New research from the journal ACS Nano Letters demonstrates the potential of laser-induced graphene (LIG)/gold nanoparticles (NPs) composite electrode-based transient enzymatic biofuel cells (TEBFCs) as a power source in transient electronics and implantable medical devices (IMDs).
Study: Transient, Implantable, Ultrathin Biofuel Cells Enabled by Laser-Induced Graphene and Gold Nanoparticles Composite. Image Credit: OSORIOartist/Shutterstock.com
Importance of Transient Power Sources in IMDs
In IMDs, miniaturized batteries containing corrosive liquid electrolytes are typically used as power sources, which can lead to transplant rejection and cause damage to the surrounding tissues upon leakage of electrolytes. Thus, a second surgical procedure is often performed after the initial implantation of IMDs to replace the entire IMDs after failure or treatment, or the exhausted batteries, which causes further damage to the implantation site.
Transient power sources with excellent bioresorbability and biocompatibility can effectively address these issues. EBFC is a suitable device that can effectively convert biochemical energy into electricity and supply stable and continuous power outputs. Additionally, EBFC can be developed in the form of an ultrathin and soft device to reduce the foreign-body sensation in the implantation site. Thus, EBFCs have gained considerable attention for implantable and wearable devices applications.
Carbon nanomaterials are used extensively as electrodes in EBFCs due to their excellent electrical properties, good biocompatibility, and large surface area. LIG can be obtained by fabricating a thin film of graphene from polyimide (PI) using an infrared carbon dioxide (CO2) laser.
In EBFCs and enzymatic sensors, certain functional materials such as nano-metal particles and conductive polymers, are used as dopants to improve the output performance or sensitivity of the sensors. Among these doping materials, gold NPs are the most suitable compared to other dopants owing to their large surface area, conductivity, and biocompatibility.
In this study, researchers synthesized ultrathin, implantable TEBFCs based on gold NPs/LIG composite electrodes and evaluated their performance.
Synthesis of the TEBFCs Array
A quartz glass served as a temporary supporting substrate during the synthesis. The glass was cleaned using acetone, ethanol, and deionized (DI) water in a sequential manner. Poly (methyl methacrylate) (PMMA) was spin-coated for 30 seconds at 2000 rotations per minute and then heated on a hot plate for 20 minutes at 200 degrees Celsius to prepare a PMMA sacrificial layer.
Subsequently, an 80 micrometer PI tape-based sheet was laminated on the glass, and the LIG electrodes were obtained directly from the laminated PI using a 10.64-micrometer infrared CO2 laser in an ambient environment. The laser line spacing, speed, and power were fixed at 0.03-millimeter, 1000 millimeters per second, and 1.8 Watts, respectively.
The obtained LIG array was submerged in acetone for 12 hours to eliminate the residual PMMA sacrificial layer and a photoresist (PR) layer was then spin-coated on the LIG to prevent it from falling off after the dissolution of water-soluble tapes. The residual LIG electrode and PR on the glass were removed using acetone and water-soluble tapes were used to pick up the pattern for transfer printing.
An ultra-thin poly (lactic-coglycolic acid) (PLGA) film was obtained as a receiving substrate by spin coating PLGA solution on the quartz glass. The water-soluble tape with LIG array was then placed on the PLGA substrate and the sample was submerged in water for 30 minutes to peel off the PLGA film and dissolve the water-soluble tape.
Subsequently, two silver nanowire layers were scattered over the PLGA film that served as the connection wire. Eventually, another PLGA film was spin-coated on the top of the as-synthesized sample that served as the encapsulation layer.
During the synthesis of LIG/gold NPs composite electrodes, the electrodes were treated with ultra-violet ozone (UVO) to improve their hydrophilicity, and then the chloroauric acid aqueous solution was added to the LIG electrode. The electrode was heated for 20 minutes at 80 degrees Celsius to obtain LIG/gold NPs composite electrodes.
Laccase and glucose oxidase were then added to cathodes and anodes, respectively, and lastly, two microliter chitosan and one microliter bovine serum albumin (BSA) was added to every electrode to embed the enzymes.
Fabrication and Evaluation of the Synthesized Samples
Linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) were used to characterize the synthesized samples. Researchers also performed in vivo and in vitro studies to evaluate the biocompatibility and bioresorbability of the samples.
An implantable TEBFCs array based on LIG/gold NPs composite electrode was successfully fabricated. The LIG/gold NPs composite electrode demonstrated high surface area and low impedance, which accelerated the electron transfer between electrode surfaces and enzyme active sites.
Fast electron transfer exceptionally improved the output performance with an open circuit potential (OCP) of 0.77 Volts, a maximum power density of 483.1 μW/cm2, and a maximum current density of 3113.5 μA/cm2. Such output performance from LIG-based EBFCs was achieved for the first time.
Additionally, the ultrathin TEBFC demonstrated a long lifetime and quick response time by reaching the maximum OCP within one minute. The TEBFC retained its activities for more than four weeks under a biofluid environment at room temperature. The TEBFCs also displayed excellent transient performance and biocompatibility in the in vivo and in vitro tests. Thus, they can be implanted for a long duration for energy harvesting purposes.
After usage, the TEBFCs present on the PLGA substrate degraded in a phosphate buffer solution in less than one month and within 44 days in animal bodies without causing any inflammation.
To summarize, the findings of this study demonstrated that TEBFCs based on LIG/gold NPs composite electrodes possess significant potential as an effective energy solution for IMDs and transient electronics. Additionally, the synthesized TEBFCs can be customized as per the requirements of the implantation sites and output intensities.
Li, J., Li, H., Li, D. et al. (2022) Transient, Implantable, Ultrathin Biofuel Cells Enabled by Laser-Induced Graphene and Gold Nanoparticles Composite. ACS Nano Letters. https://pubs.acs.org/doi/10.1021/acs.nanolett.2c00864
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