Documentation:FIB book/Skin

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The skin is the largest human organ and functions to protect the body from mechanical injury such as friction, impact, pressure, cutting, and shear.[1] Skin wounds can generally be described using a number of terms, including puncture, tear, abrasion, etc. The following discussion will primarily focus on injuries that penetrate through all layers of the skin, commonly referred to as laceration or puncture.[2] Despite being confined to the topic of injuries to the skin as an individual organ, the scope of this review is quite broad taking into account the various tests and applications found in relevant literature.

Summary

Injuries to the skin are generally considered to be minor and of lesser importance than injuries to other organs and are normally coded as AIS 1 or occasionally AIS 2 in terms of severity.[3] However, despite often being classified as slight or minimal, injury to the skin occurs relatively frequently compared to other organs, with lacerations representing 8% of cases encountered in US emergency departments.[4] Researchers found that 47% of 1,467 normal children exhibited signs of recent abrasions or scrapes upon examination.[5] Further documentation shows that soccer players sustain more skin injuries than other serious injuries such as fractures or sprains.[6]

These high frequency injuries to the skin are associated with pain, the potential for infection, and cultural implications related to aesthetics. Furthermore, lack of proper treatment can lead to complications such as serious infection, which can have drastic consequences for a patient in the form of serious illness or death. Deep lacerations can result in permanent scarring, which, when occurring on the face, was found to be significantly associated with affective disorders such as anxiety and depression in a review of 21 studies.[7] Therefore, injuries to the skin should be evaluated and considered from a human safety perspective. Quantifying the injury potential of skin is relevant for topics such as improving vehicle crashworthiness,[8] modelling traumatic injury,[9] and evaluating consumer product risks.[10]

Previous Work

A range of methods have been implemented in the research community in attempts to standardize the testing of skin laceration. Materials such as synthetic and natural leathers, human cadaver samples, nanomaterials, and even plant structures have been used to simulate the mechanical properties of human skin.[11] Although the mechanical behavior of the skin as it relates to injury is incredibly complex and can occur through multiple mechanisms simultaneously, it can generally be thought of as occurring when the pressure of an external contacting object exceeds the tensile strength of the skin, resulting in breakage.[1] This can happen in the form of sliding, applied friction to the skin surface, or blunt trauma from a sharp object. Table 1 presents threshold values found for human skin injury found in the literature; however, it is worth noting that this data is difficult to compare as authors have tested different injury mechanisms, using different test methods and reporting results in varying units.

Table 1. Threshold values found in the literature for modelling human skin injury.
Source (First author, year) Test Specimen Injury Mode Reported Parameter Peak Value Test Method
Gadd, 1970 [12] Cadaver scalp tissue Laceration Surface pressure 90-405 psi Drop impactor with rounded metal/glass edge
CPSC, 1973 [10] Cadaver skin specimens Puncture Force to 100% cut depth 8-20 lbs Drop impactor with sheared and cut edges
Reed, 1992 [13] Live human volunteers Abrasion Surface pressure 2,489 psi Deployed airbag (blunt impact)
Ankerson, 1999 [9] Artificial chamois leather and ex vivo pigskin Penetration Force to full penetration 10-14 N Blade attached to materials testing machine
Shergold, 2006 [14] Live human volunteers Penetration Surface pressure 2,030 psi Intraject liquid jet injector device

