Development of spiropyran‑PDMS optical waveguides for sensing applications: ultraviolet light and pressure sensors

Abstract

This thesis focuses on the development of soft optical waveguides using a poly(dimethylsiloxane) (PDMS) matrix and spiropyran (SP) dyes to confer targeted functionalities for ultraviolet (UV) light dose monitoring and pressure sensing with localized threshold detection. PDMS was selected for its optical transparency, elastomeric properties, and biocompatibility, as detailed in the comprehensive literature review in Chapter 1. SP dyes were chosen for their reversible isomerization into a highly colored merocyanine form under UV irradiation or upon reaching specific mechanical stress and strain thresholds. Optical waveguide sensing was adopted for its inherent advantages, including rapid signal transmission, immunity to electromagnetic interference, and compatibility with compact optoelectronic components (e.g., microLEDs and photodetectors), enabling practical signal acquisition in real‑world settings. Despite their potential, a systematic characterization of waveguide formulations as a function of processing parameters, as well as the integration of SP responsive molecules within the context of optical waveguide design, had remained largely unexplored. Based on that, SP‑doped PDMS optical waveguides were first prepared by incorporating two SP derivatives (0.05 wt%) into PDMS by physical doping for UV dosimetry. One SP derivative yielded a functional sensor with a fully reversible, repeatable, and reproducible photochromic response. Sensor parameters remained consistent under mechanical bending and were dependent on test temperature, waveguide length, and ambient white light exposure. Next, a face‑centered central composite design was applied to neat PDMS optical waveguides to systematically map the influence of the base‑to‑crosslinker mixing ratio and curing conditions on transverse compression sensitivity, secant modulus, refractive index, and propagation loss, thus establishing the baseline optomechanical behavior of this material. The resulting response surface models revealed that the mixing ratio emerged as the primary driver of compression sensitivity and secant modulus. Meanwhile, the refractive index was predominantly governed by curing temperature, and propagation loss showed a weak linear dependence on the mixing ratio. Finally, SP was covalently incorporated into the PDMS matrix and characterized using a three‑factor Box‑Behnken design, introducing SP concentration as an additional variable. The resulting waveguides demonstrated quantitative optomechanical sensing in the low‑pressure range (0–100 kPa) alongside a strain‑driven mechanochromic visual response at higher compressions (0.9–2.4 MPa), with activation thresholds compatible with biomechanical data for human bruising. Consistent with the neat PDMS study, the mixing ratio governed compression sensitivity and secant modulus. However, SP concentration dominated the refractive index and propagation loss due to its high molar refractivity and the molecules acting as optical absorbing and scattering centers. Across all experimental designs, these material responses were interpreted within a polymer science framework, linking macroscopic responses to changes in crosslink density, network defects, material density, and characteristic changes in UV‑Vis and ATR‑FTIR absorption bands. In summary, this thesis establishes a systematic foundation for the rational design of multifunctional elastomeric optical waveguide sensors for wearable healthcare and soft robotics applications.

Date	13 Aug 2026
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Bora Ung (Supervisor)