Electrolyte-gated transistors for enhanced performance bioelectronics

This PrimeView highlights the design, fabrication and use of electrolyte-gated field effect transistors.

The biological output signature is detailed in the transfer characteristics (I D -V G ) . These curves prove the selective bio-recognition at the bio-layer on the gate electrode.
Parameters to compare EGT performance include the transconductance g m ; the threshold voltage V T ; the field-effect mobility (μ); and the gate and channel capacitances per unit area (C') or per unit volume (C*).
The intersect between the linear least squares approximation of I D and the V G axis represents the threshold voltage V T .
Transconductance g m is the slope of the I D -V G curve when the EGT operates in a linear re- A top-gated EGT model highlighting the biolayer on the gate electrode serves as an example.

Reproducibility and data deposition
A detailed outline of the EGT fabrication, the relevant EGT dimensions, the electrolyte composition and the bioelectronics experimental set-up are imperative to ensure reproducibility. No data deposition standards exist owing to the wide range of application fields covered by EGTs, although this would likely contribute to better reproducibility in the field.

Limitations and optimizations
High stability, both under operating and ambient conditions, is essential for EGTbased bioelectronics. Therefore, control experiments should be conducted before running any biological experiments. These checks include leaching; electrochemical redox reactions between the EGT materials; morphological changes to the channel; and water diffusion and ion doping. Gate functionalization is limited by coverage, low density and compactness of the bio-recognition layer. However, immobilizing bio-recognition units, such as proteins, protein-embedded membranes and cells, onto or within the channel is possible.
Faradaic side reactions should be a point of caution as they can accelerate device degradation and negatively impact the local biological environment.

Outlook
EGTs offer an excellent communication platform where biosystems can be studied via ionic and electric currents. EGT properties come down to device structure, the materials used and the functionalization patterns. Advances in materials used, fabrication methods and bio functionalization strategies will broaden the application scope of EGTs, which will have a direct impact on health-care tools. Device mechanisms will enable progress for EGTs.

Experimentation
EGT architecture (top-, bottom-, side-and extended-gate) depends on the position of the gate electrode relative to the semiconductor channel. For instance, the gate is positioned directly over the channel in topgated EGTs, which are the most commonly used EGTs across different applications. Both ion-impermeable and ion-permeable small molecules and polymers are used for organic semiconductors in EGTs. Amorphous metal oxides and 2D materials are used as the inorganic semiconductors in EGT channels. Both conventional photolithography and additive manufacturing are used to fabricate EGT across the architecture landscape. The biolayer integration is imperative for EGTs to detect the analytes of interest. EGT labelling is achieved via physical adsorption, covalent attachment or bioaffinity immobilization.

Applications
EGT-based bioelectronics have been applied broadly. For instance, planar EGTs can be used to monitor properties such as cell adhesion, growth and differentiation. Although currently limited in performance, EGTs can be used to measure properties of 3D cultures. Select configurations enable multicellular tissue and organ growth for analysis of these complex systems. Examples of ultra-sensitive biosensors can detect down to a single molecule of markers such as proteins, peptides, metabolites and nucleic acids in early diagnostic tools. Electrophysiological activity can be monitored using EGTbased biosensors both inside and outside the body. In brain studies, EGTs have been used to record epileptic seizures in rats. Owing to their large capacitance, EGTs can be used to study neurotransmitter activity by configuring them as artificial synapses. These EGT-based synaptic devices can be applied to neuromorphic signal processing by configuring multiple sensors. Such configurations can then be used in e-skin prosthetics and robotics.
Electrolyte-gated transistor (EGT)-based bioelectronics enable the detection of ions and biomolecules, which have a role in biological function. EGTs are important building blocks that are stable in an aqueous environment, operate at low voltages and can transduce and amplify biological signals into electronic signals.