Xinyue Liu
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What are hydrogels?
Hydrogels are polymer networks infiltrated with water. Examples include 
fruit jellies, and human tissues. Biological hydrogels constitute the major components of the human body. 

​What are hydrogel machines?
Owing to their superior softness, wetness, responsiveness, and biocompatibility, hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, coatings, optics, electronics, and water harvesters. A nascent field named hydrogel machines rapidly evolves, exploiting hydrogels as key components for devices and machines.
By leveraging hydrogel technologies, I am developing three types of hydrogel machines.
  • Living devices (gloves, skin patches, and tattoos) that are wearable for epidermal signal detection;
  • Ingestible devices that can retained in the GI tract for long-term GI monitoring and modulation;
  • Hydrogel optics that can be implanted in central nervous system for optical stimulation and recording.
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Living Materials & Devices

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Motivation
Living systems, such as bacteria, yeasts, and mammalian cells, can be genetically programmed with synthetic circuits that execute sensing, computing, memory, and response functions. Integrating
these functional living components into materials and devices will provide powerful tools for scientific research and enable new technological applications. However, it has been a grand challenge to maintain the viability, functionality, and safety of living components in freestanding materials and devices, which frequently undergo deformations during applications.

What we did
We designed a set of living materials and devices based on hydrogel–elastomer hybrids and hydrogel 3D printing that host various types of genetically engineered bacterial cells. The hydrogel provides sustainable supplies of water and nutrients to living bacterial cells, and the cells can communicate and process signals in a programmable manner.
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Related publications
  1. Xinyue Liu#, Tzu-Chieh Tang#, Eléonore Tham#, Hyunwoo Yuk#, Shaoting Lin, Timothy K Lu*, Xuanhe Zhao*, Stretchable living materials and devices with hydrogel–elastomer hybrids hosting programmed cells, Proceedings of the National Academy of Sciences, 114, 2200-2205 (2017) [MIT News]
  2. Xinyue Liu#, Hyunwoo Yuk#, Shaoting Lin, German Alberto Parada, Tzu-Chieh Tang, Eléonore Tham, César de la Fuente, Timothy K. Lu, Xuanhe Zhao*, 3D printing of living responsive materials and devices, Advanced Materials, 1704821 (2017) [MIT News][Cover Art]
  3. Tzu-Chieh Tang#, Eleonore Tham#, Xinyue Liu#, Kevin Yehl, Alexis J. Rovner, Hyunwoo Yuk, Farren J. Isaacs, Xuanhe Zhao*, Timothy K. Lu*, Tough hydrogel-based biocontainment of engineered organisms for continuous, self-powered sensing and computation. In submission

Ingestible  Devices

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Motivation
Devices that interact with living organisms are typically made of metals, silicon, ceramics, and plastics. Implantation of such devices for long-term monitoring or treatment generally requires invasive procedures. Hydrogels offer new opportunities for human-machine interactions due to their superior mechanical compliance and biocompatibility. Additionally, oral administration, coupled with gastric residency, serves as a non-invasive alternative to implantation. Achieving gastric residency with hydrogels requires the hydrogels to swell very rapidly and to withstand gastric mechanical forces over time. However, high swelling ratio, high swelling speed, and long-term robustness do not coexist in existing hydrogels.
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What we did
We introduced a hydrogel device that can be ingested as a standard-sized pill, swell rapidly into a large soft sphere, and maintain robustness under repeated mechanical loads in the stomach for up to one month. Large animal tests supported the exceptional performance of the ingestible hydrogel device for long-term gastric retention and physiological monitoring.
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Related publications
  1. Xinyue Liu#, Christoph Steiger#, Shaoting Lin#, Ji Liu, German A Parada, Joy Collins, Siid Tamang, Hon Fai Chan, Hyunwoo Yuk, Nhi Pham, Giovanni Traverso, Xuanhe Zhao*, Ingestible hydrogel device, Nature Communications, 10: 493 (2019) [MIT News][BBC News]​

Hydrogel Optics

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Motivation
Holistic study of neural dynamics in behaving subjects often demands simultaneous recording of neural activity across multiple organs in the nervous system. However, currently available silica-based techniques for the chronic recording of activity of specific neuronal ensembles are unsuitable for mobile regions of the nervous system such as the brain stem or spinal cord due to the rigidity of the probes.

What we did
Here we present a stretchable optical photometry platform based on hydrogels that allows chronic optical recording of neural activity across multiple regions of the nervous system in freely moving rodents during behavioral assays. We anticipate that the hydrogel photometry probes will facilitate investigation of neural circuits across central and peripheral nervous systems in freely moving subjects.
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Related publications
  1. Jingjing Guo, Xinyue Liu, Nan Jiang, Ali K Yetisen, Hyunwoo Yuk, Changxi Yang, Ali Khademhosseini, Xuanhe Zhao, Seok-Hyun Yun*, Highly Stretchable, Strain Sensing Hydrogel Optical Fibers, Advanced Materials, 28, 10244-10249 (2016) [MIT News]
  2. Siyuan Rao, Xinyue Liu,  Xuanhe Zhao, Polina Anikeeva, In preparation

Anti-Fatigue Hydrogels

Motivation
The emerging applications of hydrogels in devices and machines require hydrogels to maintain robustness under cyclic mechanical loads. Whereas hydrogels have been made tough to resist fracture under a single cycle of mechanical load, these toughened gels still suffer from fatigue fracture under multiple cycles of loads. The reported fatigue threshold for synthetic hydrogels is on the order of 1 to 100 J/m
2.
What we did
We propose that designing anti-fatigue-fracture hydrogels requires making the fatigue crack encounter and fracture objects with energies per unit area much higher than that for fracturing a single layer of polymer chains. We demonstrate that the controlled introduction of nanocrystals and aligned nanofibrils in hydrogels can substantially enhance their anti-fatigue-fracture properties. ​
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Related publications
  1. Shaoting Lin#, Xinyue Liu#, Ji Liu#, Hyunwoo Yuk, Jake Song, Hyun-chae Loh, German A. Parada, Charles Settens, Admir Masic, Gareth McKinley, Xuanhe Zhao*, Anti-fatigue-fracture hydrogels, Science Advances, 5, eaau8528 (2019) [Video]
  2. Shaoting Lin#,  Ji Liu#, Xinyue Liu, Xuanhe Zhao*, Muscle-like fatigue-resistant hydrogels by mechanical training, Proceedings of the National Academy of Sciences 116 (21), 10244-10249 (2019) [MIT News]
  3. Ji Liu#, Shaoting Lin#, Xinyue Liu#, Zhao Qin#, Yueying Yang, Jianfeng Zang*, Xuanhe Zhao*, Fatigue-resistant adhesion of hydrogels, Nature Communications. In press

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