Use of flexible sensor to characterize biomechanics of canine skin
Suture materials and techniques are frequently evaluated in ex vivo studies by comparing tensile strengths. However, the direct measurement techniques to obtain the tensile forces in canine skin are not available, and, therefore, the conditions suture lines undergo is unknown. A soft elastomeric capacitor is used to monitor deformation in the skin over time by sensing strain. This sensor was applied to a sample of canine skin to evaluate its capacity to sense strain in the sample while loaded in a dynamic material testing machine. The measured strain of the sensor was compared with the strain measured by the dynamic testing machine. The sample of skin was evaluated with and without the sensor adhered.
In this study, the soft elastomeric capacitor was able to measure strain and a correlation was made to stress using a modified Kelvin-Voigt model for the canine skin sample. The sensor significantly increases the stiffness of canine skin when applied which required the derivation of mechanical models for interpretation of the results.
Flexible sensors can be applied to canine skin to investigate the inherent biomechanical properties. These sensors need to be lightweight and highly elastic to avoid interference with the stress across a suture line. The sensor studied here serves as a prototype for future sensor development and has demonstrated that a lightweight highly elastic sensor is needed to decrease the effect on the sensor/skin construct. Further studies are required for biomechanical characterization of canine skin.
KeywordsSoft elastomeric capacitor Biomechanics Canine skin Strain measurement Biomedical measurement Polymers
Soft elastomeric capacitor
In veterinary medicine, incisional dehiscence is a known complication of wound closures under tension or in areas of high motion. Axial pattern flaps are one example of these wound closures which experience at least a partial dehiscence in 20–30% of cases [1, 2, 3]. Routine veterinary surgical procedures have dehiscence rates of 2–3% . In humans, there is a wide range of dehiscence rates from 2% of all orthopedic surgeries  to 9–10% of leg wound closures .
Many surgical devices and techniques are used for the closure and repair of skin defects in veterinary and human medicine. These various methods are often compared to each other in experimental models or laboratory settings to investigate the optimal surgical technique for wound closure [7, 8, 9, 10, 11]. However, the actual loads and displacements of those tissues are unknown in most species, even humans, making experimental performance of limited value to clinical practice. With a better understanding of the biomechanics of skin wounds, common postoperative complications such as dehiscence may be avoided or additional therapies implemented prior to occurrence.
One method for monitoring skin deformation (i.e. strain) in vivo is digital image correlation [12, 13]. While effective in obtaining full-field strain maps, this technique is not well suited for the continuous monitoring of patients during recovery, or monitoring of suture lines under bandages. The use of flexible electronics for the measuring of biomechanical movement has seen a high level of research interests in recent years [14, 15, 16, 17]. These studies focus on the development of the technology and less on the application of the technology in the medical field. In this work, a novel large area electronic that has been studied for use in the monitoring of civil infrastructure [18, 19, 20] is investigated for use in monitoring the high levels of strain present on the surface of skin. This sensor, termed soft elastomeric capacitor (SEC), is a large area electronic that is highly flexible, elastic and easily customizable in both shape and size. The future goal of this application is to understand the actual tensile forces across skin at rest and during activity. An altered Kelvin–Voigt material model is utilized to map the SEC’s measured strain to an estimated stress in a skin sample under the sensor. In this paper, we report the findings of an ex vivo study on the information obtained from a sensor adhered onto canine skin. We hypothesize that the sensor would record strain proportional to that introduced into the skin by the material testing machine used.
Soft elastomeric capacitor
The SEC is a robust, elastic, inexpensive large area electronic that is easy to fabricate and customizable in shape and size. The SEC is a parallel plate capacitor where the capacitor’s (sensor’s) dielectric is composed of a styrene-ethylene-butylene-styrene (SEBS) block co-polymer matrix filled with titania (TiO2). The titania is added to increase both its durability and permittivity. A dielectric mix is fabricated through the mixing and sonication of styrene-ethylene-butylene-styrene (SEBS) and titania into toluene. This solution is drop cast onto a flat glass plate to produce a dielectric. Once dry, two conductive plates are painted onto each side of the dielectric using a conductive paint fabricated from the same SEBS matrix, but filled with carbon black particles. Lastly, copper contacts with a conductive adhesive are added to the sensor to allow for the signal wire, and therefore data acquisition (DAQ) systems, to be connected through the use of a soldered connection. For more details on the sensor’s fabrication procedure, the interested reader is referred to reference .
The skin was mechanically excited with a displacement controlled 0.1 Hz harmonic load with 4.1 mm amplitude. As before, the gauge length of the SEC was set to 10 mm. The skin was not pretensioned in the dynamic testing machine before testing. This allowed the skin to go to slack during the lower portions of the displacement loading and also allowed the introduction of an out-of-plane deformation into the skin. To account for this out-of-plane deformation, only the part of the loading cycle where the skin is fully tensioned is considered during modeling, presented later in this work. A custom made DAQ was attached to the SEC. This DAQ consists of a capacitance measurement device and shield driver for eliminating the parasitic capacitance found in the signal wire. SEC capacitance data (Additional file 3) was sampled at 20 samples per second (S/s) and recorded on a laptop.
