Researchers have been working hard to create 3D printed robotic prosthetic hands that look, and feel like, a real human hand; a team at the University of Wollongong developed a prosthetic hand that has an interface that, when placed on a person’s arm muscles, gives it a sense of touch. But engineering researchers at the University of Minnesota’s College of Science and Engineering have gone even further: thanks to their hard work, and 3D printing, a type of bionic skin could give robots the sense of touch, without needing an interface attached to a human hand. The research team developed a groundbreaking technique to 3D print stretchable, electronic sensory devices, which is a big step in the right direction to printing electronics on actual human skin.

A one-of-a-kind 3D printer built at the University of Minnesota can print touch sensors directly on a model hand. [Image: Shuang-Zhuang Guo and Michael McAlpine, University of Minnesota]

Michael McAlpine, a mechanical engineering associate professor at the university, was the lead researcher for the study; we’ve previously covered his work in nerve regeneration through 3D printed silicon guidesprogrammable release drug delivery capsules, and a functioning, 3D printed quantum dot LED; he also made headlines back in 2013 for creating a bionic ear, using electronics and 3D printed nanomaterials. McAlpine explained that his team’s new discovery could eventually be used for health monitoring, and even have applications in the military, as soldiers could potentially wear the 3D printed electronics in the field to detect explosives or dangerous chemicals.

The team’s research will be published in the next issue of the Advanced Materials journal, in a paper titled “3D Printed Stretchable Tactile Sensors,” and is currently available online. Co-authors include McAlpine and Department of Mechanical Engineering graduate students Shuang-Zhuang Guo, Fanben Meng, Sung Hyun Park, and Kaiyan Qiu.

The abstract reads, “The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human–machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.”

Inverse engineering process and conformal multi-material 3D printing. (a) Optical image of a hand model. (b) Twelve scans obtained from various perspectives to assemble a 3D model. (c) The aligned scans are assembled into a water-tight 3D model. (d) The original scanned model in point cloud format. (e) Transformation of the model data from point cloud to polygon meshes, which provides the topological information of the hand
surface. (f) A redesigned tactile sensor that conformally fits the fingertip. (g) Optical image
showing the multi-material 3D printing setup. Images of the printed tactile sensor on the
fingertip (h) with and (i) without supporting layer.

McAlpine and his research team built a brand new multifunctional 3D printer, which was used to manufacture the unique sensing fabric.

Michael McAlpine

“This stretchable electronic fabric we developed has many practical uses. Putting this type of ‘bionic skin’ on surgical robots would give surgeons the ability to actually feel during minimally invasive surgeries, which would make surgery easier instead of just using cameras like they do now,” McAlpine explained. “These sensors could also make it easier for other robots to walk and interact with their environment.”

The printer’s four nozzles are used to print the different specialized materials that make up all of the device’s layers. There’s a base layer of silicon, while a conducting ink is used to make top and bottom electrodes. A coil-shaped pressure sensor is also printed, and a sacrificial layer actually holds the top layer in place as it sets; this supporting layer is eventually washed away.

McAlpine said, “This is a completely new way to approach 3D printing of electronics. We have a multifunctional printer that can print several layers to make these flexible sensory devices. This could take us into so many directions from health monitoring to energy harvesting to chemical sensing.”

Liquid plastic 3D printing won’t work, as the material is too rigid and hot to use on skin. The nice thing about the material layers in the stretchable sensors, which can also stretch up to three times their original size, is that they are able to set at room temperature. McAlpine says the team members are continuing to make strides in their research.

SEM images of the printing process. (a) Top view of printed bottom electrode
layer using the 75 wt% Ag/silicone ink. (b) Higher magnification of the bottom electrode
showing the spanning structure. (c) Inclined top view of the fully printed sensor.

“While we haven’t printed on human skin yet, we were able to print on the curved surface of a model hand using our technique. We also interfaced a printed device with the skin and were surprised that the device was so sensitive that it could detect your pulse in real time,” McAlpine said.

“With most research, you discover something and then it needs to be scaled up. Sometimes it could be years before it ready for use. This time, the manufacturing is built right into the process so it is ready to go now.”

The next hurdle for the research team is to take a step toward semiconductor inks, and attempt to 3D print their flexible sensory devices on an a human body.

McAlpine put it best: “The possibilities for the future are endless.”

The team used the Polymer Characterization Facility and the University of Minnesota Characterization Facility (CharFac) for testing purposes. Their research was funded by the National Institutes of Health’s National Institute of Biomedical Imaging and Bioengineering. Discuss in the 3D Printed Skin forum at 3DPB.com.

[Source/Images: University of Minnesota College of Science and Engineering]

 

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