Mechanics of Interfacial Delamination in Epidermal Electronics Systems Huanyu Cheng Departments of Mechanical Engineering and Civil and Environmental Engineering, Center for Engineering and Health, and Skin Disease Research Center, Northwestern University, Evanston, IL 60208 Shuodao Wang 1 Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801 e-mail: shuodaowang@gmail.com In order to provide continuous diagnostic and therapeutic options that exploit electrophysiological signals from the epidermis, this study discusses epidermal electronics systems (EES) that conform to the skin surface via van der Waals force alone, which is other- wise susceptible to artifacts associated with motion-induced changes. This paper not only establishes a criterion of conformal contact between the EES and the skin for both initial contact and the case where the skin is subject to external loading but also investigates the criterion to prevent any partial delamination between electronics and the skin. These results improve the per- formance of EES by maximizing intimate contact between the EES and skin, revealing important underlying physical insights for device optimization and future design. [DOI: 10.1115/1.4025305] Keywords: epidermal electronics, interfacial delamination, adhesion 1 Introduction The monitoring of physiological signals is of critical impor- tance for medical diagnosis and therapeutics [1,2]. Measurements of such signals are based on electrical coupling between biological tissues and electrodes. Exploiting this mechanism, a wide range of electronics systems can be explored to measure electrophysiological signals, such as electrocardiograms, electro- myograms, and electrooculograms. Conventional flat, rigid elec- tronics have the ability to capture these types of electrical signals and have their applications in some contexts, but they do not offer continuous, real-time, or portable operations. Fixed to the skin surface by caps, belts, conductive glues, or tapes [3], these con- ventional electronics usually yield inaccurate measurements. In addition, such devices/setup may irritate the skin [4] and modify the electrical coupling nature when the skin is mechanically loaded. In contrast, a recently developed concept of epidermal electronics [1,5,6] enables electronics to be intimately integrated onto the skin surface, such that the accuracy of measurements can be guaranteed. This noninvasive integration also blocks signal noises resulted from artifacts such as motion induced changes. Here we first introduce the model for initial contact of epidermal electronics to the skin and then discuss the criterion to prevent delamination when the system is subject to external load- ing. The latter discussion is essential for continuous, portable operations of epidermal electronics during normal human activ- ities that deform the skin to as much as 30%. 2 Initial Contact of Epidermal Electronics to the Skin Epidermal electronics systems exploit arrays of sensors for large area mapping. A representative system, shown in Fig. 1(a), places ultrathin metal electrodes/interconnects on one side of a silicone backing layer (Solaris in this study, inset of Fig. 1(b)) for improved mechanical robustness and intimate contact. The filamentary mesh design (Fig. 1(a)) minimizes the strain in both the sensors and the interconnects during deformation [1]. Flat epidermal electronics systems were laminated on a wavy skin surface, followed by pressing on the top surface of EES to initiate the contact [1]. The flexure rigidity of EES is fairly small for a thin EES, which enables it to slide along the morphology of skin to achieve conformal contact. For simplicity, the initial mor- phology of the skin is characterized by a sinusoidal function as yx ðÞ¼ h rough 1 þ cos 2px=k rough     =2, where h rough and k rough are the characteristic amplitude and wavelength of the skin, respectively. After EES conforms to the skin, the skin morphology becomes wx ðÞ¼ h 1 þ cos 2px=k rough     =2, where h is the deformed amplitude to be determined. The displacement of the skin surface is then given as the difference between the initial and deformed morphologies as u z x ðÞ¼ yx ð Þ wx ðÞ. The total energy of the EES/skin system  U conformal consists of bending energy of EES  U bending , elastic energy of skin  U skin , and surface adhesion energy at EES/skin interface  U adhesion , i.e.,  U conformal ¼  U bending þ  U skin þ  U adhesion [7]. The membrane energy of EES is not con- sidered in the analysis because it is negligible compared to the bending energy due to the sliding between EES and skin (initially at the peak of the skin, material points A and C would slide along the skin surface to points A 0 and C 0 , respectively, as shown in Fig. 1(b)). Taking L 0 as the initial length of EES, the bending energy of deformed EES is derived as  U bending ¼ 1 2 ð L0 0 EI EES w 00 ð Þ 2 dx ¼ p 4 EI EES h 2 k 4 rough L 0 (1) where EI EES is effective bending stiffness of EES. This effective bending stiffness is given as a weighted average of bending stiff- ness EI device in the device region and bending stiffness EI wo in the region without device, i.e., EI EES ¼ a EI device þ 1  a ð Þ EI wo , and the weight a is the area fraction of gold device over the entire area [7]. Gold device mesh is placed between Solaris backing layer and the skin. For a multilayer structure, the bending stiffness EI of the Fig. 1 (a) Top view of epidermal electronics system and (b) schematic illustration of initial contact (inset: cross-section layout of EES) 1 Corresponding author. Manuscript received August 7, 2013; final manuscript received August 20, 2013; accepted manuscript posted August 28, 2013; published online October 16, 2013. Editor: Yonggang Huang. Journal of Applied Mechanics APRIL 2014, Vol. 81 / 044501-1 Copyright V C 2014 by ASME Downloaded From: http://appliedmechanics.asmedigitalcollection.asme.org/ on 11/03/2014 Terms of Use: http://asme.org/terms