A Three-Dimensional-Printed Recyclable, Flexible, and Wearable Device for Visualized UV, Temperature, and Sweat pH Sensing

Abstract

Wearable devices are now recognized as a powerful tool to collect physiological and environmental information in a smart, noninvasive, and real-time manner. Despite the rapid progress of wearable devices especially wearable electronic devices, there are still several challenges that limit their further development, for example, a complicated electrical signal acquisition and processing process to eliminate the interference from the surrounding signals, bulky power supply, inevitable e-waste, and environmental pollution. Herein, we report a 3D-printed recyclable, flexible, and wearable device for visualized UV, temperature, and sweat pH sensing. Compared with wearable electronic devices, our visualized wearable device senses environmental (UV light, ambient temperature), biophysical (skin temperature), and biochemical (sweat pH) signals via stimuli-responsive color change, which does not require complicated electronic circuit design/assembly, time-consuming data processing and additional power source. In addition, this visualized wearable device is fabricated via a 3D support bath printing technology by printing UV-, temperature-, and sweat pH-sensing inks containing photochromic, thermochromic, and pH-chromic materials, respectively, into/onto sustainable starch solution, resulting in a multi-functional, recyclable, and flexible sensing device with high reproducibility. Our results reveal that UV light intensities under sunlight (0-2500 μW/cm²), ambient, and skin temperatures (0-38 °C) as well as sweat pH (4.0-7.0) can be successfully monitored.

Scheme 1. 3D-Printed Recyclable, Flexible, and Wearable Devices for Visualized UV, Temperature, and Sweat pH Sensing: (a) Fabrication of Recyclable, Flexible, and Wearable Devices by Printing Visualized Sensing Inks in a Support Bath; (b) Visualized UV, Temperature, and Sweat pH Sensing Using 3D-Printed Recyclable, Flexible, and Wearable Devices

Figure 1

Visualized UV sensing via photochromic microcapsule: (a) mechanism on visualized UV sensing through structure transformation of the photochromic microcapsule between spirooxazine and merocyanine under visible and UV lights; (b) UV–vis spectra of the photochromic microcapsule under darkness and UV irradiation (365 nm); (c) UV–vis spectra of the photochromic microcapsule after exposure to UV light (1000 μW/cm²) for various times (0, 3, 10, 20, 30, and 120 s); (d) UV–vis spectra of the photochromic microcapsule under different UV light intensities (100, 600, 1000, 1500, and 2000 μW/cm²); (e) specificity of the PDMS elastomer containing 0, 0.2, 0.5, 1.0, and 2.0 wt % of photochromic microcapsules to IR, UV, daylight (fluorescent lamp, D65), and sunlight; (f) color change of the PDMS elastomer containing 1.0 wt % of the photochromic microcapsule under different UV light intensities (0–2500 μW/cm²); (g) reversibility and stability of the photochromic microcapsule for UV sensing.

Figure 2

Visualized temperature sensing via thermochromic microcapsule: (a) schematic mechanism on visualized temperature sensing by the thermochromic microcapsule; (b) photo images and UV–vis spectra of red, green, and blue thermochromic microcapsules at various temperatures (4, 24, 30, and 38 °C); (c) photo images of hybrid thermochromic microcapsules with different red/green/blue ratios (1:1:1, 1:3:1, and 5:2:5) at various temperatures (4, 24, 30, and 38 °C) and their differences in chroma values within the CIE 1931 chromaticity coordinate; (d) optical images and infrared thermal images of thermochromic elastomer containing hybrid thermochromic microcapsules (red/green/blue = 5:2:5) at different temperatures (0–38 °C); (e) evaluation on reversibility and stability of visualized temperature sensing by measuring the wavelength of characteristic UV–vis absorption peaks of the hybrid thermochromic microcapsule at 4, 24, and 38 °C within 10 temperature cycles.

Figure 3

Visualized sweat pH sensing via methyl red: (a) molecular reaction mechanism of methyl red under acidic and neutral pH values; (b) UV–vis spectra of methyl red in ethanol, artificial sweat with pH 4.0, and artificial sweat with pH 7.0; (c) optimization of methyl red concentration for visualized sweat pH sensing; (d) color images of methyl red (0.1 wt %) under different sweat pH values (4.0, 4.5, 5.0, 5.5, 6.0,7.0); (e) reversibility and stability of visualized sweat pH sensing via methyl red.

Figure 4

3D-printed UV, temperature, and sweat pH sensing devices fabricated by printing sensing inks in a PDMS support bath: (a) schematic illustration on the fabrication of UV, temperature, and sweat pH sensing devices; (b) rheological properties of UV, temperature, and sweat pH sensing inks, which were prepared by blending photochromic microcapsules, thermochromic microcapsules, and methyl red with PDMS prepolymers, respectively; (c) color change of the 3D-printed UV sensing device under various UV light intensities; (d) color change of the 3D-printed temperature sensing device at various temperatures. English letters “SUCT” were prepared with different temperature sensing inks (“S”: PDMS prepolymer mixed with red thermochromic microcapsules; “C”: PDMS prepolymer mixed with green thermochromic microcapsules; “U”: PDMS prepolymer mixed with blue thermochromic microcapsules; “T”: PDMS prepolymer mixed with hybrid thermochromic microcapsules); (e) color change of the 3D-printed sweat pH sensing device exposed to artificial sweat with pH 4.0 and 7.0.

Figure 5

Preparation and characterization of modified starch as an alternative supporting substance for the visualized wearable devices: (a) reaction equation between STMP and starch; (b) photos and transmissivity in the visible light range via the UV–vis spectrum of modified starch; (c) live/dead staining of L929 cells cultured on the starch film surface (scale bar: 100 μm); (d) degradation behaviors of starch film in tap water.

Figure 6

Visualized UV, temperature, and sweat pH sensing via 3D-printed recyclable, flexible, and wearable devices (6 cm (L) × 2.5 cm (W) × 100 μm (H)): (a) visualized UV sensing in the daytime (8:00 am, 10:00 am, 12:30 pm, 14:30 pm, 17:30 pm) through the printed letter “U”; (b) visualized temperature sensing at various temperatures (5.8, 23.3, 33.8 °C) through the printed letter “T”; (c) visualized sweat pH sensing for males’ and females’ sweat pH through the printed circle. Five male and five female volunteers were randomly sampled for sweating pH testing.