Conformable Liquid Metal Printed Epidermal Electronics for Smart Physiological Monitoring and Simulation Treatment

Journal of Micromechanics and Microengineering PAPER Conformable liquid metal printed epidermal electronics for smart physiological monitoring and simulation treatment To cite this article: Xuelin Wang et al 2018 J. Micromech. Microeng. 28 034003 View the article online for updates and enhancements. This content was downloaded from IP address 144.92.38.232 on 17/07/2018 at 03:13 IOP Journal of Micromechanics and Microengineering Journal of Micromechanics and Microengineering J. Micromech. Microeng. J. Micromech. Microeng. 28 (2018) 034003 (10pp) https://doi.org/10.1088/1361-6439/aaa80f 28 Conformable liquid metal printed epidermal 2018 electronics for smart physiological © 2018 IOP Publishing Ltd monitoring and simulation treatment JMMIEZ Xuelin Wang1, Yuxin Zhang1, Rui Guo1, Hongzhang Wang1, Bo Yuan1 and Jing Liu1,2 034003 1 Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, People’s Republic of China X Wang et al 2 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China E-mail: [email protected] Received 24 August 2017, revised 2 January 2018 Printed in the UK Accepted for publication 16 January 2018 Published 5 February 2018 JMM Abstract Conformable epidermal printed electronics enabled from gallium-based liquid metals (LMs), 10.1088/1361-6439/aaa80f highly conductive and low-melting-point alloys, are proposed as the core to achieving immediate contact between skin surface and electrodes, which can avoid the skin deformation often caused by conventional rigid electrodes. When measuring signals, LMs can eliminate Paper resonance problems with shorter time to reach steady state than Pt and gelled Pt electrodes. By comparing the contact resistance under different working conditions, it is demonstrated that both ex vivo and in vivo LM electrode skin models have the virtues of direct and 1361-6439 – immediate contact with skin surface without the deformation encountered with conventional rigid electrodes. In addition, electrocardio electrodes composed of conformable LM printed epidermal electronics are adopted as smart devices to monitor electrocardiogram signals of 3 rabbits. Furthermore, simulation treatment for smart defibrillation offers a feasible way to demonstrate the effect of liquid metal electrodes (LMEs) on the human body with less energy loss. The remarkable features of soft epidermal LMEs such as high conformability, good conductivity, better signal stability, and fine biocompatibility represent a critical step towards accurate medical monitoring and future smart treatments. Keywords: liquid metal, epidermal electronics, physiological monitoring, simulation treatment S Supplementary material for this article is available online (Some figures may appear in colour only in the online journal) 1. Introduction bad Ohm contact which affects the measurement accuracy and even treatment effects [6, 7]. It has been shown that the poor Noninvasion and precision for medical monitoring and treat- coupling between metallic electrodes and skin may lead to ment are the latest trends in the development of advanced low signal-to-noise ratio (SNR), which causes the degradation health care devices. So far, conventional medical devices used of signal performance when measuring physiological param- in signal measurement or electrical stimulation treatment have eters [8]. In addition, low energy transfer efficiency between been mainly equipped with rigid metallic electrodes such as metallic electrodes and the target site caused by air gaps lead cooper or platinum, which are generally placed directly on the to energy accumulation and temperature rise on local skin sur- skin [1–4]. However, due to the roughness of skin and irregular face, increasing the risk of skin injury [9]; this is especially physiological profile of the human body [5], there often exist reflected in equipment with high transient voltages such as air gaps between rigid metallic electrodes and skin, inducing defibrillators [10, 11]. 1361-6439/18/034003+10$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK J. Micromech. Microeng. 28 (2018) 034003 X Wang et al To overcome the disadvantages of traditional rigid elec- 3.4 × 106 S m−1, polarization voltage dives to −0.73 V) trodes, researchers have tried to add pressure onto the [33]. Gallium and indium with purity above 99.99% were electrodes to enhance electrode–skin contact [12]. However, weighed according to the radio of 75.5: 24.5. Liquid GaIn24.5 the pressure applied may alter the electrical characteristics of was obtained by mixing and stirring the prepared Gallium the skin, causing measurement deviation. Conductive gels are and Indium at 80 °C for 30 min. In order to avoid the adverse also considered to promote the coupling performance. These effects of the oxide layer on the LM surface, 10 ml NaOH gels could be smeared on the skin surface, filling the air gap solution (0.5 mol l−1) was added, and the mixture was stirred between skin and electrode and enhancing the