Selectively Micro-Patternable Fibers via In-Fiber Photolithography

Abstract

Multimaterial fibers engineered to integrate glasses, metals, semiconductors, and composites found applications in ubiquitous sensing, biomedicine, and robotics. The longitudinal symmetry typical of fibers, however, limits the density of functional interfaces with fiber-based devices. Here, thermal drawing and photolithography are combined to produce a scalable method for deterministically breaking axial symmetry within multimaterial fibers. Our approach harnesses a two-step polymerization in thiol-epoxy and thiol-ene photopolymer networks to create a photoresist compatible with high-throughput thermal drawing in atmospheric conditions. This, in turn, delivers meters of fiber that can be patterned along the length increasing the density of functional points. This approach may advance applications of fiber-based devices in distributed sensors, large area optoelectronic devices, and smart textiles.

Figure 1

Thiol–epoxy/thiol–ene photopolymer. (a) Components of the photopolymer composite. The red and blue circles represent functional groups reacting during thermal and photocuring, respectively. (b) Schematic of the photolithographic patterning process of the thiol–epoxy/thiol–ene network. The yellow, red, and blue colors denote the noncured, the thermally cured, and the photocured photoresist. Gray color marks the substrate.

Figure 2

Characterization of thiol–epoxy/thiol–ene photopolymer. (a) Fourier-transform infrared spectroscopy (FTIR) measurements are shown for the photopolymer in the uncured (black), thermally (T) cured (red), and photocured (TP) (blue) state. Peaks in the ranges 2550–2600, 800–850, and 650–700/cm correspond to thiol, epoxy, and alkene functional groups, respectively. (b) Differential scanning calorimetry (DSC) shows the increase in the photopolymer T_g following 1 (black), 2 (purple), 3 (amber), and 4 (red) hours of thermal-cuing, respectively. (c) UV–vis transmittance spectra of the photopolymer prior to (red) and following (blue) photocuring. All samples have been thermally cured. Shaded areas denote standard deviation (n = 5 samples). (d–f) Scanning electron microscope (SEM) images of patterns lithographically defined on the free-standing photopolymer films. (d) Line patterns with 1, 10, and 100 μm thickness. (e,f) Geometric patterns.

Figure 3

In-fiber photolithography. (a) Schematic depicting thermal drawing and lithographic patterning of the multimaterial fibers. Cyclic olefin copolymer (COC), carbon-loaded polyethylene (CPE), and thiol–epoxy/thiol–ene photopolymer are marked with gray, black, and orange to red gradient colors, respectively. Red and blue colors represent thermally and photocured photopolymer, respectively. (b) Cross-sectional image of the preform prior to thermal drawing. (c) Cross-sectional image of the fiber thermally drawn from the preform in panel (b). An inset presents a higher magnification image of the photopolymer layer coating the CPE conductor. (d–g) SEM images of patterns developed directly on the thermally drawn fibers. (d) Line patterns with 1, 10, and 100 μm thicknesses. (e) Thermally drawn fiber with a line pattern of 500 μm thickness. (f,g) Geometric patterns. (h,i) Impedance spectra of the CPE electrodes within the thermally drawn fibers exposed along the length via (h) 76 μm and (i) 330 μm line patterns. The black, red, and blue curves present the mean impedance of CPE electrode under the conditions shown in the inset. (1) Open circuit measurement of the epoxy-insulated fiber tip in PBS. (2) Measurement of the CPE electrode exposed along the fiber shaft. (3) CPE electrode tip impedance. Shaded areas represent standard error (n = 5 samples).