Page 36 - Read Online

Page 2 of 34 Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38

[1]
transmitting to the receiving end in the form of optical or electrical signals. In the field of health care
sensors, the physiological signals of the human body include muscle movement, metabolic fever, cardiac
electrical signals, electromyographic (EMG) signals, urine, sweat, etc. The combination of these
physiological signals will indicate the state of health, and the long-term data collection through the sensors
will facilitate health monitoring and disease diagnosis, thus enabling early intervention in disease
[2]
treatment . This asks for functional stability and structural durability for materials. In addition, the impact
of the environment on human health should not be underestimated. Exposure to ambient noise, low air
pressure, hazardous gases, or unsuitable temperatures for a prolonged period can be harmful to humans, so
sensors are also needed to detect environmental information in real time.

However, due to the non-planar shape of the human body and frequent body movements, traditional rigid
[3]
sensors can no longer cope with such complex working conditions . Therefore, flexible and electrically
conductive materials need to be introduced to overcome this disadvantage. The sensor mainly comprises a
flexible substrate and a conductive material filling the substrate . The flexible substrate can ensure a close
[4]
fit between the sensor and the body, while the flexibility allows the sensor to bend and stretch with muscles.
However, the common flexible substrate materials [polydimethylsiloxane (PDMS) [5-12] , polyimide [13-18] ,
Ecoflex [19-21] , polyurethane (PU) [22-25] , and poly(vinyl alcohol) (PVA) [26-28] ] are usually electrically insulating,
so they need to be filled with conductive material to form a conductive path to realize signal transmission.
The universal filler materials are metal nanoparticles (NPs) [8,29-32] , metal nanowires , and carbon-based
[33]
nanomaterials, such as carbon nanotubes (CNT) [12,20,23,30,34,35] , graphene [11,13,36,37] , and carbon black [8,38] .
However, limited by the cost or inherent mechanical property defects, applying the above materials in
flexible sensors requires structure design and mechanical match. Gallium-based LMs have excellent thermal
and electrical conductivity [39-43] and find applications in energy [44,45] and soft robotics [46,47] applications.
Therefore, they are used as filler materials. LMs also exhibit excellent flexibility and stretchability [39,40] ,
reducing damage to the substrate and ensuring that the electrical conductivity of materials remains stable
during the stretching process. Gallium-based LMs are less harmful to biological tissue and, therefore, could
[48]
be used in dentistry as an alternative to mercury , indicating outstanding biocompatibility [49,50] . The rich
surface properties, including the chemical properties of the oxide film [49,51,52] and the controlled composition
of the oxide film , also allow for multiple forms of conversion of the sensing signal. Nevertheless, when
[53]
macro LMs are fabricated into composites and wrapped, leakage is difficult to avoid , and direct-written
[54]
LM circuits have poor adhesion . Microfluidic sensors have relatively complex fabrication processes , and
[55]
[56]
their integration can be challenging. Fortunately, by exploiting the liquid deformability of LMs, it is possible
to prepare macroscopic LM droplets down to the nanoscale using methods such as ultrasonics, etc. [57-59] The
resulting LMNPs not only have the basic properties of macroscopic LM droplets but also have a higher
specific surface area, thus giving the LM more reaction area . Additionally, they have properties such as
[60]
self-healing and plasmon resonance. The filling of LMNPs into flexible substrates can reduce the leakage
[61]
of LM to a certain extent while simplifying the preparation process compared to microfluidic sensors.

Based on the information presented above, the application of transformable nano-LM sensors for human
health detection appears to be essential, and therefore, this paper aims to present the progress of research on
this sensor in the field of life and healthcare monitoring. The physical and chemical properties involved in
the use of LMs as sensor materials are presented, followed by a comparison of the unique transformable
properties of LMs for sensor applications, and then the main structures of existing nano-LM sensors are
discussed. Finally, the latest research and applications of nano-LM flexible sensors in human health and the
development of nano-LM sensors are also introduced [Figure 1].