Viscosity Modeling of Silicone Rubber for 3D Printing of Medical Implants

J. Stieghorst, T. Doll
Hannover Medical School,
Germany

Keywords: 3D printing, viscosity modeling, silicone rubber printing, rheology model, individually tailored implants

Summary:

In the last years, medical rapid prototyping of personalized and individualized implants has become a major field in additive manufacturing. Several rapid prototyping techniques were developed to fabricate individualized blood vessels, bandages, bones, dental prosthesis and cranial plates made of UV-curable resins, thermoplastic polymers or metal materials. Although these techniques can be used for a wide range of experimental applications, no printing technique is available for typical silicone-rubber-based neural implants e.g. cochlear implant electrodes, electrocortical grid arrays or pacemaker electrodes. To close this gap, we presented a layer by layer 3D printing process for individually shaped neural implants. Since the standard “medical grade” silicone rubber are thermal-curing liquids, an infrared high-speed-curing system was used which heats up the printed silicone rubber instantly and thereby cures the initially viscous silicone rubber material before it spreads out during the curing process. To optimize the fabrication accuracy and resolution of this system, a time-temperature profile for the curing process should be evaluated, where the spreading of the silicone rubber material is minimal. Therefore, further knowledge about the curing mechanisms and the rheological behavior of the silicone rubber is mandatory. As the spreading dynamics of polymeric liquids depends beside the surface energies and the gravity mainly on the viscosity of the polymeric liquid, a rheology model was developed which correlates the infrared heat-related temperature-time profile of the printed silicone rubber with its curing-related viscosity rise and its temperature related viscosity fall. Two commonly used room-temperature curing silicone rubbers (Silpuran 2430, Wacker Chemie AG and Sylgard 184, Dow Corning GmbH) were characterized with a vulcameter at different isothermal temperatures (20°C - 60°C). Their isothermal viscosity curves were correlated to their temperature-time profiles via an empirical viscosity expression by using a two-stage Arrhenius equation. To cope with a realistic nonisothermal curing process, a time-temperature integral for the degree of cure was introduced into the isothermal model and tested at different heat rates (5 K/min - 60 K/min). Good correlations between the model and all vulcameter measurements were observed, giving the ability to optimize the time-temperature profile of the high-speed curing system to the rheological behavior of the used silicone rubber. Since this model can be used for conventional curing systems like convective ovens or heated molds, further manufacturing benefit should be feasible by using this analytical approach instead of the typically empirical approach to determine the optimal process parameters.