Jean Won Kwak^1,2,*, Mengdi Han^1,*, Zhaoqian Xie³, Ha Uk Chung⁴, Jong Yoon Lee⁴, Raudel Avila², Jessica Yohay⁵, Xuexian Chen⁶, Cunman Liang⁷, Manish Patel⁸, Inhwa Jung⁹, Jongwon Kim^9,10, Myeong Namkoong^1,11, Kyeongha Kwon¹, Xu Guo³, Christopher Ogle⁴, Dominic Grande⁴, Dennis Ryu⁴, Dong Hyun Kim⁴, Surabhi Madhvapathy^1,12, Claire Liu^1,13, Da Som Yang¹, Yoonseok Park¹, Ryan Caldwell^5,14, Anthony Banks¹, Shuai Xu^1,4,15, Yonggang Huang^2,12,16, Stefania Fatone⁵ and John A. Rogers^{1,2,12,13,†}

¹Querrey Simpson Institute for Bioelectronics, Northwestern University, Chicago, IL 60611, USA.
²Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA.
³State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, International Research Center for Computational Mechanics, Dalian University of Technology, Dalian 116023, China.
⁴Sibel Inc., Evanston, IL 60208, USA.
⁵Prosthetics-Orthotics Center, Northwestern University, Chicago, IL 60611, USA.
⁶Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, China.
⁷Key Laboratory of Mechanism Theory and Equipment Design of Ministry of Education, School of Mechanical Engineering, Tianjin University, Tianjin, China.
⁸University of Illinois College of Medicine at Chicago, Chicago, IL 60612, USA.
⁹Department of Mechanical Engineering, Kyung Hee University, Yongin 17104, South Korea.
¹⁰Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea.
¹¹Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843, USA.
¹²Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA.
¹³Department of Biomedical Engineering, Northwestern University, Evanston, IL 60208, USA.
¹⁴Scheck & Siress Prosthetics and Orthotics, Schaumburg, IL 60173, USA.
¹⁵Department of Dermatology, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA.
¹⁶Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA.

^†Corresponding author.
^*These authors contributed equally to this work.
 

Precise form-fitting of prosthetic sockets is important for the comfort and well-being of persons with limb amputations. Capabilities for continuous monitoring of pressure and temperature at the skin-prosthesis interface can be valuable in the fitting process and in monitoring for the development of dangerous regions of increased pressure and temperature as limb volume changes during daily activities. Conventional pressure transducers and temperature sensors cannot provide comfortable, irritation-free measurements because of their relatively rigid construction and requirements for wired interfaces to external data acquisition hardware. Here, we introduce a millimeter-scale pressure sensor that adopts a soft, three-dimensional design that integrates into a thin, flexible battery-free, wireless platform with a built-in temperature sensor to allow operation in a noninvasive, imperceptible fashion directly at the skin-prosthesis interface. The sensor system mounts on the surface of the skin of the residual limb, in single or multiple locations of interest. A wireless reader module attached to the outside of the prosthetic socket wirelessly provides power to the sensor and wirelessly receives data from it, for continuous long-range transmission to a standard consumer electronic device such as a smartphone or tablet computer. Characterization of both the sensor and the system, together with theoretical analysis of the key responses, illustrates linear, accurate responses and the ability to address the entire range of relevant pressures and to capture skin temperature accurately, both in a continuous mode. Clinical application in two prosthesis users demonstrates the functionality and feasibility of this soft, wireless system.