Automated Noncontact Facial Topography Mapping, 3-Dimensional Printing, and Silicone Casting of Orbital Prosthesis.

PURPOSE: A proof-of-concept workflow study for the fabrication of custom orbital exenteration prostheses via automated noncontact scanning, 3D printing, and silicone casting. DESIGN: Noncomparative, interventional case series. METHODS: Setting: Single-center institutional study. StudyPopulation: Three patients who have discontinued wearing of the ocularist-made exenteration prosthesis due to altered fit, discoloration, or material degradation. InterventionProcedure: A digital representation of the exenteration socket and contralateral periocular region was captured through noncontact facial topography mapping. Digital construction of the anterior prosthesis surface was based on the mirrored image of the contralateral side, and the posterior surface contour was based on orbital cavity geometry. The anterior and posterior surface details were digitally merged. A 2-piece mold was designed and produced in a 3D printer. Colorimetry was used to create a custom blend of pigments for incorporation into the Shore 40 silicone elastomer to generate a prosthesis that approximates the patient's skin tone. MainOutcomeMeasures: Prosthesis symmetry, skin tone match, comfort of wear, and appearance. RESULTS: The first copy of every 3D-printed orbital prosthesis using this fabrication workflow produced good symmetry, color match, and prosthesis fit. In one case, the recontoured second copy with improved prosthesis edge-to-skin interface was made without the patient present. CONCLUSION: A noncontact 3D scanning, computer-aided design, 3D printing, and silicone casting for fabrication of orbital prosthesis was developed and validated. This production workflow has the potential to provide an efficient, standardized, reproducible exenteration prosthesis and to overcome the principal barriers to an affordable custom prosthesis worldwide: access and cost.

Modeling the effects of lipid contamination in poly(styrene-isobutylene-styrene) (SIBS).

Lipid uptake and subsequent degradation was characterized as a function of molecular weight and styrene content in four different formulations of poly(styrene-block-Isobutylene-block-styrene) (SIBS). Mechanical testing in uniaxial tension at varying lipid concentrations showed a consistent decrease in tensile strength for all specimens due to lipid contamination. Higher styrene content was associated with an improved resistance to lipid intrusion. A decrease in elongation at break was observed for low molecular weight formulations only; an expected result of the stiffer network and local chain motion restriction due to increased entanglements in high molecular weight SIBS. A new, coupled diffusion/finite element method was used to recover the swelling coefficient of the four different SIBS formulations. The Ogden strain-density energy function recovered from unidirectional tensile testing and diffusion properties from gravimetric analysis were used to construct the finite element model. The predicted swelling behavior matched experimental data and the swelling coefficients were recovered for all formulations tested. Results indicate that the higher lipid affinity of the isobutylene phase contributed to increased swelling, as expected. This novel method to calculate swelling coefficient effectively circumvents the inability of commonly-used thermal deswelling methods to characterize lipid and oil-induced swelling behavior; enabling better prediction of long-term in vivo performance of polymer-based biomedical devices and more accurate evaluation of lipid-induced degradation and swelling.

Lipid-induced degradation in biocompatible poly(Styrene-Isobutylene-Styrene) (SIBS) thermoplastic elastomer.

The thermoplastic elastomer Poly(Styrene-block-Isobutylene-block-Styrene) (SIBS) is highly biocompatible, which has led to its use in several commercially-available implants. However, lipid-induced degradation has been previously identified as a primary cause of failure in long-term SIBS implants subject to mechanical loading. Thus, understanding the mechanisms and extent of lipid-induced damage and the role of styrene-isobutylene ratio and molecular weight is critical to improving longevity of SIBS-based implants in order to fully exploit the biocompatibility advantages. Samples of four different SIBS formulations were fabricated via compression molding, immersed to lipid saturation contents from 5 to 80% by weight, and tested in uniaxial tension, stress relaxation, and dynamic creep modes. Degradation mechanisms were investigated via infrared spectroscopy, chromatography, and microscopy. No evidence of lipid-induced chemical interactions or chain scissoring was observed. However, a decrease in tensile strength, loss of dynamic creep performance and faster relaxation with increasing lipid content is attributed to strong internal straining. The magnitude of these losses is inversely proportional to both molecular weight and styrene content, suggesting that selection of these variables during the design phase should be based not only on the mechanical requirements of the application, but the expected degree of lipid exposure.

Lipid diffusion and swelling in a phase separated biocompatible thermoplastic elastomer.

Lipid uptake was analyzed via gravimetric measurements in a biocompatible poly(styrene-block-isobutylene-block-styrene) (SIBS) copolymer. Absorption followed Fickian diffusion behavior very closely, although some deviation was noticed once saturation was reached. Diffusion parameters of three different SIBS formulations were calculated and used to predict the behavior of a fourth type based on molecular weight and relative polystyrene content. SIBS with lower polystyrene content and molecular weight showed lower physical stability and developed surface cracks that propagated with exposure to the lipid medium. Saturation lipid content varied from 45% to 63% by weight and was inversely related to polystyrene content, suggesting most of the plasticization is occurring in the isobutylene phase of SIBS. Moreover, swelling of specimens was monitored throughout the immersion in the lipid medium and ranged from 32% to 58%. Swelling in formulations with lower hard phase (polystyrene) was significantly higher than the swelling in SIBS with higher hard phase content. This is consistent with lipid-induced plasticization occurring in the soft (polyisobutylene) segments, relaxing the polymer network and leading to increased swelling and lipid uptake. The biocompatibility and tailorability of SIBS through control of hard/soft phase ratio offer significant advantages for in vivo applications. However, the lipophilic nature of the material and the associated degradation may render the polymer unusable in certain applications. The predictive model of lipid uptake introduced here will allow more accurate evaluation of lipid susceptibility during the preliminary design phase of SIBS-based in vivo structures.