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Page 18 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

HEALTHCARE APPLICATIONS
Vital monitoring devices
Thin film-based flexible sensors have opened the possibility of vital sign monitoring of skin, organs, and
body movements due to their conformal attachability to body organs [81-83] . For detecting biological problems,
several research groups have demonstrated wearable sensors for analyzing various homeostatic biomarkers
such as heartbeat, sweat ion concentration, tears, and saliva [84-88] . Kang et al. reported a low-power
[89]
heterojunction phototransistor to monitor the PPG signal [Figure 9A] . The PPG sensor was fabricated
using the poly{3-[(2,2’:5’,2’’-terthiophen)-5-yl]-2,5-bis(2-octyldodecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-
1,4-dione-6,5’’-diyl} (DPP2ODT2-T) layer with chlorobenzene (CB) and toluene (Tol), accomplishing a
stable voltage level and low power consumption (5 V) for accurate sensing under various heart rates. The
superior electrical/optical properties of the sensor provide long-term and accurate sensing capabilities under
different heart rate conditions in cardiovascular checks.

Invasive blood tests for analyzing ion concentrations, such as Na , K , and Cl or glucose, are applied in
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various fields (electrolyte analysis, pH tests, and diabetes), but they have significant issues such as skin
trouble, wounds, and pain [90-92] . The potentiometric sensors measure the potential difference between a
reference electrode and a working electrode to track the change of the ion concentrations against time. With
their simplicity, diverse ion detection capabilities and rapid response times, potentiometers have been
spotlighted in diverse fields, especially in ion sensing of body fluid [93,94] . Sweat analysis with potentiometric
sensors has been considered as an invasive vital monitoring method; however, several obstacles remain,
such as limitations of sweat collection, materials toxicity, and low multi-sensing capabilities.

Criscuolo et al. developed a multifunctional sweat analyzing system, integrating ion-selective membranes
and cotton microfluidics for stable and continuous monitoring [Figure 9B] . The system consisted of four
[95]
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ion-selective electrodes for each ion: Na , Li (therapeutic drug), Pb (heavy metals exposure), and K
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(hydration and activity). The multifunctional sweat sensors were fabricated using conventional micro-
electro-mechanical systems (MEMS) processes, demonstrating stable sensing properties in water and
artificial sweat, even under mechanical stress (180° bending). To prove real-time monitoring applications,
the sodium and potassium levels were tracked by the wearable system, showing high Pearson correlation
coefficients of about 0.97 (Na ) and 0.81 (K ) during physical exercise of five human volunteers. These
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results mean that the developed wearable sweat sensing system has the potential for the non-invasive vital
sign monitoring technology.
Since various mechanical oscillations/movements (e.g., voices, heartbeats, and tremors) occur in our body,
monitoring them is important to check bio-stability. To distinguish small bodily oscillations, high sensitivity
and signal separation properties of the devices are required. Although several researchers have reported the
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wearable strain sensor with high sensitivity over 1 kPa and GF of ~10 under 2%~6% strain, these strain
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sensors have various issues including high-cost/complex manufacturing processes, and low stability under
harsh environmental conditions [96-101] . Tolvanen et al. developed a stretchable and washable crack sensor
using silver ink-patterned silicone elastomer [Ag-dragon skin with conductive fiber (Ag-DS/CF)]
[Figure 9C] . This strain sensor was fabricated in the pre-stretching state to withstand the dynamic
[102]
loading-induced strain by forming the wrinkle of Ag-ink. Depending on the intensity of the applied strain,
the change of the device sensitivity was inversely proportional, providing action detectivity regardless of its
size. The fabricated strain sensor exhibited a high GF of 1.2 × 10 and negligible hysteresis loss under the
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repeated stretching cycles. This crack-based strain sensor with the maximum pressure sensitivity of
0.82 kPa at 0.035 kPa demonstrated the notable sensing properties under body movement and steel ruler
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oscillation [Figure 9D].