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Ma et al. Soft Sci 2024;4:26 https://dx.doi.org/10.20517/ss.2024.20 Page 9 of 34

porous LIG, achieving speech recognition and generation to rehabilitate the patient’s vocalization
[46]
capability .
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APPLICATIONS OF LIGS E FOR INTELLIGENT HEALTHCARE
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LIGS E for biophysical signals detection
Physical signals detection
A densely interconnected 3D network of graphene flakes renders LIG with conductivity and excellent
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piezoresistive response capability. Motivated by this, engineers have designed LIGS E to monitor
biophysical signals. Kaidarova et al. developed a soft pressure sensor by laser-printing conductive porous
graphene structure onto polyamide film packaged with an ultrathin coating material [Figure 6A] . The
[60]
fabricated pressure sensor exhibited an outstanding sensitivity of 1.23 × 10 kPa, a low detection limit of
-3
approximately 10 Pa, a wide dynamic range (> 20 MPa), and excellent cycling stability (> 15,000 cycles).
Profited by excellent mechanical sensing performance, the pressure sensors manage health when attached to
the skin surface, such as heart rate monitor, gait analysis, and tactile perception. In addition, Luong et al.
[36]
reported a 3D LIG foam printing process based on laminated object manufacturing [Figure 6B] . The
fabrication process of the LIG foam is simple and low-cost: firstly, the PI layer with prepared LIG was
stacked with one another using ethylene glycol as a binding agent; then the sandwiched layers were lased to
fabricate macroscale LIG foams; finally, a fiber laser was utilized to mill the bulk LIG, forming LIG foams.
The piezoresistive effect of the LIG foam was investigated by applying stress, which exhibited a significant
gauge factor (GF) of approximately 40. In addition, the LIG foam presented an increased GF overuse while
maintaining full stretchability. The developed pulse sensor based on LIG/PDMS precisely captured pulse
waveforms from wrist arterial sites.

Learning from the bean pod structure, Tian et al. showcased a self-healable pressure sensor, which was
composed of a polystyrene (PS) microspheres-based micro-spacer layer sandwiched between two laser-
induced graphene/polyurethane (LIG/PU) sheets [Figure 6C] . The developed devices presented an
[92]
outstanding sensitivity of 2,048 kPa , a short response time of 16 ms, and self-healable performance. The
-1
sensors demonstrated several healthcare application scenarios, including human arterial pulse detection and
gaits monitoring. Besides, a pressure sensor array (4 × 4) was fabricated, realizing a mapping of a two-
dimensional spatial of pressure. Flexible pressure sensor arrays face an urgent challenge during practical
applications, i.e., the realization of high spatial resolution and low crosstalk interference. To tackle this
problem, Li et al. proposed a high-resolution pressure sensor array through a simple laser processing
method, where the individual sensing elements were prepared by laser-induced graphitization and
interconnects were created by laser-induced ablation to reduce crosstalk interferences [Figure 6D] . A
[93]
0.7 mm-resolution pressure sensor array validated that the crosstalk coefficient reduced from -8.21 to
-43.63 dB. Besides, the individual sensing element inside the pressure sensor array presented a wide pressure
-1
working range of 80 kPa, a remarkable sensitivity of 1.37 kPa , and a short response time of 20 ms.
In addition to compressive strain detection, the LIG functional materials are sensitive to tensile strain.
Benefiting from this, engineers have developed various strain sensors based on LIG for healthcare. Carvalho
et al. employed an UV laser to fabricate a flexible strain sensor, achieving a fourfold decrease in the
penetration depth (5 μm). At the same time, the spatial resolution is doubled compared with the devices
prepared by the typical IR source [Figure 6E] . The developed sensor showcased a mechanical sensitivity
[37]
(GF) of approximately 20 in a 0%~1% strain range. Furthermore, when integrated into the skin surface, the
sensor could detect radial and carotid pulse waves with detailed features. To increase the stretchability of the
LIG/PI composites, researchers have attempted to prepare LIG on the PI/PDMS composites directly. For
instance, Wang et al. employed a PDMS/PI composite substrate to fabricate LIG under the irradiation of an