conductivity for 1 min. [13]. However, the resistance of the conductive gel itself cannot be ignored. Furthermore, the electrode cannot main- tain good contact for long, which requires people to reconnect 2.2. GaIn24.5 electrode–skin coupling model the electrodes, after monitoring, every 24 h. To compare the differences between rigid Pt electrode, gelled Since flexible circuits can perfectly match the body curve Pt electrode, and LME, the contact impedances of Pt electrode– and have excellent performance in electrophysiological and skin, gelled Pt electrode–skin, and GaIn24.5 electrode–skin biochemical detection [14, 15], they are becoming more and in different frequencies from 1 to 105 Hz were measured on more popular in wearable medical device fabrication for in vivo skin surface of 8 week BALB/c Nude mice by elec- tasks such as body temperature measurement [16–18], blood trochemical workstation (CHI600E, Chenhua, China). In the flow measurement [17], and sweat analysis [19]. Due to their Pt electrode–skin model, the Pt electrode was directly placed good electric coupling with skin, flexible circuits can achieve on the skin surface. In the gelled Pt electrode–skin model, the higher SNR, better security, and a more robust anti-motion Pt electrode was placed on skin covered with conductive gel effect [20]. Notably, existing flexible devices can fit the mac- (ECG gel, Jinnuote). In the GaIn24.5 electrode–skin model, the rophysiological curve but cannot closely fit the microstructure Pt electrode was placed on skin covered with GaIn24.5. When on the skin surface. measuring contact impedances (figure S2), the working (green With the capability of being printed directly on the skin, probe) and ground (black probe) electrodes are connected gallium-based liquid metals (LMs) show great potential for with the gelled Pt electrode or GaIn24.5 electrode (point A in overcoming the technical challenges conventional rigid elec- figure 1(b)) via Pt wire; the reference (white probe) and aux- trodes have faced [21–23]. Owing to the merits of non-toxicity iliary (red probe) electrodes are connected with skin surface and benign biocompatibility, LMs have made remarkable pro- (point B in figure 1(b)) via Pt wire. The epidermis of the nude gress in bone cement [24], drug delivery nanomedicine [25], mouse was very thin and the wire probe placed below the epi- implantable devices [26], stretchable wireless sensors [27–29], dermis can be seen clearly. Therefore, both wire probes were electrical skin [30, 31], and wearable bioelectronics [32–35]. visible and easily located. All data were measured at the same The favorable conformability [32, 33, 36] of LMs allow them position to avoid individual difference. to form a direct contact with the skin and even spontaneously To illustrate the effect of LME oxide skin on contact resist- fill the microstructure on the skin surface without external force ance, 0.1 mol l−1 HCl solution (Beijing Chemical Works), [30, 31]. For instance, Yu et al have printed LM ink on a biolog- 0.1 mol l−1 NaCl solution (Sigma), and 0.1 mol l−1 NaOH ical surface as a drawable electrocardiogram (ECG) electrode solution (Sigma)) were respectively treated on the in vivo skin to demonstrate its conformability and attachment [31]. surface of 8-week-old BALB/c Nude mice. A LM droplet was In this paper, a model is established to simulate the perfor- dripped on the mice skin first, and then the HCl, NaCl, or mance of the LM electrodes directly attached on the skin. Ex NaOH solution was dripped on the droplet. Four measuring vivo and in vivo animal experiments of liquid metal electrodes electrodes were connected according to figure S2, and 5 min (LMEs) are also taken to evaluate the electrode–skin coupling later, the contact resistances were measured by electrochem- impedance in comparison to rigid electrodes. Additionally, ical workstation. stable ECG signals are detected to present the performance For the evaluation of contact situation, a CCD camera of physiological parameter monitoring. More importantly, (JC2000D3, Shanghai) was used to capture the contact angles a simulation treatment for smart defibrillation using these between the LM and skin surface. The contact angles of the soft electrodes is done by analyzing the distribution of elec- LM droplet on the ex vivo skin surface of 8-week-old BALB/c tric field and energy on the human body. The feasibility of Nude mice with different treatments (dry, wet (0.01 ml water highly conformable LM epidermal printed electronics meets on ex vivo skin surface), oil (0.01 ml oil on ex vivo skin sur- the demands of medical applications, and provides a possible face)), was further measured and recorded (figure S1). approach to future smart, accurate, and noninvasive physiological monitoring and treatment. 2.3. In vivo LME experiment GaIn was prepared as described in section 2.1. 2. Materials and methods 24.5 Polydimethysiloxane (PDMS) [39] was fabricated as a mixture of main and hardening agents (Dow Corning Sylgard 184, 2.1.

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