Skin Injury Criteria

Methods Using Human or Animal Test Specimens

Most of the earlier research related to testing laceration originated in the automotive industry. Gadd et al. attempted to characterize the skin laceration of the scalps of unembalmed cadaver test subjects by conducting a series of tests using an instrumented free-fall device. Affixed to the drop weight was an edge block of various sizes and geometries, which were dropped from “heights sufficient to produce varying degrees of visible damage.”[12] Total penetration of skin was achieved with impacts producing surface pressures between 90-405 psi, depending on the amount of fatty tissue beneath the location at which the test impacted.[12] To measure the potential skin abrasion resulting from airbag kinematics, other researchers developed a test fixture allowing airbag deployment forces to impact a target surface within a rectangular frame.[13] Peak surface pressure on the skin of live human volunteers was measured using Fuji Prescale film, and the results indicated that skin abrasion can be expected with surface pressure levels over 175 kg/cm2 (or 2,489 psi). Although their use of cadaver specimens may not be optimal due to the potential for altered soft tissue properties. The Gadd test method is appealing as they characterize skin injury based on a wide variety of impact shapes and geometries, which offers a wider perspective on potential injury modes and can be applied to numerous situations. A strength of the second study mentioned is their use of live human volunteers, which, despite the inherent ethical considerations, is ideal for such testing to observe the effect that biological tissue response has on injury mechanism. Furthermore, this study quantifies pressure using a more robust measurement study than the Gadd study, which calculates applied pressure simply using the applied force and impact area.

The studies presented above implemented either cadaver or human volunteer tissue, which may not always be feasible considering financial and ethical constraints. Another option for quantifying skin injury would be a test method that utilizes a synthetic cut indicator, which would then need to be compared to data collected on human tissue for validation purposes. A comprehensive project carried out by the US Consumer Product Safety Commission (CPSC) aimed to investigate quantitative data of skin injury and develop relevant test methods for the purpose of distinguishing between hazardous and non-hazardous toys.[10] Human skin specimens excised during autopsy were cut with various sheared and ground edges at a range of forces and velocities using a repeatable custom cutting test apparatus. Synthetic materials such as paper, foams, silicone gels, and rubbers were also tested using binary yes/no evaluators to assess the likelihood of potentially behaving as a cut indicator. Depending on edge characteristics, the force required to cause 100% depth of cut ranged from 8-20 lbs; however, none of the tested synthetic materials proved fully suitable as cut indicators.[10] A major strength of the CPSC work is that they present a highly repeatable test methodology that can be implemented with a variety of test specimens and impact conditions and is a more feasible test set-up for other test locations to incorporate. However, the use of binary yes/no cut indicators may not be the most objective or reliable quantification method available.

Another alternative to cadaver tissue or artificial options is animal tissue. Trauma due to assault can unfortunately occur through multiple mechanisms, but stabbing in particular is often studied. Ankersen et al. aimed to quantify the force needed to penetrate human tissue using synthetic chamois (goat leather, wet and dry conditions) and pigskin (ex vivo, <4 hours after death) as skin simulants.[9] Stab penetration tests were conducted on rectangular skin simulant specimens by way of a blade attached to a materials testing machine in a custom configuration. Despite differing tensile strengths, the two materials exhibited similar penetration forces in a range of 10-14 N.[9] Strengths of this study include the inclusion of animal tissue in a short window after death, minimizing the likelihood of posthumous material property changes. Although their test method did not include human donors, it is generally thought in biomechanics that pigskin well represents the mechanical properties of human skin tissue.[15] The test method presented by Ankersen's group is relatively limited in application, but offers a repeatable way to biomechanically observe a very specific injury scenario.

The force needed to penetrate human skin is also of interest to medical researchers for the purpose of drug delivery. To further understand the biomechanics behind liquid jet (needle-free) injections, researchers at Cambridge University used a custom instrumented test setup to measure the pressure required to penetrate the skin of live human volunteers. Their research found this pressure value to be approximately 14 MPa (or 2,030 psi).[14] This work concerns a very specific injury scenario relevant for biomedical device development as opposed to more traditional injury biomechanics scenarios. Furthermore, pressure values and results from penetrating human skin with liquid may not necessarily be transferable to injuries occurring from interacting with solid matter. The use of live human volunteers as a test specimen is a major strength of this study.

