2.1.

Corolling Technique, Multilayer Fibers

We first describe a corolling technique²⁴ to fabricate multilayered all-polymer preforms containing several distinct materials in their cross section [Fig. 1a]. Thus, two or more polymer films having the desired thickness ratios are placed on top of each other and then jointly rolled around a mandrel. The rolled films are subsequently consolidated together by heating above the melting temperature in a vacuum oven. After cooling down, the mandrel is removed from the preform. Generally, the multilayer formed by the polymer films can be repeated after each turn around the mandrel, forming a periodic multilayer stack with any desired number of layers. During drawing, the central hole can be either collapsed or left open to result in the solid or hollow core multilayer fibers, known as Bragg fibers. Practical applications of Bragg fibers include light guidance in the hollow, solid, or liquid-filled cores for sensing and power delivery.^{23, 24, 25}

The principal advantage of the corolling method is the simplicity with which a multimaterial periodic structure can be created, while the principal disadvantage of the method is the difficulty in managing contamination and nonuniformities due to the mechanical nature of the rolling process.²⁵ Moreover, in order to avoid delamination of the layers during drawing, it is desirable, although not critical, that the materials have similar melting temperatures. Another practical disadvantage of a corolling method is its reliance on the availability of high-quality polymer films. Thus, whereas most conventional polymers are commercially available in the form of films, for most biomaterials, such films are difficult to find.

A two-layer fiber preform was ultimately prepared by corolling the higher refractive index cellulose acetate (CA) film with the lower index poly(L-lactic acid) (PLLA) film around a $2\text{-}\mathrm{cm}$ -diam Teflon rod. For preform fabrication, we used commercial $127\text{-}\mu \mathrm{m}$ -thick CA films purchased from McMaster-CARR. However we had to cast our own PLLA films. Several methods were tested to get thin films of PLLA. We first attempted to extrude the polymer but were only able to get thick films on the order of $1\mathrm{mm}$ . Therefore, we had to resort to solution casting of a $100\text{-}\mu \mathrm{m}$ film. The solution of PLLA was prepared by dissolving polymer pellets in a methylene chloride. PLLA pellets were purchased from NatureWorks. A solution was cast onto an $8\times 13\mathrm{in.}$ glass sheet, with a Teflon’s seal around the edge to prevent leakage. The thickness of the polymer film was controlled by the concentration of the solution. The film was dried at $40°\mathrm{C}$ in a nitrogen atmosphere until it could be removed from the glass. The freestanding transparent PLLA film was subsequently dried in a vacuum oven at $120°\mathrm{C}$ for three days.

We then proceeded with fabrication of a single bilayer fiber presented in Fig. 1a. The rolled preform was mounted in the drawing tower and preheated at $150°\mathrm{C}$ for $1\mathrm{h}$ . Thereafter, the temperature was ramped over $1\mathrm{h}$ to the drawing temperature of $225°\mathrm{C}$ . Care should be taken not to overheat the preform because, at higher temperatures, PLLA tends to degrade with apparition of a brown color. The fiber was drawn at $225°\mathrm{C}$ to a diameter of $\sim 860\mu \mathrm{m}$ . During drawing, the bilayer structure has a tendency to collapse [see Fig. 1a] apparently due to the high surface tension of the polymers in a melted state.

Figure 2 reports measured transmission spectra of the studied fibers for the light launched either in the fiber core, cladding, or air hole. In the insets, the photos of the intensity distribution in the fiber cross sections are shown for different launch conditions. In all the optical characterization measurements, the fibers were cleaved with a hot razor blade cutting at a constant speed and temperature. A supercontinuum white-light source was coupled into the fibers using a microscope objective. The output was sent into an Oriel Cornerstone monochromator, and the power at each wavelength was measured using a Si detector and a Newport powermeter. The resulting transmission spectra were normalized with respect to the spectrum of the supercontinuum in the absence of a fiber and the length of each fiber segment. The resulting total loss spectra, including the coupling loss, were plotted as a function of wavelength in Fig. 2. Ideally, the fiber losses would be measured by the standard cutback technique; however, the short length of the heavily multimode fibers makes this technique impractical.

