The problem

Silicone is one of the most versatile materials we have. It stretches without breaking, works from −120°C to 300°C, resists chemicals, and is biocompatible enough for medical implants. There's just one problem: it's an electrical insulator. Its conductivity sits around 10−12 S/m — twelve orders of magnitude below what you'd need for a sensor or a heater.

The conventional fix is brute force: pack the silicone with enough conductive filler until electrons can hop from particle to particle. But carbon nanotubes are expensive, metallic fillers make the material brittle, and either way you need a lot of filler — typically 5–15 vol%. At those loadings, the silicone loses the flexibility that made it attractive in the first place.

A few groups have tried smarter architectures — localizing filler at interfaces rather than dispersing it uniformly — but these approaches required 16.5 vol% graphene, over 2,500 hours of solvent interdiffusion, or vacuum-based CVD synthesis. The cost and complexity defeats the purpose of starting with cheap graphite.

The approach — Solvent Interface Trapping

Our lab's insight is that graphene doesn't need to be forced into position. It wants to go to oil-water interfaces. When graphite flakes encounter a boundary between an oil phase and water, thermodynamic forces drive them to exfoliate into graphene sheets and sit at the interface, acting as two-dimensional surfactants that stabilize the emulsion. This is the Solvent Interface Trapping Method (SITM), developed in the Adamson Lab.

The synthesis is a one-pot process. Silicone resin, water, heptane, and natural graphite flakes are combined in a glass vial and shaken vigorously for about one minute. The agitation creates a water-in-oil emulsion where graphene sheets self-assemble at every droplet boundary, forming a continuous conductive network. A platinum catalyst initiates silicone crosslinking, and after the water is removed by drying, the result is a porous silicone foam with graphene confined to the cell walls.

The material achieves 0.43 S/m conductivity at just 1.37 vol% graphene — a 12-fold reduction in filler compared to prior interfacial assembly methods that needed 16.5 vol% to reach similar performance.

The Sylgard problem

One of the more interesting findings came from trying to use commercial silicones. When we replaced our additive-free Gelest formulation with Sylgard 184 — the most widely used PDMS in research — the resulting foams looked identical but were completely insulating. The culprit: Sylgard 184 contains 30–60 wt% nanoscale fumed silica. These amphiphilic nanoparticles compete with graphene for space at the oil-water interface through a mechanism we call competitive adsorption. The silica physically crowds out the graphite, preventing the conductive network from forming. This explained why SITM needed additive-free silicone formulations — a finding that came from systematically testing every variable in the system.

What one material can do

Because the conductive network sits at the foam's cell walls rather than in the bulk, the same architecture enables two distinct functions from a single material.

Strain sensing

When the foam is compressed, the graphene-lined pore walls come into closer contact, lowering resistance. Past a critical strain (~6.7%), the pores close fully and lateral deformation causes graphene junctions to slip apart, increasing resistance. This U-shaped response is stable over 5,000 compression cycles at 10% strain, with resistance baseline stabilizing after an initial break-in period.

Cut into a thin strip and placed on the wrist, the foam resolves individual heartbeats as 12% swings in relative resistance. Fast Fourier transform of the signal identified a heart rate of 94 BPM, validated against an Apple Watch Ultra.

Electrothermal heating

Running current through the same foam produces Joule heating. At 125 V, the material reaches 143°C with uniform surface temperature confirmed by infrared imaging. The foam operates at milliamp-scale current densities — roughly 1,000 times lower than continuous CVD graphene networks — because the segregated architecture creates high-resistance pathways that dissipate power efficiently at higher voltages.

In a de-icing test, a foam precooled to −17°C reached 0°C in 28 seconds at 15 W. The porous, air-filled structure means low thermal mass, so the surface heats quickly without needing to warm a dense bulk material. The hydrophobic silicone surface releases ice once the interface melts — no need to melt through the full ice layer.

The bigger picture

The silicone foam work is part of a broader effort in the Adamson Lab to build useful materials from cheap, abundant graphite using interfacial self-assembly. The same SITM platform underlies several related projects:

  • Transparent conductive films — PEDOT-graphene composites achieving 1,070 S/cm with ~75% visible transparency, demonstrated for gas sensing. Published in Synthetic Metals (2025).
  • Moisture-driven power generation — Graphene-based hydrogels that produce 0.78 V from ambient humidity gradients, sustained for 120+ hours. Enough to power a calculator, humidity sensor, and LEDs.
  • Depletion-driven conductivity — During polymerization, growing polymer chains are excluded from narrow gaps between graphene sheets, compressing the conductive network and dropping resistance 17-fold.

Different applications, same starting material: natural flake graphite that costs a few dollars per kilogram.