Step-Index Optical Fiber Made of Biocompatible Hydrogels

Abstract

A biocompatible step-index optical fiber made of poly(ethylene glycol) and alginate hydrogels is demonstrated. The fabricated fiber exhibits excellent light-guiding efficiency in biological tissues. Moreover, the core of hydrogel fibers can be easily doped with functional molecules and nanoparticles for localized light emission, sensing, and therapy.

Keywords: biocompatible; hydrogels; implantable; optical fibers; sensing.

Figure 1

Optical properties of bulk hydrogels in cuvettes. (a, b) Measured attenuation coefficients, (a), and refractive indices, (b), of PEG hydrogels made with a monomer size of 700 Da at concentrations of 15-90% w/v. (c, d) Absorption spectra, (c), and refractive indices, (d), of alginate hydrogels at concentrations of 1-4% w/v.

Figure 2

Fabrication of core-cladded fibers. (a) Fabrication steps. Step 1, PEG hydrogel is formed by photocrosslinking in a tube mold. Step 2, the core is extracted from the tube by swelling the tube in dichloromethane. Step 3, the alginate hydrogel clad layer is added by dip-coating. Step 3 can be repeated to obtain a desired clad thickness. (b) Phase-contrast images of four hydrogel fibers with different sizes, immersed in distilled water. Arrows indicate the outer clad-water interface and arrowheads indicate interfaces between alginate layers formed by multiple dip coating. The numbers on top represent the thickness of the core and the clad in μm (i.e. 250/60 indicates 250-μm core diameter and 60-μm clad thickness; the outer diameter is thus 370 μm). The third image (550/200) shows a four-layer cladding. (c) A photograph of a 1-meter-long fiber. (d) Light guidance in air. Blue laser light (492 nm) was coupled to a hydrogel fiber (800/100) through a 10× objective lens (obj). (e) Light guidance of a fiber (800/100) sandwiched between two thin porcine tissue slices. Scale bar, 1 cm. (f) Propagation loss of three different fibers. (g) Corresponding 1/e propagation distances of the bare (800/0) and shelled (800/100) fibers.

Figure 3

Functionalized hydrogel fibers. (a) A schematic for functionalization of the core. Small-molecular fluorophores can be loaded through passive diffusion. Immobilization of the fluorophores can be achieved by introducing reactive groups (e.g. avidin) in the core. Larger objects, such as gold nanoparticles (GNP) can be physically trapped during the fabrication of the hydrogel core. (b) A fluorescence image of a dye-doped fiber. Rhodamine-6G was loaded in the fiber by dipping the tip in a solution of the fluorophore. When blue laser was coupled, the dye-doped region emits yellow fluorescence from excited rhodamine-6G molecules. (c) A fluorescence image of a fiber doped with three different fluorophores, Atto 488, 520 and 565, respectively, along the fiber. The biotin-conjugated fluorophores were incorporated into the avidin-encapsulated core. (d-e) Photothermal operation of a GNP-doped fiber. Green laser (532 nm, 800 mW) was coupled to the fiber to induce photothermal heating. Temperature was measured using a thermocouple in contact with the fiber.

Figure 4

Light amplification in a dye-doped fiber. The core was doped with rhodamine-6G and pumped with a Q-switched laser at 535 nm. (a) A setup for amplified spontaneous emission (ASE). Approximately 5 mm length of the fiber was pumped, and the guided ASE output at the end of the fiber was analyzed. (b) Output spectra measured at two different pump levels. (c) Output energy (red) and spectral width (blue) as a function of pump fluence. (d) A setup for whispering-gallery-mode (WGM) lasing. The red ring illustrates the optical paths of the laser modes oscillating along the circumferential interface between the core and the cladding. (e) Output spectra measured at three different pump levels. (f) The output power curve showed a distinct lasing threshold about 80 μJ/mm². Inset, a side-view image of the fiber above laser threshold. The red region indicates the laser light that was leaked from the fiber by scattering and recorded in a camera.

Figure 5

Demonstration of uses in vivo. (a) Pictures of a mouse with a hydrogel fiber administered to the colon through the rectum. The emission of red laser (640 nm) at the distal end of the fiber is seen through the skin and confirmed by abdominal laparotomy. (b-c) Reflectance oximetry of tissues. (b) Two hydrogel fibers—one for excitation and the other for collection—were implanted subcutaneously in an anesthetized mouse to measure the intrinsic optical absorption signal in response to oxygen/nitrogen ventilation. (c) Typical time-lapse traces of the calculated concentrations of oxy-hemoglobin (HbO), deoxy-hemoglobin (HbR), and total hemoglobin (HbT= HbO + HbR).