Methods Using Synthetic Materials

To further understand stabbing mechanics, Gilchrist et al. developed a biaxial measurement device to simulate this injury using a homogeneous synthetic material with similar stress-strain characteristics to that of skin tissue (polyurethane, Shore hardness 40 A).[16] These tests were supplemented with the addition of either ballistic soap or compliant synthetic foam underneath the test specimens to further understand the influence of underlying tissues with different stiffnesses. Although the results in this paper cannot be applied to the properties of human skin, the authors present a device for the characterization and quantification of parameters associated with stabbing mechanics. Similar to the Ankersen stabbing study, Gilchrist's method appears to present a repeatable test set-up able to be relatively easily reproduced in other settings. Although their use of a synthetic skin specimen is not ideal in terms of biofidelity, the supplementation of their skin specimen with additional subcutaneous material is an interesting way to replicate mechanics of underlying tissue.

Another mechanism that causes injury to the skin is abrasion. A study that considered consumer safety as it relates to daily-use products analyzed the damage to the skin caused by the wiping of dry tissue. Researchers evaluated the coefficient of friction and perceived abrasion associated with the repeated wiping of a simulated skin material (VITRO-SKIN®) using five commercially available bath tissues.[17] To describe the degree of gentleness, tissues were ranked by static friction value as a method of quantifying damage to the skin. Strengths of this test-up include the inexpensive and instrumented measurement aspects, as well as the incorporation of microscopic examination to quantify degree of abrasion. However, skin abrasion due to tissue wiping would not be expected to be a typical concern for injury researchers, and more applicable for consumer product development. Furthermore, degree of injury was determined by quantifying transepidermal water loss on live subjects, and then correlating this injury to friction levels associated with similar surface patterns of the VITRO-SKIN. The comparison of ex and in-vivo results is a unique aspect of this paper, despite its limited applications for traditional injury modes.

Development of Injury Scales

The AIS considers skin injury at the lower severity levels of most of their body regions; however, type of injury is most often simply described as "abrasion", "deep laceration", or in other qualitative terms. The index lacks a clear quantifiable definition of these types of injury in terms of the forces, pressures, or other mechanisms in which they are produced, and how to measure degree of severity. Furthermore, except for burns and some other modes, skin injuries are grouped into their respective body regions instead of existing in an individual, uniform section.

A series of laceration indices based on chamois material testing were proposed by members of the automotive industry in the 1960s and 1970s. The Corning scale considered cuts on two pieces of chamois placed on an ATD headform by correlating the level of injury severity with a qualitative assessment of the number of cuts and their location on the two layers.[18] A more quantitative laceration scale was suggested by Pickard et al. in 1973 and recommends estimating laceration potential by analyzing the number, length, and depth of cuts in two adjacent chamois layers in a mathematical manner.[19] The Triplex Laceration Index was introduced by researchers at Wayne State University and fits a regression to the Corning method, similarly using two layers of chamois on a dummy head, but with the additional consideration and inspection of the headform rubber underneath.[20]

The skin can also be damaged during a contact mechanism like sliding, which is common in sports such as soccer, especially when they occur on a playing surface like artificial turf. Researchers in the Netherlands proposed a Skin Damage and Severity Index (SDASI) for the characterization of sliding-induced skin lesions.[21] Their assessment was based on a common dermatological tool used for classifying Psoriasis severity, using the sum of damage characteristics multiplied by the affected area score. Although this method is not instrumental and may be subjective, it provides a non-invasive way to quantify skin injury due to sliding in human subjects.

Discussion

Current controversy on the topic of skin injury exists primarily over the optimal way to simulate human tissue in vivo. Skin is a complex material, with anisotropic properties and many layers that interact with each other in a complicated manner. Furthermore, the skin of different individuals can exhibit varying properties based on thickness, roughness, and overall strength, due to factors such as age or lifestyle. The use of animal or human cadaver tissue to model skin biomechanics would be expected to be the most biofidelic method. However, this often results in scattered data, increasing the attractiveness of using synthetic skin surrogates in injury quantification to produce more reliable data.[16] Using in vitro studies to approximate an in vivo situation is problematic, thus, in vivo testing of animals may be another improved option for biofidelic modeling of human skin injury mechanisms. It seems that the most accurate quantification methods are those carried out using live human volunteers when possible, or using biofidelic synthetic skin simulation materials as an alternative.

