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Stretchable circuit technology is an interesting offshoot of flexible circuit technology. It has grown steadily in the last few years, propelled – at least in part – by the European Union’s funding of research in the area through such initiatives as the STELLA project.
STELLA, which ran from February 2006 to January 2010, involved 11 participants, representing the electronics industry, academia and research institutes from four EU countries. It was funded to the tune of over 13 million euros, with 7 million euros in funding contributed by the European Commission. That jump start yielded a number of interesting technology demonstrators. The mixture of stretchable substrates and electronic components and circuits has resulted in the coining of the term “elastronics” which seems apropos given the evolving description.
Actually, the stretching of circuits to alternately increase and decrease length of a circuit has proven a useful feature for electronic products and assemblies for many years. In the past this has typically been accomplished by folding a flexible circuit in an accordion-like manner. This type of stretching is accomplished in an “out-of-plane” fashion as illustrated in Figure 1.
This approach to stretching a circuit has been successfully used in a number of different applications over the years, such as drawers that contain electronics, allowing easier access for repairs, replacements or upgrades for the system. The resilience of construction of the type illustrated is limited because the metals used (normally copper) will tend to plastically deform. More springy metals will improve this, but they require trade-offs that must be considered. While this out-of-plane solution is viable for many applications, it is of limited value in other applications which require the stretching of circuits having a low (i.e., thinner) profile and such solutions contrast with the current effort to stretch the circuit in plane and not just in one direction but potentially in all directions at once.
In response to this anticipated future need for thin stretchable circuits (i.e., stretchable in-plane), there is now an evolving branch of electronic interconnections designed to resiliently interconnect electronic elements of every type. This includes, but is not limited to: Components such as printed circuits having various electronic devices including integrated circuits, discrete devices such as capacitors, resistors and the like, light emitting components (e.g., LEDs) and/or sensing devices including micro-electromechanical systems (MEMS). Applications being explored for use with or enabling by stretchable circuits range from fanciful fashion apparel to medical monitoring and diagnostic equipment.
In terms of materials that are useful for such applications, the most important attribute is resilience, and resilience is a key property of rubbers and elastomers. The ability to stretch and recover is obviously an imperative property for any stretchable circuit. In general, a desirable polymer is one with elasticity, which is characterized mechanically as having a low Young’s modulus, greater elongation at break and a higher Poisson’s ratio than other materials. The list of materials that are prospective candidates is thus, in general, limited by such properties.
However, there are likely to be applications where full recovery is not required and where the material may be stretched plastically to a desired limit, with just enough remaining elastic strength to perform its function. This is referred to as plastic deformation – the material is stretched by not broken. The list of materials that are being investigated is relatively short and includes nitrile rubbers, silicone rubbers and thermoplastic polyurethanes.
More challenging is finding stretchable conductive materials. For metal conductors, this is normally addressed by designing the circuits in a serpentine manner. More recently there has been effort to use nano materials, especially nano carbon fiber-filled elastomers to make the circuits.
In summary, the stretchable circuit is an interesting and promising new branch on the flexible circuit tree. For those interested in learning more, the about-to-be published fourth edition of Flexible Circuit Technology will feature a full chapter devoted to this topic.
Verdant Electronics Founder and President Joseph (Joe) Fjelstad is a four-decade veteran of the electronics industry and an international authority and innovator in the field of electronic interconnection and packaging technologies, with more than 250 U.S. and international patents issued or pending. He is also the author of "Flexible Circuit Technology" and author, co-author or editor of several other books and more than 300 technical papers, articles and columns. To contact Joe, click here.
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