The multi-material, multiscale and multifunctional process has the potential to open opportunities in applications such as health monitoring, medical devices, orthotics and flexible electronics. It also has far-reaching implications for robotics, as 3D printed stretchable electronic sensory devices could give robots the ability to “feel” their environment, which could be relevant to many robotics processes from manufacturing to remote surgery by robotics (in which the surgeon is in a different location than the patient).
To date, 3D printing directly onto human skin has been impossible because of the temperatures usually involved in the process. The materials used by the UM researchers set at room temperature, making it possible to build devices directly on skin. The customized sensors, once printed, are capable of detecting and differentiating human movements and are so sensitive they can monitor pulse. The sensors are also able to stretch up to three times their original size.
The research opens new routes for the biointegration of various sensors in wearable electronics systems and advanced bionic skin applications. The new process could be used to develop wearable technology for military personnel in the field to detect dangerous chemicals or explosives.
It works like this: the multifunctional printer has four nozzles that print the various specialized “inks” that make up the layers of the sensory device. The four materials include a base layer of silicone, top and bottom electrodes made of a conducting ink, a coil-shaped pressure sensor, and a “sacrificial” layer that holds the top layer in place while it sets. The supporting sacrificial layer is later washed away in the final manufacturing process.
|The team’s process is the first to enable printing directly on human skin. Photo credit: Shuang-Zhuang Guo and Michael McAlpine, University of Minnesota.|
University of Minnesota Department of Mechanical Engineering graduate student Shuang-Zhuang Guo, a co-author of the study, told Design News that the printer platform is extremely versatile and can process any kind of printable inks, including a diverse range of materials from semiconductors to conductive nanoparticles and biological hydrogels. This makes it stand out from commercially available multi-material printers.
“Stratasys Ltd. has commercialized several PolyJet based 3D printers such as J750, Objet 100 Plus, and Connex3, which offer the processes capabilities for multi-material,” said Guo. “However, the inks for the PolyJet printers are limited in UV curable materials and purchased from the vender of the printer.”
The printer developed by the University of Minnesota researchers has the capability to print with a very broad and very specialized range of materials.
“Four different inks were employed to print my tactile sensors, which were prepared using silver nanoparticles, silicone rubber, polymer surfactant and solvent,” Guo told Design News.
There is very little post-processing involved, with the exception of a step to wash out the supporting layer using water. This is important since the researchers hope to be able to print bionic skin under mild conditions directly onto biological surfaces. The new process is also novel because it enables the printer to work on freeform surfaces of different shapes.
“One of the important application is that we can directly print various electronic devices such as tactile sensors, strain sensors, optical sensors and LED light conformally on curved surfaces,” Guo told Design News.
The new process will also be particularly relevant to robotics, according to the researchers. The platform means manufacturers can design and fabricate much more complex robots, going far beyond traditional fabrication methods. It could also potentially be cost-effective, as it’s unique in that the manufacturing is built right into the process.
“With most research, you discover something and then it needs to be scaled up. Sometimes it could be years before it’s ready for use,” said McAlpine. “This time, the manufacturing is built right into the process, so it is ready to go now.”
The research is a natural progression for McAlpine, who was recognized in 2013 for developing a process that involved 3D printing electronics and nanomaterials to create a “bionic ear.” The researchers used facilities at the University of Minnesota Characterization Facility and Polymer Characterization Facility for testing.
The new printing platform will play a a key role in all the team’s currently active group projects. Going forward, the group plans to embark on a process that will allow for the printing of integrated multi-sensors to mimic human skin functions. The team says its discovery could also pave the way toward semiconductor inks.
The research, which was funded by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health, has been published by Advanced Materials.