The literature reviewed shows poor alignment in the form of standardization and results agreement for injury criteria, with authors using varying test methods and reporting results in different units and gathered in a range of ways, as can be seen in Table 1. Another major issue encountered in the quantification of skin injury is achieving coherence in defining the multiple modes of injury and their anatomical and biomechanical differences; because injury modes such as abrasion, laceration, or puncture occur due to different mechanics, it may be desirable to have a specific test method, injury scale, and associated criteria specific to each mode of injury. Inherent to this issue are the key differences in the various ways that these modes have been simulated in test methods, whether it be by free-fall drop, a materials test machine, or customized methods.

It can be clearly observed from the diversity in the test methods present in the discussed literature that the quantification of skin injury is a broad field with various applications. However, from a biomechanics perspective, it should theoretically be conceivable to harmonize force and kinematics values correlated to certain defined skin injury mechanisms. Skin injury as a whole is usually less of a priority for biomechanics researchers compared to other, more life-critical organs, which may explain the lack of aligned research and attention to standardization in this field.

Finally, there has yet to be a truly objective injury scale for human skin presented in the literature. Even the three scales developed for the automotive industry, although similar to one another in scope and assessment aspects, lack a quantitative method of interpretation that minimizes the subjectivity of the assessor and tests in a realistic manner.

Future Work

The skin is generally less well-researched from an injury biomechanics perspective than other organs due to a lower likelihood of death associated with such injuries. This likely contributes to the lag in the development of agreed-upon methods and standards. However, as skin injury is a relatively common and potentially serious matter, there remains a need for the establishment of a universal and standardized injury scale and threshold criteria for skin injury. Achievement of these objectives relies on a validated test method, appropriate test specimens, and agreement or differentiation of various modes of skin injury.

Researchers agree that safety criteria for skin injury must be chosen, one that considers both a tolerable injury level and a force level that is representative of real-world consumer behavior.[10] It would be beneficial for multiple industries, especially automotive and consumer products, to collaborate in the development of a test method for quantifying skin injury and agreeing on an injury scale and criteria for the purpose of standardization and comparison of results. Furthermore, the resources made possible by modern imaging systems and computer vision[22] could enable relatively straightforward development of automated detection of skin injury, regardless of injury mode.

