Page 31 - Read Online

Page 2 of 28 Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34

INTRODUCTION
The recording of high-quality neural signals, including electroencephalogram (EEG), electrocorticogram
(ECoG), and local field potential (LFP), is of great importance for the exploration of working mechanism of
brain activity, which is considered as the foundation for the development of the artificial intelligence and
[1-3]
the diagnosis and treatment of brain-related disorders . In addition, brain-machine interaction also has
various applications in the fields of aerospace, surgery, and the military. Therefore, the acquisition of neural
[4-6]
signals with non-invasive or invasive detection methods has received extensive attention .
Firstly, despite the rapid development of material science, the acquisition of high-quality EEG signals
through non-invasive methods remains a challenging task [7-12] . The existing non-invasive EEG electrodes are
often rigid, which leads to a mismatch in electrical and mechanical properties between human tissue and
electrodes, and they can not be used in a wearable form and stably record the EEG signals. Also, the
electrodes suffer from high noise due to poor resistance to environmental interference [13-19] . As a result, it is
difficult to record high-quality EEG signals with the non-invasive method [20-25] . Therefore, it is urgent to
develop interface materials that can adapt to the human skin to improve the performance of non-invasive
EEG electrodes. Among various materials, viscoelastic materials, such as hydrogels, are able to conformally
adhere to the skin and improve the interfacial quality [26-33] . In addition, hydrogels have a unique selective
frequency damping effect, which can minimize environmental noise. In contrast to filtering the noise with
signal processing after acquisition, electrodes with selective frequency damping effect can effectively obtain
signals with a high-to-noise ratio at low cost and with a simple interface [34-38] . As a result, the hydrogel-based
electrode is desired since it can record the EEG signals with a high signal-to-noise ratio (SNR) even in noisy
conditions [39-45] .

In the application of scientific research and exploration of the working mechanism of brain activity, an
invasive neural electrode is more attractive since it is able to monitor single-neuron activity [46-52] . ECoG and
LFP signals can be recorded by the invasive method, as shown in Figure 1. However, gradual encapsulation
of the implanted electrodes by glial scars often occurs for long-term implantation, which will lead to the loss
of recording and stimulation capabilities of the electrodes [53-59] . These phenomena result from the
mechanical and biological incompatibility between the implanted electrodes and the nerve tissue, which
causes chronic tissue damage. Also, the mismatch in mechanical properties, such as bending stiffness and
elastic modulus between the implants and biological tissues, will also lead to imperfect conformal contact
and may exert pressure on the organ. As a result, permanent deformation of the organ will be caused [60-62] .
Therefore, the development of an implantable electrode based on viscoelastic materials that match the
neural tissue in both biological and mechanical properties is a promising method to minimize the damage
to the tissues and realize long-term implantation.

Furthermore, conventional invasive devices can only monitor unimodal neural signals, which lack spatial or
[48]
temporal resolution and cannot meet the requirements of precision medicine . Hydrogel, a soft material
with high conductivity and transparency, is compatible with magnetic resonance imaging (MRI), computed
tomography (CT) and optogenetics [63-68] . Thus, hydrogel-based electrodes can be integrated with
optogenetic, CT, and MRI for multimode signal requisition, which is of great significance in brain science
and neurological disease diagnosis [69-72] . As a result, it can provide a new way and method for clinical
diagnosis and treatment of neurological diseases [73-75] .

Although both non-invasive and invasive neural electrodes have been widely investigated, the dehydration
of hydrogel when applied non-invasively and the swelling of hydrogel for invasive devices are still crucial
issues, especially for long-term applications . Therefore, wet, semi-dry and dry electrodes were proposed
[76]