Mechanical system modeling
where a, b, c, x, y, and z, are solved for using a particle swarm algorithm . These parameters are solved for both the canine skin without the SEC sensor and the canine skin with the SEC sensor. Once solved, the strain-dependent Kelvin-Voigt material models are defined by the parameters E skin-data, ηskin−data, E skin-SEC-data, and ηskin−SEC−data where the subscript indicates that they are the values associated with the data set for either the canine skin or the canine skin with an SEC attached.
Once all the material properties have been solved for, the stress in the canine skin under the SEC can be computed using Eq. 2 and the strain data measured through the SEC.
Parameters used for the nonlinear Kelvin-Voigt model, as expressed in Eq. 4
Canine skin & SEC sensor
In this study, we presented the feasibility of estimating strain and stress in a canine skin sample with a highly elastic sensor termed soft elastomeric capacitor (SEC) sensor. This non-destructive method requires the use of models to obtain parameters about the skin because the sensor alters the response of the skin due to the direct-contact nature of the method. This method could be useful in tracking biomechanical properties that are unknown for most soft tissues in most species. While the modulus of elasticity is likely variable between species and even breeds, the integration of nondestructive methods to compute a range of possible values would enable further characterizations of such biomechanical properties for canine skin. Monitoring soft tissues over time and during various activities would allow a more evidence-based recommendation to activity restrictions, suture patterns, and tension-relieving techniques.
Our approach is achieved through testing of a canine skin sample both with and without an SEC sensor attached. Once the canine skin has been characterized, the strain in the skin is measured through the SEC sensor. Next, an estimated stress value can be obtained through the assumption of a modified Kelvin-Voigt model, whose material parameters were obtained in prior testing. These calculations assume that a simple Kelvin-Voigt model is sufficient to capture the complex biomechanics of canine skin, assuming the skin does not undergo any out-of-plane deformations during modeling and that the attachment of the SEC to the canine skin does not affect the gauge factor of the SEC. This novel approach to measuring the strain experienced in the skin may be applied with further refinement to a live patient. Variables such as age, breed, and hydration that have been noted to affect the elasticity of skin will pose difficulty in exactly mapping the biomechanics of skin . However, a range of reference values would be useful for designing future surgical implants and performance of ex vivo testing. This information would quantify and further validate Karl Langer’s long accepted skin tension lines map in humans and those adapted for veterinary patients [31, 32]. However, limitations to this proposed method include the measurement and monitoring of out-of-plane motion, challenges with adhering the sensor to a live patient and the interference between bandages and the sensor. Further research is needed on the effects of various adhesives to the skin to determine optimal bonding with minimal effect on stiffening of the skin.
Due to the ex vivo nature of the study, readers are cautioned to utilizing our measurements to represent in vivo biomechanics of skin. Specifically, the sample preservation prior to investigation always affects the mechanical behavior of skin samples. Previous studies noted that freezing skin at -20 C preserves the normal elasticity of the specimen . However, it is also noted that any method of conservation of samples will change the biomechanics of the tissues [22, 23, 24]. In most cases, the behavior of a preserved specimen will be similar, yet exaggerated or dampened from the normal mechanical behavior of fresh samples.
With the assumption of the appropriate biomechanic models, these strain measurements can then be related to the stress present in the tissue. With stress representing force per area, this study provides evidence that a sensor may be capable of monitoring stress in skin in vivo. Knowing the actual force in live canine skin under normal conditions is necessary for proper interpretation of the ex vivo studies evaluating suture materials and patterns in skin. At this moment, ex vivo study results are limited to comparing sample groups only to each other instead of in vivo data [7, 8, 9, 10, 11]. If canine skin can be biomechanically characterized, reference values of tensile forces may be utilized in evaluation of currently used suture materials and techniques in laboratory settings.
This study documents the use of an SEC sensor for measurement of strain in canine skin. The values in this study are not intended to be utilized as actual values of canine skin as the sample was not fresh and the effects of the freeze-thaw cycle on skin is unknown. Further investigation of flexible sensors is needed for characterization of canine skin on the live patient.
This section introduces the methodology used for estimating the forces present in a canine skin sample. First, the calibration procedure for the SEC sensor is described. Second, the experimental setup involving the canine skin is introduced. Lastly, the mechanical models used for estimating the forces in the canine skin are presented.
This work is also partly supported by the National Science Foundation Grant No. 1069283, which supports the activities of the Integrative Graduate Education and Research Traineeship (IGERT) in Wind Energy Science, Engineering and Policy (WESEP) at Iowa State University. Their support is gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The funding agency had no participation in study design, data collection or analysis of data.
AD and JY performed the experimental investigation, processed the data, and developed the manuscript. EZ and KK provided the skin sample, performed the experimental investigation, and provided expertise on the biomechanics of skin. IR developed the experimental procedure and studied the material properties of the sensor. SL provided the sensor, performed the experimental investigation, and processed the data. All authors were involved in data interpretation, preparing the manuscript, and final manuscript approval.
Ethics approval and consent to participate
This project used tissues from animals euthanized for reasons unrelated to this project and approved by Iowa State University’s Institutional Animal Care and Use Committee. This is in accordance with ethically sourced guidelines previously published. (Martinson, S. and Jukes N. Ethically sourced animal cadavers and tissue: Considerations for education and training. Proc. 6th World Congress on Alternatives & Animal Use in the Life Sciences. Alternatives to Animal Testing and Experimentation 14: 265–268).
Consent for publication
The authors declare that they have no competing interests.
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