Different engineered approaches have led the design of implants with controlled physical features to minimize adverse effects in biological tissues. Aiming to prevent infection, similar efforts have focused on optimizing the design features of drivelines used to transfer power to percutaneous ventricular assistance devices (VAD), omitting however a thorough look on the implant-skin interactions that govern local tissue reactions. Here, we utilized an integrated approach for the biophysical modification of transdermal implants and their evaluation by chronic sheep implantation in comparison to the standard of care VAD drivelines. We developed a novel method for the transfer of breath topographical features on thin wires with modular size. Moreover, we examined the impact of implant’s diameter, surface topography, and chemistry on macroscopic, histological, and physical markers of inflammation, fibrosis, and mechanical adhesion. All implants demonstrated infection-free performance. The fibrotic response was enhanced by the increasing diameter of implants but not influenced by their surface properties. The implants of 0.2 mm diameter promoted mild inflammatory responses with improved mechanical adhesion and restricted epidermal downgrowth, in both silicone and polyurethane coated transdermal wires. On the contrary, the VAD drivelines with larger diameter triggered severe inflammatory reactions with frequent epidermal downgrowth [1].
Furthermore, we performed COMSOL simulations to investigate the electrothermal implications of conductive wires with different sizes for the power transfer to VADs. In our model, we simulate the electrical properties of the prototype’s wires, to confirm that it does not produce a significant body temperature rise. The skin model (Fig. 1a) mimics the multiple skin layer’ properties of epidermis, dermis, fat and muscle [2]. Also, we include a PDMS layer (5 mm thickness) that represents the silicon-based material of the conductive skin. During the study, different thicknesses of the polyurethane (PU) insulating coating were tested for the wire of 0.2 mm diameter. However, no significant improvement was observed when increasing the insulation layer, since the temperature difference in the model was due to the temperature skin gradient and not the electric current. In this model, the dimensions of both the inner copper diameter and the PU coating thickness were obtained from the manufacturer’s specifications (0.2030 mm and 0.0105 mm, respectively). Our results show that when the wires are subject to the peak voltage for VADs (~14.5 V) and a steady-state current of 1.2 A (Fig. 1b), the temperature increases 0.65°C in the core of the copper wires (Fig. 1c), due to the inrush current. Nonetheless, the surface temperature of the patch in the steady state remains around 0.02°C, indicating that there is no significant risk for skin injury from heat dissipation. This combination of experimental and computational findings will enable the design of new percutaneous medical devices to support therapies that require safe exchange of power, signal, and mass through the human body
