(Nanowerk Spotlight) As we are moving into an era of flexible and wearable electronic devices, one challenge that arises is an increased vulnerability to mechanical failure. Relatively small damages such as tiny cracks along the
electrical conductive pathway caused by the bending, twisting and folding of such devices could easily cause the entire gadget to fail.
Researchers in Korea have now demonstrated light-powered healing of an electrical conductor. Reporting their findings in the September 15, 2014 online edition of Advanced Functional Materials ("Light-Powered Healing of a Wearable Electrical Conductor"), a team led by Seungwoo Lee, a professor at the SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering at Sungkyunkwan University, showed that green light even with low intensity has potential to provide fast (less than 3 minutes) and repetitive recovery of a damaged electrical conductor without any direct invasion.
SEM images taken at various stages of light-powered (s-polarization) healing of linearly cracked PDO 3 film (15 µm width and 5 µm depth). (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
While structural recovery of materials by using light as a stimulus has been explored previously, this new work further extends the concept to build up on-demand, light-powered healing of wearable electronic devices. Its novelty is the use of a directional photofluidic diffusion of a silver nanowire mesh/azobenzene material.
Most of the existing strategies for healing electronic components depend on capsules, heat, and water – approaches that could be heavily invasive due to the use of mechanical forces or liquid-mediated partial dissolutions.
These strategies also haven’t been amenable to the recovery of the irregular multiple cracks which could be generated particularly by the vigorous mechanical motions of a wearable device (e.g., bending and twisting).
"The mechanical failure along a conductive pathway in a wearable electronic device should be addressed in a fast, noninvasive, and on-demand way," Lee remarks to Nanowerk. "In contrast to other techniques, non-invasive light-powered healing provides the potential for fast and remote access to the recovery of mechanical failure, which is difficult to achieved with any other method."
In their work, the team makes use of a photochromic soft material (i.e., an azobenzene material), which can be directionally diffused along the light polarization.
"This unique directionality of the material diffusion with respect to light polarization enables an efficient healing process regardless of crack propagation directions, light incident angles, and the number of cracks," explains Lee. "Furthermore, thanks to the high quantum yield of azobenzene, which is used in this work as photochromic molecule, directional molecular diffusion even with low light intensity (at room temperature) is achieved in a fast (within a few minutes) and repeatable (several times) manner."
By depositing silver nanowires as conducting material onto the top layer of the flexible photochromic soft material, the researchers extended this optically healable material to have fully functional electrical conductivity.
Lee points out that the silver nanowires were found to maintain conformable contact with the photochromic soft material even during the diffusible optical healing process.
The researchers also investigated the capability of simultaneous healing of multiple, irregular cracks. While the cracks did heal, healing times increased significantly (from 120 seconds to 350 seconds) as did the recovered resistance.
Lee notes that repeatability of this novel system still poses a challenge. Currently, experiments have shown that about 3 times repeatability of healing of a wearable electrical conductor can be performed.
The current system is based on the light-powered, directional delivery of a conductive silver nanowire mesh; this is achieved by the use of a photofluidic diffusible polymeric backing layer (i.e., azobenzene materials) as a cargo of delivering silver nanowire mesh.
"What we found out in our experiments is that the photofluidic polymer can be easily penetrated into the silver nanowire mesh during the repetition of healing process," says Lee. "However, the junction between silver nanowires becomes weakened after performing the healing processes several times until, after a certain number of repetitions, the electrical conductivity disappeared altogether."
The team believes that this problem can be addressed by solely using conductive materials with photofluidic diffusibility rather than a two-layered system (conductive silver nanowire/photofluidic polymer layer). The results of this work will be published separately.