Agarose-based structured optical fibre

Abstract

Biocompatible and resorbable optical fibres emerge as promising technologies for in vivo applications like imaging, light delivery for phototherapy and optogenetics, and localised drug-delivery, as well as for biochemical sensing, wherein the probe can be implanted and then completely absorbed by the organism. Biodegradable waveguides based on glasses, hydrogels, and silk have been reported, but most of these devices rely on complex fabrication procedures. In this sense, this paper proposes a novel structured optical fibre made of agarose, a transparent, edible material used in culture media and tissue engineering. The fibre is obtained by pouring food-grade agar into a mould with stacked rods, forming a solid core surrounded by air holes in which the refractive index and fibre geometry can be tailored by choosing the agarose solution composition and mould design, respectively. Besides exhibiting practical transmittance at 633 nm in relation to other hydrogel waveguides, the fibre is also validated for chemical sensing either by detecting volume changes due to agar swelling/dehydration or modulating the transmitted light by inserting fluids into the air holes. Therefore, the proposed agarose-based structured optical fibre is an easy-to-fabricate, versatile technology with possible applications for medical imaging and in vivo biochemical sensing.

Figure 1

(a) Fabrication of structured agarose fibre: boiled agar solution is poured into the mould, and the fibre is released after solidification in a refrigerator. (b) Detail of the fibre inside the mould with cross-section views of the mould edges and centre, as well as the fibre end face: the glass tube supports the fibre structure whereas air holes are formed by the rods.

Figure 2

(a) Agarose-based structured optical fibre: (b) cross-section view of the end-face and (c) output speckle field of the core-guided modes. The fibre has 60 mm length, diameters of 0.64 mm, 2.5 mm, and 0.5 mm for core, cladding, and holes respectively, and bridges of ~0.08 mm width.

Figure 3

Optical characterization: (a) refractive index of bulk agarose samples; (b) refractive index as a function of sucrose addition in a 2% w/v agarose samples; (c) output power spectra for agarose bulk and fibre samples, the inset shows the output spectrum of a 2% w/v agarose fibre; and (d) fibre optical loss at 633 nm obtained by the cut-back method.

Figure 4

Sensing with the agarose structured fibre: (a) effect of surrounding fluid on the EZNCC. The sample dripping event is indicated by the vertical line. (b) Relative intensity values of the light transmitted by fibre core, cladding, and holes. Solid lines are guides to the eye, whereas the vertical line indicates the RI of 2% w/v agarose.

Figure 5

Experimental setups for fibre sensing: (a) measurement of surrounding fluid droplet by speckle field analysis; (b) assessment of fluids inserted into all the fibre holes based on the average intensity of projected speckle pattern.