Thermoplastic Polyurethane Foams for Wearable Electronics

Foam Additive Manufacturing tunes TPU foam conductivity for flexible electronics by adjusting temperature and print speed.

Conductive polymer foams provide a lightweight, low-density material choice for flexible and wearable electronics applications. When printing conductive foams, printing parameters can influence the foam’s electrical and mechanical properties. In a recent study, researchers fabricated conductive TPU foams using FAM. This process leverages CO₂ absorption and fused-filament foam (FFF) printing. Through this study, they developed a reproducible framework for controlling the foaming process for optimized conductive performance.

Conductive Foams: How They Function

Conductive foam composites function due to the addition of fillers such as carbon black, Carbon Nanotube (CNT), graphene, or metallic nanoparticles. Increasing filler content can increase conductivity, but can elevate viscosity and lead to agglomeration. Thus, strategies to enhance electrical performance while maintaining processability and mechanical integrity must be employed.

You can also read: AM and Conductive Polymers: Next-Gen Aerospace Electronics.

Effects of Bubbles and Foam Density

Gas bubbles from the polymer foaming process can significantly alter the behavior of conductive polymer composites. Bubbles can change the orientation and interconnection of conductive fillers, promoting interconnectivity and network formation. This can enhance the material’s electrical conductivity and is affected by bubble size. Excessive void fractions can, in turn, disrupt the filler network, decreasing conductivity. Large cells can cause fillers to orient tangentially, limiting interconnectivity. On the contrary, excessively small cells may cause opposing forces to be exerted, thus negating the benefits of reoriented filler. Low-density foams with high porosity and thin cell walls may not form a continuous conductive network at all.

Optimizing Conductive TPU Foam

During extrusion, researchers observed whether viscoelastic die swell occurred under the processing conditions for filaments with and without foaming. For filaments without foaming, the filament diameter remained nearly constant. Thus, the die swell observed in foamed filaments originated from the foaming process rather than viscoelastic contributions. Foamed filaments showed an increase in expansion ratio at intermediate inlet velocity values, demonstrating gas expansion’s role in filament swelling. Quantifications of die swell can thus aid in process parameter selection.

Researchers used scanning electron microscopy (SEM) cross-section images to determine the effect of process parameters on foam morphology. They observed that porosity was higher at 200 °C and 230 °C than 220 °C. Viscosity and melt strength decrease with increasing temperature. This leads to bubble expansion and increased nucleation rates but weakening mechanical properties. At 220 °C, trade-offs among mechanical properties lead to coalescence and partial collapse, thus lowering porosity. At a high inlet velocity, cell walls are thinner and more prone to collapse. This reduces the quality and stability of the cellular network.

Foam Filaments for Soft Electronics

Additive manufacturing benefits engineers by enabling rapid iteration cycles and greater design freedom. For emerging markets such as wearable electronics and conductive fabrics, innovative approaches like this enable new design opportunities. Achieving tailored conductivity by adjusting process parameters and cell morphology shows potential for use in these applications. This framework could influence design approaches for distributed resistive networks, soft interconnects, or heating elements.