Scientists at École Polytechnique de Montréal have demonstrated what they believe is the first biodegradable microstructured (holey) optical fiber that is capable of multiple optical and/or microfluidic functionalities. They envision many diverse applications for such a fiber. For example, its small inner core might deliver laser power to tissue, while its larger outer core could efficiently collect fluorescence and, simultaneously, its porous structure would allow microfluidic delivery of therapeutical substances.Figure 1. The inner core of the dual-core, coaxial fiber is suspended inside the outer core by grains of polydisperse hydroxypropyl cellulose powder, and the inset is a cross section of the preform (a). The cross section of the fiber itself was slightly asymmetric and varied from one sample to another (b). After propagating through 3 cm of fiber, the power emerging from the fiber is distorted chiefly by an imperfect cutting of the fiber end (c). Reprinted with permission of Optics Letters.The scientists fabricated the fiber’s preform by suspending one cellulose butyrate tube coaxially inside another and filling the space between them with a polydisperse hydroxypropyl cellulose powder for mechanical support (Figure 1a). The tubes formed the inner and outer cores of the fiber. The powder, whose melting temperature was significantly higher than that of the tubes, remained in powder form as the researchers heated the preform and drew it down to a 450-μm-diameter fiber (Figures 1b and c). The powder-filled region formed a porous inner cladding whose refractive index was considerably smaller than that of the core.In practice, the inner core could be solid if the core’s function was only to guide light, or it could be left hollow for microfluidic drug delivery. Additionally, as hydroxypropyl cellulose powder is dissolvable in aqueous solutions, drugs might be embedded in the porous inner cladding, where they would enter the surrounding tissue as the cladding itself dissolved.By cutting back the fiber to ever-shorter lengths and measuring the transmission after each cut, the scientists estimated the fiber’s transmission loss at 630 nm to be between 1 and 2 dB/m, depending on the sample. Figure 2. The experimental apparatus immersed one end of the fiber in deionized water while keeping the other end dry. The loss of bulk cellulose butyrate, about 0.4 dB/cm, accounts for nearly half the fiber’s (lowest) measured loss. The bulk material’s transparency window, defined as less than 10 dB/m loss, extends from 700 nm up to 1100 nm, a range in which several medical lasers operate. To determine the fiber’s behavior in vivo, the scientists set up a test apparatus to immerse one end of the fiber in deionized water (Figure 2). They observed that the transmission gradually increased over the course of a day and, eventually, leveled off (Figure 3). By examining the fiber after it had been under water more than 24 h, they confirmed that the powder grains in the inner-cladding region had dissolved.Figure 3. Transmission through the fiber gradually increased over a 24-h period and then leveled off.The investigators hypothesize that the increase in transmission results from water (n = 1.33) replacing the irregular mixture of powder grains (n = 1.337) and air (n = 1). The uniform refractive index of water in the cladding region reduced side scattering of light.Optics Letters, Jan.15, 2007, pp. 109-111.