Percutaneous implants are a family of devices that penetrate the skin and all suffer from the same problems of infection because the skin seal around the device is not optimal. Contributing to this problem is the mechanical discontinuity of the skin/device interface leading to stress concentrations and micro-trauma that chronically breaks any seal that forms. In this paper, we have quantified the mechanical behavior of human skin under low-magnitude shear loads over physiological relevant frequencies. Using a stress-controlled rheometer, we have performed isothermal (37°C) frequency response experiments between 0.628 to 75.39rad/s at 0.5% and 0.04% strain on whole skin and dermis-only, respectively.Step-stress experiments of 5 and 10Pa shear loads were also conducted as were strain sweep tests (6.28rad/s). Measurements were made of whole human skin and skin from which the epidermis was removed (dermis-only). At low frequencies (0.628 to 10rad/s), the moduli are only slightly frequency dependent, with approximate power-law scaling of the moduli, G′ ~ G′ ω β , yielding β = 0.05 for whole skin and β = 0.16 for dermis-only samples.Step-stress experiments revealed three distinct phases. The intermediate phase included elastic "ringing" (damped oscillation) which provided new insights and could be fit to a mathematical model. Both the frequency and step stress response data suggests that the epidermis provides elastic rigidity and dermis provides viscoelasticity to the whole skin samples. Hence, whole skin exhibited strain hardening while the dermis-only demonstrated stress softening under step-stress conditions. The data obtained from the low magnitude shear loads and frequencies that approximate the chronic mechanical environment of a percutaneous implant should aid in the design of a device with an improved skin seal.

A percutaneous, osseointegrated (OI) prosthetics are alternative docking systems for upper- and lower-extremity prostheses. Persistent inflammation and micro-motion are known to cause negative soft-tissue adaptation in wound healing and may also be detrimental to implant longevity. In this study, a unique single-stage sheep amputation and implantation model was developed to assess the efficacy of a porous coated sub-dermal fixation surface in the prevention of skin regression around a percutaneous osseointegrated prosthetic implant. Porous coated and smooth sub-dermal fixation surface prosthetics were implanted in the right forelimb of skeletally mature sheep for up to 12 months. Skin regression kinetics and sub-dermal fixation surface coverage were measured from histological samples. Quantitative measurements of porous coated surfaces yielded skin migration rates of 0.90 ± 0.23, 0.56 ± 0.15, 0.44 ± 0.22 mm/month for the 6, 9, and 12 month animals, respectively. In addition, three load dependent regions of skin adaptation were identified; an interface, a transition, and a stress absorbance region. Immediate post-implantation immobilization of the skin may foster improved load-bearing percutaneous device outcomes. The skin adaptations reported here will aid in informing the design and optimization of future percutaneous, OI devices intended for the treatment of upper- and lower-extremity amputees.

Percutaneous medical devices are indispensable in contemporary clinical practice, but the associated incidence of low to moderate mortality infections represents a significant economic and personal cost to patients and healthcare providers. Percutaneous osseointegrated prosthetics also suffer from a similar risk of infection, limiting their clinical acceptance and usage in patients with limb loss. We hypothesized that transepidermal water loss management (TEWL) at the skin-implant interface may improve and maintain a stable skin-to-implant interface. In this study, skin reactions in a 3-month, pig dorsum model were assessed using standard histology, immunohistochemistry, and quantitative image analysis. Immunohistochemical analysis of peri-implant tissue explants showed evidence of: continuous healing (cytokeratin 6+), hypergranulation tissue (procollagen+), hyper-vascularity (Collagen 4+), and the presence of fibrocytes (CD45+ and procollagen type 1+). Importantly, the gross skin response was correlated to a previous load-bearing percutaneous osseointegrated prosthetic sheep study conducted in our lab. The skin responses of the two models indicated a potentially shared mechanism of wound healing behavior at the skin-implant interface. Although TEWL management did not reduce skin migration at the skin-implant interface, the correlation of qualitative and quantitative measures validated the pig dorsum model as a high-throughput platform for translational science based percutaneous interface investigations in the future.