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Li et al. Soft Sci 2023;3:37 https://dx.doi.org/10.20517/ss.2023.30 Page 13 of 20

Figure 8. Implantable flexible electrodes for sensing biosignals. (A) The exploded-view drawing of the ultra-large tunable stiffness
electrodes enabled by LMs. Reproduced with permission [132] . Copyright 2019, Elsevier B.V; (B) The stiff sate electrode can translate into
soft sate due to the melting of LMs, and the insert picture shows the outlet of the drug delivery channel and Pt electrodes of the probe
tip. Reproduced with permission [132] . Copyright 2019, Elsevier B.V; (C) Schematic illustration of the highly stretchable electrodes for in
vivo epicardial recording on a rabbit. Reproduced with permission [134] . Copyright 2022, American Association for the Advancement of
Science; (D) The electrodes can be conformally attached to the right ventricle for long-term monitoring of the electrocardio; and (E)
shows the representative electrogram for 20 min of monitoring. Reproduced with permission [134] . Copyright 2022, American
Association for the Advancement of Science; (F) Photo of the stretched neural electrode arrays prepared by depositing Au film on LM-
PDMS composite. Reproduced with permission [135] . Copyright 2022, All authors; (G) Intraoperative image of the neural electrode arrays
for in vivo recording of ECoG signals, showing the high flexibility of the electrode to be in contact with the cerebral cortex of the rat; and
(H) ECoG signals of a healthy rat under normal state and epileptic state. Reproduced with permission [135] . Copyright 2022, All authors.
ECoG: Electrocorticogram; LM: liquid metal; PDMS: polydimethylsiloxane; PEDOT:PSS: poly 3,4-ethylene dioxythiophene : polystyrene
sulfonate; SEBS: styrene-ethylene-butylene-styrene.

Although electrode arrays show stable electrical properties under high strain, the deposited solid-state
conductors at the sensing site significantly affect the long-term stability due to the mechanical mismatch of
the interface between the metal and polymer. Therefore, deposited conductive polymers have been adapted
to encapsulate LM interconnects inside the polymer [133,134] . Wang et al. selected carbon nanotube composites
and microcracked conductive poly 3,4-ethylene dioxythiophene : polystyrene sulfonate (PEDOT:PSS)
polymer as the sensing sites of electrodes, as shown in Figure 8C . Combining LM interconnects with a
[134]
highly stretchable styrene-ethylene-butylene-styrene (SEBS) elastomer allows the electrode array to exhibit
ultrahigh stretchability (up to 400% tensile strain) and a low interfacial impedance. In addition, the long-
term in vivo epicardial recording on a rabbit [Figure 8D and E] further demonstrates the vast potential of
the electrodes for health monitoring applications. Li et al. fabricated a stretchable 8-channel neural electrode
array by depositing an Au film onto the surface of an LM-PDMS composite [Figure 8F] . The electrode
[135]
array can conformally match the cortical surface of a rat due to its high flexibility and stretchability
[Figure 8G]. The electrode array was effectively utilized for the in vivo identification of epilepsy by