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Page 2 of 10 Zhu et al. Soft Sci 2024;4:21 https://dx.doi.org/10.20517/ss.2024.01

[7,8]
and decode intercellular and intracellular messages , leading to the establishment of platforms for early
diagnosis and treatments of cardiovascular and neurodegenerative diseases [9,10] . Most electrophysiological
techniques, e.g., electrocardiograms (ECGs) [11-13] , electroencephalograms (EEGs) [14,15] , electrocorticography
[18]
(ECoG) [16,17] , etc., capture signals by situating bioelectrodes on the tissue surface . Conventional electrodes,
typically termed “passive electrodes” , have their own sets of challenges. For instance, metal electrodes
[19]
frequently result in undesired immune responses, making them unsuitable for chronic applications in the
biotic environment. Organic electrodes, on the other hand, generally exhibit a low signal-to-noise ratio
[20]
(SNR) resulting from their limited conductivity .
Transistors [21,22] , particularly the organic electrochemical transistors (OECTs) [23,24] , hold advantages in
detecting low-amplitude signals within physiologically relevant time frames owing to their intrinsic
amplification capabilities. OECTs excel in on-site signal processing, design flexibility, and biocompatible
characteristics. Their unique mechanism, bulky modulation of the active layer, results in a volumetric
capacitance, endowing them with high transconductance (g ) under low-operation voltage, enabling high
m
sensitivity and safe operation for biosensing applications [25,26] . Leveraging these benefits, in vitro [27,28] and
in vivo [29-31] signals have been successfully recorded using OECTs [Figure 1A].

The form factors of OECTs are evolving towards increased stretchability and conformability to the tissue.
This necessitates using interfacial materials that are both adhesive and porous, retaining excellent
conformability without forming interfacial air gaps, ensuring high oxygen and water vapor permeability,
and allowing the underlying tissue to breathe freely [Figure 1B]. Future advancements in implantable
devices also demand innovations to engineer these systems to be programmable for enhanced
spatiotemporal resolution and biodegradable to reduce the need for subsequent surgical interventions. For
chronic application scenarios, including self-healing property further boosts the durability, practicality, and
lifespan of OECTs [Figure 1C]. Additionally, integrating self-powered features, such as coupling with
energy ultraflexible energy harvesters , paves the way for eco-friendly bio-integrated systems. Despite the
[32]
rapid evolution of this field, achieving high-quality recordings with superior spatiotemporal resolution and
stability still presents a formidable challenge.

RECENT PROGRESS IN ELECTROLYTE ENGINEERING OF OECTS
One primary issue arises with the use of liquid electrolytes. Their tendency to evaporate might decrease the
reliability of in vitro applications , and they pose a barrier to individual gating in integrated circuits/arrays
[33]
for in vivo applications in an ion-rich medium. While organic liquid electrolytes demonstrate superior ionic
conductivity, many have not been proven to be biocompatible, and fluidic issues persist . Solid-state
[34]
electrolytes, i.e., ion gels [35,36] based on polymers blended with ionic liquid or based on biomaterials such as
[37]
[39]
hydrogels , levan polysaccharide , etc., offer improved nonvolatility and facilitate stable operation of the
[38]
devices under dynamic conditions. This is attributed to the crosslinked polymer structure that enhances
stability while maintaining ionic conductivity by incorporating ionic-conducting components. Interfacial
intimacy can be facilitated through in-situ polymerization, while biocompatibility further necessitates
careful material selection. Besides, the electrolyte materials should be mechanically soft to maintain
conformal contact between the skin and devices, which is crucial to decrease the impedance for efficient
gating and realize stable recording.

The radar chart in Figure 2 provides a comparative analysis of key performance metrics across diverse
electrolytes currently in use. These encompass aqueous electrolytes such as NaCl solution and phosphate-
buffered saline (PBS), organic liquid electrolytes such as 1-ethyl-3-methylimidazolium tetrafluoroborate
([EMIM][BF ]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) ,
[34]
4