A Spiropyran-Based Hydrogel Composite for Wearable Detectors to Monitor Visible Light Intensity to Prevent Myopia

Abstract

A wearable detector to monitor visible light intensity is realized by the restrained photochromism of a hydrogel composite containing light-responsive spiropyran with hydroxyl groups (SPOH). When exposed to visible light, the SPOH experiences a ring-opening to a ring-closed transition accompanied by discoloration from red to yellow. Unlike in the solution, the photochromism/discoloration rate is strongly correlated to the cross-linking points. By reducing the amount of cross-linker from 40 to 5 mg, the photochromism rate of SPOH is 300% faster. Inspired by the Chinese Jade Loong from Hongshan, the hydrogel composite is shaped into a Loong to monitor the light intensity. By increasing the amount of cross-linker in the head, body, and tail, the photochromism/discoloration rate sequentially turns slower from one region to the other. Higher light intensity is required to realize the discoloration in the hydrogel composite containing a larger amount of the cross-linker. Because the initial colors are identical, the light intensity can be easily traced by checking the discoloration of these pieces containing different amounts of cross-linker. Based on this unique and reversible photochromic capability, the present hydrogel composite can be used for monitoring the visible light intensity to prevent myopia, especially for children and students.

Keywords: hydrogel composites; myopia prevention; reversible photochromism; spiropyran; visible light detector.

Figure 1

SEM images of hydrogel composites with different amounts of MBA: (a) 5 mg, (b) 20 mg, and (c) 40 mg. (d) Porosity in hydrogel composites with different amounts of MBA.

Figure 2

Photochromic behavior of hydrogel composites with the different amounts of MBA (a: 5 mg, b: 20 mg; and c: 40 mg). (d) The absorbance value of hydrogel composites containing different amounts of MBA (5, 20, and 40 mg) at the maximum absorption peak before and after exposure to the xenon light. The light intensity applied is fixed as 637 W m^–2.

Figure 3

Comparison of curve areas of hydrogel composites with different amounts of MBA (a) during visible light illumination and (b) recovery in hot water. The light intensity applied is fixed as 637 W m^–2. Solid lines are guides for the eye.

Figure 4

Temporal discoloration in the hydrogel composites containing different amounts of MBA (5, 10, 20, 30, and 40 mg) after illumination for 0, 6, 10, 15, and 60 s. The light intensity applied is fixed as 637 W m^–2.

Figure 5

Optical image of the photochromic behavior of the Loong-shaped detector illuminated by a xenon lamp with different light intensities.

Figure 6

Cycling performance of the hydrogel composite containing 5 mg of MBA. The light intensity applied is fixed as 637 W m^–2.

Figure 7

Photographs of (a) the wearable bracelet based on the hydrogel composite with different amounts of MBA (5, 20, and 40 mg) for the visible light intensity monitoring, (b) before and (c) after xenon lamp illumination with a light intensity of 600 Lux (640 W m^–2).