1. Introduction

Biocompatible photonic components are of great interest for biomedical applications as they bridge the gap between optical and biological worlds. A number of materials can be listed as biocompatible photonic material in terms of biodegradability and transparency including synthetic polymers and natural polymers. Many of them have been widely studied and are very popular in various applications, such as polylactic acid (PLA) [1], collagen [2], cellulose [3], chitosan [4] and their blends [5]. However, many of such materials suffers from some problems such as toxic fabrication process, poor mechanic strength and low transparency.

Among various biocompatible photonic materials, silk fibroin is a distinct one. It is a natural polymer and has been used in textiles for centuries. Owing to its excellent biocompatibility, silk fibroin has been employed in pharmaceuticals and as biomedical sutures for decades. Silk fibroin is highly transparent in visible and near infrared regions with a refractive index of about 1.54 and characterized by high strength of about 3 GPa which is mechanically stable up to 200 °C [6]. Moreover, aqueous silk fibroin solution can be obtained by an environmentally benign fabrication process. With aqueous solution, bioactive elements can be incorporated into silk fibroin biophotonic components regenerated from the aqueous solution. Therefore silk fibroin have raised many attentions these years for biophotonic applications [7].Currently, some bioactive sensors based on regenerated silk fibroin films have been demonstrated [8,9].

Optical fiber provides a convenient way to manipulate light with extremely low loss. Combined with biomaterials, miniature fiber-optic biosensors can be fabricated. Among various schemes, diaphragm-based fiber-tip Fabry-Perot sensors are very popular for its simplicity. Pressure sensor is one typical application of such type of sensors, which is useful in medical and many other areas. Many materials have been employed as the diaphragm such as metal, graphene and polydimethylsiloxane (PDMS) although normally complicated micro-electro-mechanical (MEMS) techniques have to be employed for fabrication [10–13]. For biomedical applications, biocompatible material is more favored. Therefore, chitosan diaphragm-based Fabry-Perot acoustic sensor has also been proposed [14]. However, the porous feature of chitosan makes the sensor not ideal for pressure sensing.

In this paper, we use regenerated silk fibroin film as a new diaphragm material and propose a new biocompatible fiber-tip Fabry-Perot pressure sensor. The excellent membrane-forming ability and good adhesion of silk fibroin makes the sensor fabrication very easy and the resulted sensor shows good airtightness with the help of surface processing technique. The proposed sensor shows a response of 12.3 nm/kPa in terms of cavity length change.

2. Preparation of aqueous silk fibroin solution

The aqueous solution of silk fibroin is obtained by dissolving B. mori silkworm cocoons in a process shown in Fig. 1. Silk fibers of silkworm cocoons are in a core-cladding structure with silk fibroin as the core covered by a cladding of sericin which is a kind of glue to hold silk fibers together. Therefore, silk fibers have to be degummed at first to remove sericin and leave silk fibroin for further processing. For our experiments, the silk fibers were boiled in 0.5% (w/w) Na2CO3 solution for 30 minutes to degum followed by rinsing twice in water over 60 °C for three times. The weight ratio between the silk fibers and the Na2CO3 solvent is 1:20. The degummed silk fibers were then extensively washed twice with ultrapure distilled water and dried at 125 °C for four hours to obtain pure silk fibroin fibers. The extracted fibroin was dissolved in LiBr solution (LiBr: C2H5OH:H2O = 44:45:11(w/w)) at 75 °C with stirring for four hours. The solution was then dialyzed using dialysis cassettes (MWCO 3500) against flowing water for two days followed by being dialyzed against ultrapure distilled water for one day to remove the salt. The resulted solution from this process was an aqueous solution of silk fibroin with concentration of approximately 7 wt%. The solution was then ready for use and stored at 4 °C before use. No toxic or hazardous material is used in the process and hence the entire process is environmentally benign.

 

Fig. 1 The preparation of aqueous silk fibroin solution. (a) Silk fibers of silkworm cocoons. (b) Degumming. (c) Dialysis of dissolved silk fibroin. (d) The resulted aqueous silk fibroin solution.

Download Full Size | PDF

3. Fabrication of silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor

The proposed fiber-tip Fabry-Perot pressure sensor is based on a commonly used ferrule with an inner diameter of 127 μm for standard single mode fibers. As shown in Fig. 2(c), the ferrule is sealed by a thin silk fibroin film at one end. A cleaved standard single mode fiber is inserted from the other end of the ferrule and sealed and fixed by epoxy, leaving a gap of a few tens of micrometers between the cleaved end of the fiber and the silk fibroin film to form a Fabry-Perot cavity.

 

Fig. 2 (a) Attaching a silk fibroin film to one end of the ferrule by dipping the ferrule in the silk fibroin solution. (b) The surface processing of the ferrule using APTES. (c) The schematic of the fabricated silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor. (d) The image of the ferrule end-face covering a silk fibroin film.

