Batteries contribute to the overall weight and size of implantable devices such as cardiac peacemakers and neuro-stimulators. The rigid encapsulation and requirement for proper insulation from corrosive materials limit the widespread utility of batteries for implantable devices. Besides weight and design aspects, the batteries have to be biocompatible and offer a high performance. Hence, development of light weight, physically flexible, biocompatible, high performance batteries are highly warranted to meet the demands of advanced personalized health care applications.
Researchers at King Abdullah University of Science and Technology (KAUST), Saudi Arabia, lead by Prof. Muhammad M. Hussain, have demonstrated a transfer-less method to develop a flexible, light-weight, biocompatible, high performance lithium ion batteries (LIBs) for implantable devices. They have also proposed a strategy for integrating the LIBs with flexible electronics and embedding them in a three-dimensional (3D) printed dental brace for orthodontics application (npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7)
Polydimethylsiloxane (PDMS) was used as the carrier substrate. A bulk LIB (thickness: 130 μm) was flipped on to the PDMS substrate. The Si (at the base of the LIB) was etched using xenon difluoride (XeF2) at a rate of 67 nm/s for 36 min (50 cycles). The surface roughness was monitored using atomic force microscopy (AFM) to optimize the conditions of XeF2 etching. The thinned LIB was removed from the PDMS (Fig. 1(a)). Complete removal of the Si substrate from the thinned LIB results in a free standing and physically flexible active stack (thickness: 30 μm). It consists of SiO2 (insulation layer), Al (cathode current collector), lithium cobalt oxide (cathode), lithium phosphorous oxynitride (electrolyte), Ti (anode current collector) and protective layers at the top surface (Fig. 1(b)). In spite of a large reduction in thickness (from 130 μm to 30 μm), the flexible LIB experienced only a lower strain (five times less) when compared to the bulk, for a 10 mm bending. Cell cultures grown on LIB for 3 to 5 days exhibited a healthy proliferation of the cells, which confirmed the biocompatibility of LIB. The flexible LIB possess a light weight (236 μg for each microcell of 2.25 × 1.7 mm) and exhibits a very high energy density (200 mWh/cm3) with a capacity retention of up to 70% after 120 cycles.
Fig. 1 (a) Schematic of the fabrication of flexible LIBs; and (b) cross sectional morphology of the flexible thinned battery and its components
Polyethylene terephthalate (PET) was used as the base for the flexible electronic device. It was metallized with Al (thickness: 0.023 mm). Interconnections were patterned using 1.06 μm ytterbium-doped fiber laser. Flip-chip technology and stencil printing allowed placement of the components. A conductive silver epoxy was used to bond the LIBs and LEDs. The LIB in the flexible electronic device exhibits minimal strain since most of the stress is experienced by the PET film. Transparent orthodontic brace was prepared by 3D printing using a clear resin. Finally, the flexible electronic device is integrated with the 3D printed orthodontic brace. The schematic of the integration of the flexible thinned LIBs with the flexible electronics and 3D printed dental braces is shown in Fig. 2(a). A pictorial representation of how the device fits in conformably onto the human dental arch is shown in Fig. 2(b).
Fig. 2 (a) Schematic of the integration of the flexible thinned LIBs with flexible electronics and 3D printed dental braces; and (b) Pictorial representation of how the device fits in conformable manner onto the human dental arch.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Arwa T. Kutbee et al., npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7
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