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Electronic devices which stretch like rubber bands: a holistic approach to materials and fabrication methods for stretchable electronics

Book - Dissertation

This thesis revolves around a vision of electronic circuits which are mechanically soft and can be stretched and bent to certain degrees. These stretchable (soft) electronics first of all offer great advantages over the traditional, rigid devices with which we are surrounded in terms of integration: circuits and sensors that can stretch and bend can now be indiscernibly integrated on for instance textiles or the human skin. In general any conformable surface or soft object really. Furthermore these circuits exhibit unseen reliability due to their ability to withstand unconventional mechanical deformations. As the technology behind them matures rapidly from lab-based workflows to industrially applicable production principles, stretchable electronics have become increasingly relevant. A first and more traditional approach to achieve stretchable electronic circuits is the one of composite materials. Either by accommodating stretch within a conductive material through inclusion of elastomeric fillers, or by rendering a stretchable base material conductive through the addition of conductive fillers. Both approaches are explored within this thesis whereby results emphasize a characteristic trade-off: an increase in stretchability is coupled to a decrease in conductivity and vice versa. Percolation theory in this context is used as a tool to clarify the mechanism by which a composite blend transitions into conductive behavior. Aside from the characteristic trade-off between conductivity and stretchability, extreme viscosities reached at elevated filler weight fractions pose a main practical impediment to the pursuit of blends which display both satisfactory conductivity and stretchability. A novel approach, then, to achieve stretchable electronic circuits is by exchanging first traditional copper circuit traces for liquid metal and second the rigid board which holds components for a silicone elastomer. The result contains commercially available off-the-shelf components: the very same as in traditional rigid circuits, however integrated into a soft and stretchable entity since at its core is a sheet of silicone elastomer. One of the contributions of this thesis entails an approach to chemically bond off-the-shelf components to their silicone encapsulant. Without this Abstract xii chemical bond, components delaminate from their silicone cover layer which causes their liquid metal interconnects to short circuit. As the interconnections are composed of a liquid metal they do not break under deformation, instead the metal just flows inside its silicone container and ensures proper electrical conductivity. In its current state, the Silicone Devices fabrication workflow can be used to convert any arbitrarily complex rigid circuit design into a soft circuit implementation. These circuits can span multiple layers which are interconnected by VIA’s. The workflow excels in its proportionality between the ease of creating a soft circuit and the corresponding performance exhibited this circuit. All precision steps are thereby outsourced to a computer controlled laser cutter, reducing the possibility of human error. As a Do-It-Yourself (DIY) research tool, Silicone Devices serves anyone who wants to explore the (application) possibilities of stretchable electronic circuits. Significant emphasis was therefore placed on exclusively resorting to accessible materials and processes. Due to the disruptive nature, performance of early proofof-concepts, and interest shown from industry, an opportunity to commercialization was concluded. In sequence, the Silicone Devices fabrication approach’s Technology Readiness Level (TRL), System Readiness Level (SRL) and Demand Readiness Level (DRL) are established. By further tailoring the underlying materials system and developing this DIY approach to industrially viable fabrication steps, the process behind Silicone Devices can be further refined to match the production quality of conventional rigid circuits.
Number of pages: 210