Fig. 2

Transmission spectra of the biodegradable optical fibers with images of the transmitted light as insets. The fiber geometries are identified in Fig. 1. The lengths of the measured segments are 3.2, 6.95, 3.35, 3.25, and $8.05\mathrm{cm}$ respectively.

In the inset of Fig. 2a, we can see that the collapse of the bilayer structure has created a higher refractive index CA core region surrounded by the lower index PLLA cladding. Light guides much more efficiently in the CA than in the PLLA. From the normalized transmission data, and from the fiber length of $3.2\mathrm{cm}$ , we estimate the total loss in the CA region of the PLLA/CA fiber to be $9.8\mathrm{dB}∕\mathrm{cm}$ at wavelength $633\mathrm{nm}$ . These losses are attributable mostly to the structural inhomogenities of the PLLA layer in the drawn fiber resulting from the low quality of a solution cast homemade PLLA film.

2.2.

Powder Filling Technique, Single Core Fibers

We now discuss fabrication of the single-core step-index fibers [Fig. 1c] by filling a lower refractive index polymer tube [Fig. 1b] with a higher refractive index polymer powder. Cellulose butyrate (CB) was chosen as a tube material mainly because of its commercial availability. As a filling material for the interstitial region, higher refractive index poly( $\epsilon$ -caprolactone) (PCL) was chosen. Practical applications of a single-core step-index biodegradable fiber include disposable fibers for in vivo light delivery, or a controlled drug delivery, where fiber core is made of a water-soluble material doped with pharmaceuticals.

For the following experiments PCL powder was purchased from Sigma-Aldrich, while CB tubing was purchased from McMaster-CARR. We began by drawing single CB tubes [Fig. 1b] into capillaries to find the appropriate range of drawing parameters. Particularly, the CB capillaries of $393\mu \mathrm{m}$ diam at a temperature of $178°\mathrm{C}$ , and a drawing speed of $1\mathrm{m}∕\min$ were fabricated. Interestingly, over the distances of several centimeters, such capillaries can guide light in their hollow cores as seen in Fig. 2b with an estimated loss of $1\mathrm{dB}∕\mathrm{cm}$ . CB appears to be the most transparent of the biodegradable polymers indicated in Table 1. Further measurement of the light guidance in a CB cladding of a $6.95\text{-}\mathrm{cm}$ -long capillary lead to a $2.2\text{-}\mathrm{dB}∕\mathrm{cm}$ fiber-loss estimate at wavelength $633\mathrm{nm}$ .

To fabricate a step index fiber preform [Fig. 1c], a CB tube having an inner and outer diameter of $3∕8$ and $5∕8\mathrm{in.}$ was tightly packed with a higher refractive index PCL powder. The preform was solidified in a vacuum oven at $110°\mathrm{C}$ . It was then preheated in the draw tower at $130°\mathrm{C}$ for $3\mathrm{h}$ and drawn at $176°\mathrm{C}$ into $420\text{-}\mu \mathrm{m}$ -diam fiber.

Using this fabrication approach, it was difficult to prevent the formation of trapped air bubbles in the preform. An air-hole defect, which spans the entire length of the fiber segment, can clearly be seen in the resulting fiber [Fig. 1c]. Figure 2c shows the transmission spectrum of the step-index fiber. Whereas the air defect can guide some light, the PCL regions appear to be optically opaque and true guidance can only be observed by coupling into the CB regions. We estimate the loss in the outer CB cladding of this fiber of length $3.35\mathrm{cm}$ to be $6.7\mathrm{dB}∕\mathrm{cm}$ at wavelength $633\mathrm{nm}$ . We ascribe nontransparency of a PCL fiber core to the high degree of crystallinity in this material after preform consolidation, which can be also confirmed as a milky color of the resulting fibers. Better processing conditions of a PLC polymer to suppress its crystallization during preform fabrication are needed to perfect this approach.

2.3.

