Skin Inspires Creation of 3D Printable, Conductive Self-Healing Hydrogel

The ability to heal was once thought to be relegated to living things only, but that’s not the case anymore, thanks to technology. A great deal of work has been done in the development of self-healing materials, with the goal of applying them to everything from electronics to medical devices to even construction. Despite the research that has gone into those and other applications, however, there’s still a lot of work to be done. When bringing self-healing materials into real-world applications such as robots, artificial skin, etc., other properties need to be cultivated, such as conductivity, and not a lot of research has been done into how to make that happen.

Most self-healing materials so far have been designed to repair themselves in response to external stimuli such as light, temperature and electricity. These non-autonomous self-healing systems differ from living tissue in that they still need some form of intervention before they can heal, and the goal is to create materials that can fix themselves without any help. For that, conductivity is a key property. Autonomous conductive self-healing materials have been created, but they haven’t performed very well in practical applications due to lack of characteristics like stretchability, pressure sensitivity, bulk conductivity or overall mechanical performance.

A team of researchers at the University of Manitoba, led by Malcolm Xing, Assistant Professor of Mechanical Engineering, Biochemistry and Medical Genetics, have come up with a new method of designing self-healing materials to make them mechanically stable, conductive and autonomously self-healing. The research was published in a paper entitled “Skin-Inspired Multifunctional Autonomic-Intrinsic Conductive Self-Healing Hydrogels with Pressure Sensitivity, Stretchability, and 3D Printability,” which you can access here. The method involves a combination of physical and chemical cross-linking, as the researchers explain:

“The autonomous intrinsic self-healing of the hydrogel is attained through dynamic ionic interactions between carboxylic groups of poly(acrylic acid) and ferric ions. A covalent cross-linking is used to support the mechanical structure of the hydrogel. Establishing a fair balance between the chemical and physical cross-linking networks together with the conductive nanostructure of polypyrrole networks leads to a double network hydrogel with bulk conductivity, mechanical and electrical self-healing properties (100% mechanical recovery in 2 min), ultrastretchability (1500%), and pressure sensitivity.”

A two-step process is used to prepare the hydrogel. Polypyrrole (PPY) is linked to double-bond decorated chitosan (DCh), forming PPy-grafted chitosan (DCh-PPy). Acrylic acid (AA) monomers are polymerized in DCh-PPy using iron ions, which produces the double network hydrogel. The autonomous self-healing properties of the material are produced by the reversible ionic interactions between the AA and the DCh-PPy.

Preparation of a 3D printed wearable sensor

As described in the paper’s abstract, the material is able to fully repair itself within two minutes, and it can also recover 90% of its electrical properties within 30 seconds, without any external stimuli. This autonomous healing ability, as well as shear thinning behavior, make the hydrogel an ideal material for 3D printing, according to the researchers, and potential applications include soft robotics, biomimetic prostheses, and health monitoring systems.

“The practical potential of CSH hydrogels is further revealed by their application in human motion detection and their 3D-printing performance,” the researchers add.

Authors of the paper include Mohammad Ali Darabi, Ali Khosrozadeh, Rene Mbeleck, Yuqing Liu, Qiang Chang, Junzi Jiang, Jun Cai, Quan Wang, Gaoxing Luo, and Malcolm Xing.

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