored in the wrapping process. The constraint conditions included: fSear g \ fSskin g ¼ Φ
ð1Þ
max f gap ðPear ; Pskin Þoe
ð2Þ
PðiÞ ¼ 10; 135 Pa ¼ 1 atm; i ¼ 1; 2:::n
ð3Þ
In the equations above: fSear g and fSskin g represent the element entities of the scaffold and the skin. Eq. (1) ensures that the elements on the scaffold and the skin do not penetrate each other; and f gap ðPear ; Pskin Þ is a function describing the distance between the scaffold and the skin. Eq. (2) defines that the distance should not exceed the tolerance e; PðiÞ is the pressure added to the No. (i) surface of the skin. Eq. (3) shows 1 atm is added to the entire surface of the skin.
Fig. 2 – Offset–error graph.
399
Several studies have laid the foundation for FEA of soft tissue materials (Zhang and Zheng, 1997; Xu et al., 2011). It has been discovered that for bio-materials, ultrathin examples possess a few folds of Young's modulus compared to thicker ones. The work done by Xu et al. (2011) further suggests that if the strain is larger than 45% of the original thickness, Young's modulus value will experience an abrupt increase in ultrathin polymer films. Hence, it becomes a typical non-linear problem. This study assumed that large deformations would occur in the process so Young's modulus increased with the deformations. The properties of the silicone materials were acquired from experimental data and clinical results from the Fourth Medical Military University (FMMU) and Xijing Hospital. The models were meshed to tetrahedrons (scaffolds) and quadrilateral dominant (skin) elements through ANSYS Workbench, and augmented Lagrange formulations were defined for the contact between the scaffold and skin. Hyper-elastic material was defined for the scaffold and the skin. As the skin would deform greatly in the vacuuming process, large deflection switch was turned on and deformable mesh was defined. The edges at the bottom of the skin and around the cochleae were fixed to simulate surgical sutures. Initially, the skin remained in its original drum-like shape and the scaffold was inside the “drum”, and from 0 to 0.5 s, pressure was linearly added to 1 atm onto the outer surface of the skin. The pressure was consistently maintained on the surface even when the surface suffered deformation. The variation of pressure and deformation on the scaffold was achieved after iterative computations (see Fig. 3). The procedure was repeated, and the relation between deformation and offset distance was obtained (see Fig. 4). Fig. 4 shows that deformations of the scaffolds were slight if the offset value did not exceed 1.75 mm. For example, when the offset value was 1.75 mm, the deformation was merely 1.8967 mm, which accounted for only 3.0% of the total dimension of the auricle in the deforming direction, and the shape of the helix basically remained unchanged. When the offset value reached 2 mm, however, the scaffold experienced an abrupt increase in deformation, reaching 15.986 mm. The minimal thickness of the auricular concha was only 0.16 mm when the offset value reached 2 mm, and the auricular concha, which is a supporting part of the auricle, became unable to hold the pressure. In this case, the cavum conchae bent significantly and the total depth of the scaffold decreased drastically. To further study the relation between the deformation and the minimal thickness of the supporting areas and feature areas, thickness analyses for the helix, auricular concha, cavum conchae, and cymba conchae were performed for each offset scaffold (results are shown in Fig. 5). A conclusion could be drawn based on the results that the auricle would experience large amounts of deformation if the thickness of the helix, auricular concha, and cavum conchae was less than 0.38 mm, 0.14 mm, and 0.17 mm respectively. Hence, it was reasonable to set 0.4 mm as the minimal required thickness criterion for these three areas. For the cymba conchae, however, its thickness did not have an obvious influence on the auricular deformation because it is a feature structure rather than a supporting structure.
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Fig. 3 – The transplant process simulated by ANSYS Workbench: (A) geometrical modeling; (B) mesh tetrahedron elements to the auricle; (C) deformation results of scaffold with offset value of 1.75 mm.
Fig. 4 – Offset–maximum deformation graph.
Though non-linear models were introduced to improve simulation accuracy, the actual situation still required experimentation. Notably, some factors were ignored in the simulations, such as the frictional co-efficiency between the skin and the scaffold, because this was unable to be obtained and it varies depending on the specific case. The simulations illustrated that the offset value of 1.75 might be a transition value for this auricular scaffold, and hence not all scaffolds were to manufacture.
3.
Experiments and discussion
3.1.
Manufacture of scaffolds
Based on the analysis results, solid silicone auricular scaffolds were manufactured. Given the complexity of the auricle, conventional cutting methods were not the best options; 3D printing was more suitable in this case. Poly-Lactic Acid was used as the printing material for the construction of the auricular molds. SY-1 Type medical silicone was poured into the mold and cured, the mechanical properties of which are shown in Table 1.