Download Full Size | PDF

Silk fibroin shows excellent membrane-forming ability. As shown in Fig. (a), a silk fibroin film can be formed simply by dipping the ferrule in the silk fibroin solution. However, to make the ferrule be sealed tightly, before a silk fibroin film can be attached to one end of the ferrule, the surface of the ferrule should be properly treated. The ferrule was thoroughly cleaned at first and then one end of it was covered by a monolayer of aminopropyl groups through interaction with γ-aminopropyltriethoxysilane (APTES). To prevent APTES from blocking the hollow core of the ferrule, the ferrule was filled by a single mode fiber before the ferrule had touched APTES and then the single mode fiber was drawn out from the end touching APTES. As shown in Fig. 2(b), after modified by APTES, the ferrule was linked with silk fibroin via the reaction between –NH2 at its end and –COOH on the chains of the latter [15, 16] by dipping the ferrule in the aqueous silk fibroin solution. The silk fibroin solution on the ferrule was then dried in air and formed a membrane covering the ferrule and tightly sealing the hollow core. Figure 2(d) shows the microscopy image of the ferrule surface covering the silk fibroin film. The resulted membrane is basically quite smooth over the surface of the ferrule except some imperfection at the lower left corner which we believe to be due to the flow of the silk fibroin solution during the desiccation process. At the position of the hollow core of the ferrule, an interference pattern is observed, showing that the inner and outer surface of the silk fibroin membrane are very smooth. Therefore the obtained aqueous silk fibroin solution demonstrates excellent membrane-forming ability even on a surface with a hole. A single mode fiber with one end cleaved was then inserted into the hollow core of the ferrule. Epoxy was used to fix the fiber in place, which also made the whole assembly airtight and finally form a silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure senor as shown in Fig. 2(c).

4. Measurements and results

The experiment setup is shown in Fig. 3 with the inset showing the picture of the fabricated silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor. A superluminescent light-emitting diode (SLED) with output wavelength from 1200 - 1700 nm was used as the broadband light source to characterize the silk fibroin diaphragm-based Fabry-Perot cavity. The sensor was place in a pressure chamber and a reflective system was employed for measurement. Under the illumination of the SLED, the reflection spectrum of the cavity was recorded and analyzed by an optical spectrum analyzer (OSA). Figure 4 shows a typical reflection spectrum without pressure applied. A spectrum with highest contrast reaching 15 dB was observed over a wavelength range of 400 nm, suggesting a very close reflectivity for both silica-air and air-silk fibroin interface. The excellent optical properties of silk fibroin in visible region has long been known. Our measurements show that in near infrared region, its optical properties are also excellent. The Fourier transform of the curve shown in Fig. 4 directly gives the reflections at different places. Figure 5 shows the Fourier transform of measured curves under various pressures. Note that the curve in Fig. 4 was resampled at an equal frequency spacing as required by Fourier transform to obtain Fig. 5. The values shown in Fig. 5 are normalized according to the value at the origin. Besides the peak at origin, Fig. 5 shows a peak at around 80 μm which gives the cavity length directly as a strong reflection is identified there. A cavity length change under various pressures is also clearly observed from Fig. 5. Another small peak at around 100 μm is also presented, suggesting a fairly small reflection from the outer surface of the silk fibroin film. This gives the thickness of the film, that is, about 20 μm. One should note that Fig. 5 shows only one sideband of the Fourier transform. Due to the symmetry in Fourier transform for real signals, the actual relative intensity shown in Fig. 5 should be double except the one at the origin. Therefore, the maximum relative reflection at the inner surface of the silk fibroin film approaches 0.9. With greater pressure applied, the reflection degrades, which is reasonable as the deformation of the film results in more reflection loss. The relative reflection at the outer surface of the silk fibroin film is much smaller than that of the inner side, which may be attributed to the uneven surface at the outer side, leading to more reflection loss.

 

Fig. 3 Experiment setup of the pressure measurement by a silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor. The inset shows the picture of the fabricated sensor.

Download Full Size | PDF

 

Fig. 4 The measured reflection spectrum silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor without pressure applied.

Download Full Size | PDF

 

Fig. 5 Fourier transform of measured reflection spectrums for various applied pressures.

Download Full Size | PDF

Cavity length changes at various applied pressures are plotted in Fig. 6 which shows a quite good linearity with a response of 12.3 nm/kPa. The value is fairly low because the silk fibroin film in our experiments is actually quite thick. The mechanic strength of regenerated silk fibroin, about 3 GPa as reported in literatures, is quite high in polymers. With a thickness of about 20 μm, the response of the proposed sensor is not very high but it can measure a larger range of pressure. Moreover, the thickness makes the two reflections from the inner and outer side of the film distinguishable, which demonstrates the transparency of the film in the measured near infrared region and establishes the connection between the film thickness and the solution concentration. For higher response, the thickness should be lowered, which can be realized by diluting the concentration of the aqueous silk fibroin solution. The airtightness of the sensor has also been tested. A pressure of 45 kPa was applied and the results of the cavity length change as a function of time are shown in Fig. 7. The initial cavity length was about 550 nm as soon as the pressure was applied and then gradually increased about 100 nm in the first 10 minutes. After the first 10 minutes, the cavity length change became very stable and fixed at about 670 nm for the rest of our measurements. The sensor then demonstrates a quite good airtightness which confirms the good adhesion of silk fibroin and the effectiveness of surface processing using APTES.

 

Fig. 6 Measured cavity length changes at various applied pressures. The solid line shows a linear fitting.

Download Full Size | PDF

 

Fig. 7 Cavity length change versus time for an initially applied pressure of 45 kPa. The solid line shows an exponential fitting.

Download Full Size | PDF

5. Conclusion

A new diaphragm-based fiber-tip Fabry-Perot pressure sensor is demonstrated by attaching a silk fibroin film onto a ferrule. The silk fibroin film was regenerated from aqueous silk fibroin solution which was prepared in a nontoxic and environmentally benign process. The silk fibroin shows good optical and mechanic properties in reflectivity, transparency and strength. With good adhesion and the help of proper surface processing by APTES, the airtightness of the sensor is guaranteed. With a film thickness of 20 μm, the sensor demonstrates a response of 12.3 nm/kPa in terms of cavity length change which can be enhanced by diluting the aqueous silk fibroin solution to obtain a thinner film. The excellent biocompatibility of silk fibroin makes the senor a promising biocompatible photonic component.