Soft electronics allow for tracking of multiple biophysical vital signs (e.g. heart rate, blood pressure, body temperature) and biochemical signals (e.g. ions and metabolites in body fluids) in a continuous manner. Recent developments in advanced materials enable a diverse set of soft electronics, including conductive polymers, nanomaterials, hydrogels, liquid metals, and organic semiconductors.
Soft electronics constructed from the above materials exhibit an extremely soft form factor to mitigate the mechanical mismatch at the interfaces between electronics and biological tissues, thereby expanding the modality and improving the fidelity in sensing. However, such physical properties of soft electronics pose difficulties for connecting them with conventional electronic devices. Although various methods adopt interpenetrating polymer/metal nanostructures, stretchable anisotropic conductive films or mechanically interlocking microbridges to realize solderless and fast interconnections of flexible electronics, the interconnections are irreversible, resulting in limited reconfigurability of the functions, sensitivities, and many other features of the soft electronics.
To address the above restrictions, Mengdi Han resreach group at Peking University developed hard magnetic graphene nanocomposite (HMGN). The preparation of HMGN contains a process flow including laser inducing graphene, drop casting a mixture of neodymium iron boron/ polydimethylsiloxane (NdFeB/PDMS), and peeling off for transfer and magnetization. Dropping casting the mixture of NdFeB/PDMS allows NdFeB particles to penetrate into the porous structure of graphene. Results in Figure 1 demonstrateexcellent mechanical, electrical, magnetic and biocompatible properties of HMGN.
Figure1.Fabrication and characterization of the HMGN.
The research group fabricated multimodal sensors and studied the modulation of sensing performance by magnetic particles and magnetic domains. For voltammetric electrochemical sensor, the magnetic particles exert Lorentz force to induce convection via magnetohydrodynamic and micro-magnetohydrodynamic effects, promoting rapid redox reactions and electron transfer. For electrophysiological sensor, doping NdFeB particles enhances the hydrophilicity of HMGN by reducing the air gap and enhancing surface wetness to lowering interface impedance. For temperature sensor, the synergistic effect of the thermal motion of electrons inside magnetic particles and the thermal motion of magnetic domains enhances sensitivity (Figure 2).
Figure 2. HMGN for enhanced, multimodal sensing.
In addition, the research group also proposedreversible, and self-aligned electrical interconnection based on HMGN. Ordered magnetic domains allow the HMGN sensors to self-assemble onto or detach from HMGN substrate with magnetic and conductive interconnections. Magnetic particles in the sensor and substrate exhibit opposite polarity to attract each other, and form continuous conductive path from N-pole of the sensor to S-pole of the substrate. The current-voltage (I-V) curve of the interface exhibits a linear relationship, suggesting an Ohmic contact. Compared with other techniques for interconnection, such as wire bonding and conductive adhesives, the interconnection formed from magnetic attraction is reversible and self-aligned, and does not require external pressure or heat (Figure 3).
Figure 3. HMGN for reversible and self-aligned electrical interconnection.
Experiments on healthy human subjects demonstrate that the HMGN-based soft electronics can measure electrocardiogram, skin impedance, skin temperature, and concentrations of ions and metabolites in sweat, with reconfigurable sensitivities, spatial coverages, and sensing modalities (Figure 4).
Figure 4. Applications of HMGN in reconfigurable soft electronics.
In summary, HMGN can achieve the reconstruction of flexible electronics for various biomedical applications, and show the potential to assist in the diagnosis and treatment of many human diseases. Further opportunities lie in the expansion of sensing modalities of HMGN to target at more diverse biophysical and biochemical conditions, the improvement of self-aligned interconnection to assist the assembly of small-scale devices across large areas, and the development of fully integrated reconfigurable soft electronics that comprise front-end sensing modules and back-end circuit modules for wireless operation.
This work was published in Advanced Materials titled “Hard Magnetic Graphene Nanocomposite for Multimodal, Reconfigurable Soft Electronics”Zehua Xiang from Peking University is the first author, and Mengdi Han from Peking University is the corresponding author.
Link to the paper: https://onlinelibrary.wiley.com/doi/10.1002/adma.202308575