Fig. 5 – Relation between offset and minimal thickness of helix, auricular concha, cavum conchae, and cymba conchae. Table 1 – Mechanical properties of the SY-1 silicone (Shao, 2000). Mark of product Mass ratio of curing (base silicone:curing agent) Curing time – curing temperature After curing Shore A hardness Elasticity modulus Density Tearing strength Breaking strength
SY-1 100:1 60 min – 80 1C 27 2.14 MPa 1.17 g/cm3 18.0 kN/m 4.2 MPa
The mold was generated by a subtraction of boolean operation of a cube and the scaffold, and optimized according to 3D printing principles by SolidWorks. The 3D printer CubeX (3D Systems, USA) was utilized for the printing process. A thin silicone film was used as the skin in the experiments which wrapped the auricular scaffold and then was vacuumized. According to the results of finite element analysis, only those with offset values of approximately 1.75 mm needed to be studied; hence, three auricular silicone scaffolds with offset values of 1.5 mm, 1.75 mm,
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Table 2 – Deformation results of the scaffolds with different offset values Area
Offset value
Helix Cymba conchae Auricular concha Cavum conchae
1.5 mm
1.75 mm
2.0 mm
No obvious deformation
Slight bend No obvious deformation
Slight bend Slight deformation Severe deformation Severe deformation
Fig. 6 – Deformation of scaffolds with different offset values after wrapping in thin film and vacuumizing: (A), (B), and (C) show the scaffold with offset value of 2 mm, large deformations occur in the areas circled; (D), (E), and (F) show the scaffold with offset value of 1.75 mm, no large deformations occur. and 2 mm were manufactured according to the method described above.
3.2.
Results and discussion
In the process of vacuumizing, the scaffold with the offset value of 2 mm folded quickly at the back side of the auricular concha, and the cavum conchae suffered great deformation. The scaffold with the offset of 1.75 mm saw a slight bend at the top side of the helix, but the entire structure was mostly maintained. The scaffold with the offset of 1.5 mm had no obvious deformation during the process. The experimental results are presented in Table 2, and the deformations can be more clearly seen in Fig. 6. To summarize, the experiments demonstrated that the maximum offset value of the auricle should be 1.75 mm so that no large deformations occur, meaning the scaffold possesses an appropriate intensity and hardness for the transplant process. The scaffold with an offset value of 2 mm, however, was not capable of resisting the pressure of the process. Although it is rather difficult to measure the precise value of deformation of the solid scaffold, the severe deformation of the 2 mm scaffold in the area of auricular concha and cavum conchae matched the FEA results in its trend to some extent. A more exact optimized point between 1.75 mm and 2 mm could be found, but the step accuracy in this paper (0.25 mm) is sufficient for medical transplant usage; hence, no further study will be performed. It is of importance to note that because different individuals have different auricle features, the maximum offset
value will differ slightly, and therefore individual experimentation is the best method to find the appropriate maximum offset value for each case. However, for the thickness of supporting structures such as the helix, auricular concha, and cavum conchae, this paper suggests that the value should not be less than 0.4 mm, as was discovered in the above analyses and experiments. It is also noteworthy that silicone may bring in side effects like exposure or extrusion. Hence for further clinic usages, animal experiments and long term bio-stability experiments are necessary. This publication mainly focused on providing a method of reversed modeling and reconstruction of auricular, and thickness optimization of the scaffold, so no further experiments will be executed here.
4.
Conclusion
With the developments of 3D printing and bio-engineering technology, the replacement of autologous tissue transplants with manufactured bio-materials will soon be the norm in the medical field. Through simulation, analyses and experimentation, this paper accomplished the following: (1) an auricular model with high accuracy was built using CT scans and reverse modeling. The error of the thinned scaffold increased with the offset value, but within an acceptable range; (2) finite element analysis was successfully used to simulate the transplant process, which provided a tangible
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reference for the offset value; (3) the auricular silicone scaffold was found to deform very easily in the transplant process if the thickness of the supporting areas was less than 0.4 mm; (4) for the auricle described in this paper, the maximum offset value was 1.75 mm, in which case the auricle maintained the basic features of a normal auricle, and had sufficient intensity and hardness to resist deformation.
Acknowledgment The authors thank academics and staff from mechanical design laboratory in department of Mechanical engineering for guidance on reversed modeling and 3D printing. Tao Jiang is also grateful for the medical experimental support from Prof. Shu-zhong Guo from the Fourth Military Medical University and Xijing Hospital.
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