Materials Horizons Emerging Investigator Series: Xinyu Liu, University of Toronto, Canada

Xinyu Liu is an Associate Professor and the Percy Edward Hart Professor in the Department of Mechanical and Industrial Engineering at the University of Toronto, Canada. Prior to joining U of T, he was an Associate Professor and the Canada Research Chair in Microfluidics and BioMEMS in the Department of Mechanical Engineering at McGill University, Canada. He obtained his BEng and MEng from Harbin Institute of Technology in 2002 and 2004, respectively, and his PhD from the University of Toronto in 2009, all in Mechanical Engineering. He then completed an NSERC Postdoctoral Fellowship in the Department of Chemistry and Chemical Biology (with George Whitesides) at Harvard University from 2009–2011. At U of T, his research activities focus on micro and soft robotics, microfluidics, and wearable electronics, with their applications primarily in medicine and biology. He has received the Canadian Rising Star in Global Health Award (2012), the Douglas R. Colton Medal for Research Excellence (2012), the Award of Excellence for Basic Science Research of the McGill Surgery Department (2013), the McGill Christophe Pierre Award for Research Excellence (2017), the MINE Outstanding Young Researcher Award (2018), and 7 best paper awards at major engineering and biomedical conferences. He is a co-inventor of 16 US/PCT patents (issued or pending). He serves as a Senior Editor of IEEE Robotics and Automation Letters and an Associate Editor of IEEE Transactions on Automation Science and Engineering, IEEE Transactions on Nanotechnology, IET Cyber-Systems and Robotics, Journal of Advanced Robotic Systems, and Journal of Sensors.

Read Xinyu Liu's Emerging Investigator Series article “An ambient-stable and stretchable ionic skin with multimodal sensation” and read more about him in the interview below:

MH: Your recent Materials Horizons Communication focuses on a novel artificial ionic skin that has promising applications in sensing, human-machine interaction and walking energy harvesting. How has your research evolved from your first article to this most recent article and where do you see your research going in the future?

XL: In the past several years, my research group has been working on soft robotics and wearable electronics, with the focus on new functional materials, sensing mechanisms and their device applications. The development of stretchable sensors made from materials compatible with soft robots and human skin could enable new designs of self-sensing soft robots and wearable physical and biosensors. Our major motivation is to develop such an artificial ionic skin (AIskin) with multi-modal sensing capability.

In this article, we propose a novel design of a hydrogel-based artificial ionic skin (AIskin) that mimics the transmembrane ion transport of sensory neurons in human skin for multi-modal sensation. The AIskin is made from a bilayer double-network hydrogel ‘doped’ with oppositely charged polyelectrodes, and is highly stretchable, tough, transparent, and ambient stable. It can sense force, pressure, deformation, and humidity, and provide multiple readout types such as resistance, capacitance, open-circuit voltage and short-circuit current. Because its sensing mechanism is based on input-induced ion transport across the hydrogel bilayer, the AIskin is self-powered and does not require a power supply during operation. Based on its unique mechanical properties and sensing capabilities, we demonstrated the application of the AIskin to sensing, human–machine interaction, and walking energy harvesting.

In the near future, we plan to develop a fabrication process for patterning millimeter- to sub-millimeter-sized bilayer hydrogel structures, which will enable the fabrication of an AIskin integrating a large number of sensing modules. This will reduce the device footprint and provide better sensing spacial resolution. We will apply the improved AIskin to soft robot designs, and demonstrate more precise environment perception and human–robot interaction. We would also integrate chemical and biosensing functionalities into the AIskin material system by ‘doping’ the double-network hydrogel with corresponding reagents. This will further enrich the AIskin sensing modalities, and enable new types of wearable applications, such as smart bandages for wound management.

MH: What aspect of your work are you most excited about at the moment?

XL: I am particularly excited about the unique combination of excellent mechanical and optical properties (high stretchability, toughness, and transparency) and the self-powered sensing capabilities of the AIskin, as these are highly desired features of functional materials for soft robotics and wearable electronics. For instance, the AIskin could be applied to the design of hydrogel-based, self-sensing soft robots with multiple sensor types, or used to construct highly compliant wearable medical devices such as rehabilitation gloves and ‘smart’ bandages. In addition, the double-network hydrogel material system leaves ample space for us to add other types of sensing capabilities to the AIskin, for example, temperature, chemical and biomolecule sensing.

MH: In your opinion, what are the most important questions to be asked/answered in this field of research?

XL: This research falls into the emerging field of hydrogel ionotronics, in which an ionic hydrogel is used to construct devices with electrical functions. In my opinion, one of the most important questions to be asked in this field is how to push these promising hydrogel ionotronic devices toward real-world applications. Fully addressing this question will involve research efforts in many aspects such as further improving the mechanical and chemical stability of hydrogels, expanding the application of these hydrogel ionotronic devices to new fields, developing highly conductive and stretchable electrodes as electrical interfaces, and inventing batch microfabrication and integration strategies for mass production.

MH: What do you find most challenging about your research?

XL: During our development of the AIskin, we found two technical challenges that must be addressed. One is to integrate stretchable and highly conductive electrodes with the AIskin. The self-powered sensing modes of the AIskin require high conductivity of the interfacing electrodes, but we found that existing stretchable electrode designs based on conductive nanomaterials such as carbon nanotubes and silver nanowires cannot provide efficient measurement of the open-circuit voltage and short-circuit current of our AIskin. The other challenge is to micro-pattern the bilayer hydrogel for high-density integration of sensing modules under a small device footprint. As a follow-up study, we are now developing a microfabrication strategy to fabricate an AIskin touchpad with an array of many pressure sensors for improved sensing spacial resolution.

MH: At which upcoming conferences or events may our readers meet you?

XL: We will present this work at the MRS 2020 Spring Meeting & Exhibit in Phoenix. I look forward to meeting colleagues who are interested in our research along this direction.

MH: How do you spend your spare time?

XL: I spend most of my spare time with my family. I also enjoy reading books and catching up with friends on social media.

MH: Can you share one piece of career-related advice or wisdom with other early career scientists?

XL: For those of you pursuing interdisciplinary research, working at the interface of different areas opens doors to many unique opportunities and outside-the-box solutions. But bridging multiple fields also tends to make you lose the focus of your research program. Always ask yourself what is the overarching theme of your research, and try to find your own unique ‘label’ in your home discipline and community.

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