For both biomedical engineering and fundamental biological studies, electronic devices constitute one of the most important tool-sets. In order to address more complicated health conditions and diseases, a new generation of electronics that can be seamlessly merged with human bodies/tissues to realize long-term stable and independent operations is highly desired, which requires both biomechanically-adaptive formfactors (i.e. soft and stretchable) and biomechanically-powered energy supplies. Therefore, this new technology for future bioelectronics will be based on new types of tissue-like materials, and guided by fundamental understandings at the interface of semiconductor physics, solid mechanics and energy sciences.
1. Intrinsically stretchable electronic materials
Skin-like electronics capable of seamless attachment on human skin or within the body are highly desirable for applications such as health monitoring, medication therapy, implantable medical devices, and biological studies. An desirable path towards this is to use polymer materials to impart intrinsic stretchability onto electronics, which could potentially afford high mechanical deformability, improved skin compatibility, and high device density. As the ground-breaking development for stretchable semiconductors, we have explored a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, the fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. As for stretchable conductors, we have been able to impart highly stretchability onto graphene electrode, by creating graphene nanoscrolls in between stacked graphene layers, so that its extraordinary electronic properties can be taken advantage of in stretchable electronics.
2. Intrinsically stretchable polymer devices
Despite the development of intrinsically stretchable electronic materials, the realization of functional intrinsically stretchable electronics have is restricted by the lack of a scalable fabrication technology for intrinsically stretchable polymer devices, especially transistor arrays as the device building-block for electronics. We developed a fabrication process that enables high yield and uniformity, and is applicable to a variety of intrinsically stretchable polymer electronic materials. We demonstrated the first intrinsically stretchable polymer transistor array, with an unprecedented device density. The constituent transistors have an average mobility comparable to that of amorphous silicon and showed only small variations within one order of magnitude when subjected to 100% strain for 1000 cycles. Taken together, the developed transistor arrays enabled the first demonstration of intrinsically stretchable skin electronics, including an active matrix for sensing arrays as well as analog and digital circuit elements. More importantly, this fabrication process constitutes a general platform for incorporation of other intrinsically stretchable polymer materials toward the fabrication of next-generation stretchable skin electronic devices. This research provides the technological platform for intrinsically stretchable electronics.
1) Nature, 555, 83-88, (2018).
3. Nanogenerators for bio-mechanical energy harvesting
The fundamental sciences and applicable technologies for harvesting environmental energy are not only essential in realizing the self-powered electronic systems, also tremendously helpful in meeting the rapid-growing world-wide electronic energy consumptions. Mechanical energy is one of the most universally-existing, diversely-presenting, but usually-wasted energies in the natural environment. We developed triboelectric nanogenerators as a new technology for mechanical energy harvesting, which can efficiently convert mechanical motions into electricity based on the coupling of triboelectrification and electrostatic induction. We also designed the electrochemical energy storage processes for nanogenerators, and proposed a new energy device concept–self-charging power cell, based on the hybridization with Li-ion batteries.
1) Fundamental working mechanisms for triboelectric nanogenerators
Through enabling different types of coupling between the triboelectrification and the electrostatic induction, I established two fundamental working mechanisms for triboelectric nanogenerators: lateral-sliding mode, and freestanding-triboelectric-layer mode, which are among the four basic working modes/mechanisms for triboelectric nanogenerators.
2) Enhancement of triboelectric nanogenerators’ output performance
For the development of any energy harvesting technology, including triboelectric nanogenerators, the enhancement of the power output (i.e. energy conversion efficiency) is the most important research theme, which determines the practical applicability of this technology. Our research has been addressing this through innovative conceptual material developments with extraordinary triboelectric performance, and the device structure developments to maximize the electrostatic induction.
3) Hybridized electrochemical energy storage for nanogenerators–self-charging power cells
Energy conversion and energy storage are two distinct processes that are accomplished through two different and separated physical units. These two categories of technologies both have their own limitation in serving as sustainable power sources for electronic devices/systems. We have developed new fundamental science and invented a new type of energy devices—self-charging power cells/units that hybridize the nanogenerator-based mechanical energy harvesting and the Li-ion-battery-based energy storage process into a single-step or in a single device.