Flexible electronic parts could significantly improve medical implants. However, electroconductive gold atoms usually hardly bind to silicones. Researchers from the University of Basel have now been able to modify short-chain silicones in a way, that they build strong bonds to gold atoms. The results have been published in the journal Advanced Electronic Materials.

Extracted dielectric functions of Au nanoparticles grown on PDMS. The imaginary part ε” (a), the real part ε’ of the dielectric function (b), the refractive index n (c), and the extinction coefficient k of thermally evaporated Au (d) on UV cured DMS-V05 are extracted from spectroscopic Ψ- and Δ-spectra and at characteristic deposition times of 120, 240, 480, 840, 1,300, 1,800, and 2,400 s with corresponding film thicknesses of 2.7, 5.3, 7.1, 9.5, 12.2, 14.6, and 16.8 nm, respectively, colorcoded from blue to red. (Credit: Advanced Electronic Materials/Wiley-VCH Verlag GMBH & Co. KGaA)

Ultrathin and compliant electrodes are essential for flexible electronic parts. When it comes to medical implants, the challenge lies in the selection of the materials, which must be biocompatible. Silicones were particularly promising for application in the human body because they resemble the surrounding human tissue in elasticity and resilience. Gold also provides excellent electrical conductivity, but only weakly binds to silicone, which results in unstable structures.

Molecular Conductive Glue

An interdisciplinary research team of the Biomaterials Science Center and the Department of Chemistry at the University of Basel has developed a procedure that allows binding single gold atoms to the ends of polymer chains. This procedure makes it possible to form stable and homogeneous two-dimensional gold films on silicone membranes. Thus, for the first time, ultrathin conductive layers on silicone rubber can be built.

The novel approach: First, the thermal evaporation of organic molecules and gold atoms under high-vacuum conditions enables preparing ultrathin layers. Second, their formation from individual islands to a confluent film can be monitored with atomic precision by means of ellipsometry. Using masks, the sandwich structures that are fabricated can convert electrical energy into mechanical work similar to human muscles.

Energized Silicone Rubber

These dielectric artificial muscles could simultaneously serve as pressure sensors and, in the future, may even be used to harvest electrical energy from body movement. For this purpose, the silicone membranes are sandwiched between electrodes. The relatively soft silicone then deforms according to the applied voltage.

So far, the silicone membranes were several micrometers thick and required high voltages to reach the desired strain. These new nanometer-thin silicone membranes with ultrathin gold electrodes allow operation through conventional batteries. To bring such a product to the market, the production costs would have to be reduced drastically. However, Dr. Tino Topper, first author of the study, is optimistic: “The perfect experimental control during the fabrication process of the nanometer-thin sandwich structures is a sound basis for longterm stability — a key prerequisite for medical applications.”

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