Stretchable Circuits with Horseshoe Shaped Conductors Embedded in Elastic Polymers Amir Jahanshahi 1 , Mario Gonzalez 2 , Jeroen van den Brand 3 , Frederick Bossuyt 1 , Thomas Vervust 1 , Rik Verplancke 1 , Jan Vanfleteren 1 , and Johan De Baets 1 1 Center for Micro Systems Technologies (CMST), ELIS Department, Ghent University–imec, Technology Park 914, B-9052 Ghent, Belgium 2 Interuniversity Microelectronics Center (imec), Kapeldreef 75, B-3001 Leuven, Belgium 3 Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands E-mail: Jan.Vanfleteren@elis.ugent.be Received November 7, 2012; accepted December 26, 2012; published online May 20, 2013 Conformable electronics, i.e., electronics that can be applied on curved surfaces, is demanded nowadays in place of conventional rigid printed circuit board (PCB) based electronics for a number of applications. In the field of stretchable electronics there has been a swift progress in recent years. In this paper we are presenting our contribution to this ever growing topic, including thin-film based polyimide (PI), supported Au stretchable meanders as well as PCB based Cu meanders. These meanders are supported by PI or poly(ethylene naphthalate)/poly(ethylene terephthalate) (PEN/PET) films. Thin-film based stretchable interconnects is targeting mainly the biocompatible environments with demands for strong miniaturization while the PCB based technology is used more for large area applications. Both approaches are reviewed in this paper in terms of fabrication processes, materials and cyclic fatigue reliability. For each technology fabricated demonstrators are presented as well. # 2013 The Japan Society of Applied Physics 1. Introduction Stretchable electronics has received a tremendous attention as can be observed by the number of recent scientiﬁc publications. Many research institutes delve deeply on the realization of stretchable circuits at this stage which makes it impossible to address all of them in a single publication any more. In this technology various materials and fabrication methods are explored. All of them are used to realize conformable electrical circuits which can be used in various emerging applications as discussed also in this work to some extent. Stretchable neural probes, 1) artiﬁcial skin, 2,3) organic transistors, 4) solar cells, 5) batteries, 6) antenna, 7) and piezo- electric energy harvesters 8) are just a few out of many current envisaged applications. In academia, there are several ways to fabricate stretch- able circuits: carbon nanotube (CNT) based, 9–13) thin-ﬁlm based, 14–16) printed circuit board (PCB) based, 17,18) con- ductive polymers using prominently silver nano wires 19,20) and other creative ways. 21–24) CNT based stretchable interconnects have been used to fabricate several applica- tions. 4,6) The feasibility of CNT stretchable interconnects is extensively investigated in literature (e.g., Ref. 25). In this technology maximum elongations as high as 100% are reported, 26) however, track conductivity and fatigue relia- bility are still somewhat of a challenge for demanding applications. The same shortcomings still apply for the conductive polymers. On the other hand, thin-ﬁlm based technologies have proven to be promising in terms of fatigue reliability. In the state of the art literature for this technology, fatigue tests until failure for almost 100 000 cycles at 40% elongation have been reported. 14,15,27,28) This particular brand of technology aims to be suitable for biocompatible applica- tions as thin ﬁlm Au and poly(dimethylsiloxane) (PDMS) provide excellent biocompatibility. In this paper, our recent activities in this technology are presented. There is another trend in fabrication of stretchable interconnects which is based on conventional PCB board technology for electronics. 17) Cu is used as the metal in this technology providing higher conductivity compared to thin ﬁlm based interconnects. This type of technology has shown to be a very good candidate for large area applications. 29) Furthermore, this technology has a high potential to be transferred from the research labs to the industrial world, as the processing methods are very close to standard PCB fabrication technology. In this contribution, we are going to review the current status at our lab in the ﬁeld of stretchable electronics with the focus on PCB based and PCB compatible technologies. 2. Design Stretchable interconnects presented in this contribution are based on spatially periodic conducor shapes, e.g., triangle, serpentine, or horseshoe. Then these meander lines are embedded into elastomer that can handle relatively large elongations (e.g., >300% for PDMS elastomer). In order to ﬁnd out the optimum shape for the embedded interconnects, the commercial ﬁnite element code MSC.MARC Ò is used to simulate the amount of plastic strain which is applied to the metal. This is one of the major causes of failure for the stretchable interconnects. 2.1 Stress–strain comparison of different conductor shapes The ﬁnite element mesh of the triangular metal interconnects is shown in Fig. 1(a). The areas with high strain concentra- tion are highlighted in the image; they are located on the sharp angles of the arms of the spring. As in the case of simple bending of a cantilever beam, the inner side of the arm is under tensile stress while compressive stress is induced on the outer side. Generally the interconnect fails at the onset of highest concentration of stress and strain. Therefore, it is a good practice to avoid sharp angles, as they are the points where the maximum concentration of stress/ strain occurs. Figure 1(b) shows the results of the same simulation but for the meander with rounded edges. Using this type of shape, stress concentration does not appear any Japanese Journal of Applied Physics 52 (2013) 05DA18 05DA18-1 # 2013 The Japan Society of Applied Physics REGULAR PAPER http://dx.doi.org/10.7567/JJAP.52.05DA18