Powder Filling Technique, Multiple Core Fibers

We now discuss fabrication of the multiple-core fibers (Figs. 1d and 1e). Such fibers can be fabricated by inserting a small-diameter tube within a larger diameter tube, and by filling the interstitial region between the tubes with an optically different material. The central hole of the inner tube can alternatively be filled, be left as an air hole, or be collapsed during the drawing process. As filling materials for the interstitial regions, higher refractive index PCL and lower refractive index hydroxypropyl cellulose (HPC) were used. Potential application of double-core fibers include enhanced in vivo sensing, where high intensity light is launched through a smaller core, while reflected light is collected efficiently through a larger core. Incorporation of the hollow or water-soluble microstructure into such fibers can also enable environment sampling and drug-delivery functionalities.

First multiple-core step-index fiber was created by inserting a small CB tube into a larger CB tube and by filling the interstitial region and the inner tube hole with higher refractive index PCL powder, thus forming two concentric PCL fiber cores. The smaller CB tube had inner and outer diameters of $1∕8$ and $1∕4\mathrm{in.}$ , whereas the larger tube had inner and outer diameters of $3∕8$ and $5∕8\mathrm{in.}$ , respectively. The PCL powder was tightly compacted. and care was taken to keep the inner tube centered. The preform was solidified in a vacuum oven at $80°\mathrm{C}$ . The consolidated preform was then drawn at $178°\mathrm{C}$ into a $410\text{-}\mu \mathrm{m}$ -diam fiber. Again, air-hole defects are clearly seen in the PCL regions of the resulting fiber [see Fig. 1d], and we were unable to observe any light transmitted through the highly crystalline PCL core [see Fig. 2d]. The loss in the outer CB cladding of this multiple-core fiber of length $3.25\mathrm{cm}$ was estimated to be $8.3\mathrm{dB}∕\mathrm{cm}$ at wavelength $633\mathrm{nm}$ .

For the second design of a two-core fiber, we filled the space between the CB tubes with a lower refractive index material, leaving the central CB tube empty. In this case, these are the CB tubes that form the two higher refractive index cores. For the lower index material, we chose HPC, which is a water-soluble derivative of cellulose commonly used in ophthalmology. HPC powder was purchased from Sigma-Aldrich. Interestingly, high-molecular-weight HPC has a higher glass transition temperature than CB, which opens the door to the innovative fiber designs. For example, by filling the space between the CB tubes with an HPC powder, and then by drawing the thus prepared preform at temperatures below the softening temperature of an HPC powder, one can create a fiber where the inner CB core is suspended inside of the outer CB core by a porous water-soluble network of HPC particles as shown in Fig. 1e.

The preform was made by filling the interstitial region between the two CB tubes with a polydispersed HPC powder of molecular weight 80,000. Note that the preform was not consolidated prior to drawing. The preform was preheated in the drawing tower furnace for $4\mathrm{h}$ starting at $140°\mathrm{C}$ with a $10°\mathrm{C}∕\mathrm{h}$ increase until drawing temperature of $180°\mathrm{C}$ was reached. Fibers of $415\text{-}\mu \mathrm{m}$ -diam were drawn at $180°\mathrm{C}$ . The fiber $\sim 1–4\mathrm{dB}∕\mathrm{cm}$ transmission loss is mostly attributable to the HPC particles forming a highly porous network of scatterers lining the inner fiber core. Transmission through such fibers suspended in the aqueous environment was detailed in Ref. 11, where the fiber potential for monitored drug release applications was demonstrated.

2.4.

Solution Casting Technique

Alternatively, to realize the multiple-core fiber preforms discussed in the previous section, HPC can be dissolved in water, then filled between the CB tubes, and finally, reconstituted in a solid form by water evaporation in the vacuum oven. This is a solution casting method. Figure 2e shows thus fabricated fibers and the corresponding transmission spectra. We note that losses through the inner and outer cores are similar, with an estimated loss of $3.1\mathrm{dB}∕\mathrm{cm}$ at wavelength $633\mathrm{nm}$ as measured using an $8.05\text{-}\mathrm{cm}$ -long fiber.