References

  1. ↑ 1.0 1.1 Edwards, C., and Marks, R., 1995, “Evaluation of Biomechanical Properties of Human Skin.” Clinics in Dermatology, 13(40), pp. 375-380.
  2. ↑ American College of Surgeons, Wound Home Skills Kit: Lacerations & Abrasions. Sample Booklet, pp. 4-5.
  3. ↑ Schmitt, K.-U., Niederer, P. F., Cronin, D.S., Muser, M. H., and Walz, F. Trauma Biomechanics: An Introduction to Injury Biomechanics. 2014. Springer. 2.5: Methods in Trauma Biomechanics, Experimental Models. 31.
  4. ↑ Almulhim, A. M., Madadin, M., 2019, “Scalp Laceration.” StatPearls, StatPearls Publishing, Florida, USA. Accessed at: https://www.ncbi.nlm.nih.gov/books/NBK541038
  5. ↑ Labbe, J., and Caouette, G., 2001, “Recent Skin Injuries in Normal Children.” Pediatrics, 108(2), pp. 271-276.
  6. ↑ van den Eijnde, W. A.J., Peppelman, M., Lamers, E. A.D., van de Kerkhof, P. C.M., and van Erp, P. E.J., 2014, “Understanding the Acute Skin Injury Mechanism Caused by Player-Surface Contact During Soccer: A Survey and Systematic Review.” Orthopedic Journal of Sports Medicine, 2(5).
  7. ↑ Gibson, J. A.G., Ackling, E., Bisson, J. I., Dobbs, T. D., and Whitaker, I. S., 2018, “The association of affective disorders and facial scarring: Systematic review and meta-analysis.” Journal of Affective Disorders, 239, pp. 1-10.
  8. ↑ Jettner, E., and Hiltner, E., 1986, “Facial Laceration Measurements.” SAE Technical Paper 860198. SAE International, USA.
  9. ↑ 9.0 9.1 9.2 9.3 Ankerson, A., Birkbeck, A.E., Thomson, R.D., and Vanezis, P., 1999, “Puncture resistance and tensile strength of skin simulants.” Proc Inst Mech Eng H. 213, pp. 493-501.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Consumer Product Safety Commission, 1973, “Some Cutting Experiments on Human Skin and Synthetic Materials,” NBSIR 73-262.
  11. ↑ Dabrowska, A. K., Rotaru, G.-M., Derler, S. et al., 2016, “Materials used to simulate physical properties of human skin.” Skin Research and Technology. 22, pp. 3-14.
  12. ↑ 12.0 12.1 12.2 Gadd, W. G., Nahum, A. M., Schneider, D. C., and Madeira, R. G., 1970, “Tolerance and Properties of Superficial Soft Tissues in Situ.” SAE Technical Paper 700910. SAE International, USA, pp. 356-368.
  13. ↑ 13.0 13.1 Reed, M. P., and Schneider, L. W., 1992, “Assessing the Skin Abrasion Potential of Driver-Side Airbags.” UMTRI-92-6, University of Michigan Transportation Research Institute.
  14. ↑ 14.0 14.1 Shergold, O. A., Fleck, N. A., King, T. S., 2006, “The penetration of a soft solid by a liquid jet, with application to the administration of a needle-free injection.” Journal of Biomechanics, 39, pp. 2593-2602.
  15. ↑ Ranamukhaarachchi, S., Lehnert, S., Ranamukhaarachchi, S. et al. A micromechanical comparison of human and porcine skin before and after preservation by freezing for medical device development. Sci Rep 6, 32074 (2016) doi:10.1038/srep32074
  16. ↑ 16.0 16.1 Gilchrist, M.D., Keenan, S., Curtis, M., Cassidy, M., Byrne, G., and Destrade, M., 2008, “Measuring knife stab penetration into skin simulant using a novel biaxial tension device.” Forensic Science International, 177, pp. 52-65.
  17. ↑ Koenig, D. W., Dvoracek B., and Vongsa, R., 2013, “In vitro prediction of in vivo skin damage associated with the wiping of dry tissue against skin.” Skin Research and Technology, 19, pp. E453-3458.
  18. ↑ Blizard, J. R., and Howitt, J. S., 1969, “Development of a Safe Non-lacerating Automobile Windshield,” SAE Technical Paper 690484. SAE International, USA.
  19. ↑ Pickard, J., Brereton, P. A., and Hewson, A., 1973, “An Objective Method of Assessing Laceration Damage to Simulated Facial Tissues - The Triplex Laceration Index”, Proceedings of the 17th Annual Conference of American Association of Automotive Medicine.
  20. ↑ Kay, S. E., Pickar, J., and Patrick, L. M., 1973, “Improved Laminated Windshield with Reduced Laceration Properties,” SAE Technical Paper 730969. SAE International, USA.
  21. ↑ van den Eijnde, W., Peppelman, M., Weghuis, M. O., and van Erp, P. E.J., 2014, “Psychosensorial assessment of skin damage caused by a sliding on artificial turf: The development and validation of a skin damage area and severity index.” Journal of Science and Medicine in Sport, 17, pp. 18-22.
  22. ↑ Banchev, Borislav (28–29 June 2013). "Automated abrasion segmentation in medical images" (PDF). International Conference on Computer Systems and Technologies Proceedings: 17–21.CS1 maint: date format (link)


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