Researchers create biosensor by turning spider silk into optical fiber

"Glucose sensors are crucial to people with diabetes, but these devices tend to be invasive, uncomfortable and not cost-efficient," said research team leader Cheng-Yang Liu from National Yang Ming Chiao Tung University in Taiwan. "With spider silk attracting attention for its superior optomechanical properties, we wanted to explore using this biocompatible material to optically detect various sugar concentrations in real-time."

Liu and colleagues from Taiwan Instrument Research Institute and Taipei Medical University describe their new sensor in the Optica Publishing Group journal Biomedical Optics Express. They show that it can be used to determine concentrations of fructose, sucrose and glucose sugars based on changes in a solution's refractive index. Spider silk is ideal for this application because it can not only transmit light like an optical fiber but is also very strong and elastic.

"Our new spider silk-based fiber optic sugar sensor is practical, compact, biocompatible, cost-effective and highly sensitive," said Liu. "With further development, it could lead to better at-home medical monitoring devices and point-of-care diagnostic and testing devices."

From silk to sensor

To make the sensor, the researchers harvested dragline spider silk from the giant wood spider Nephila pilipes, which is native to Taiwan. They enveloped the silk, which is just 10 microns in diameter, with a biocompatible photocurable resin and cured it to form a smooth protective surface. This created an optical fiber structure that was 100 microns in diameter, with the spider silk acting as the core and the resin as the cladding. They then added a biocompatible nano-layer of gold to enhance the fiber's sensing abilities.

This process formed a thread-like structure with two ends. To use the fiber to take measurements, the researchers immersed one end in a liquid sample and connected the other end to a light source and a spectrometer. This allowed the researchers to detect the refractive index of the solution and use it to determine the type of sugar and its concentration.

"The spider silk-based sugar sensor is reusable, cost-effective, easy to use and offers real-time detection," said Liu. "Moreover, because it is compact it could allow access to hard-to-reach areas such as the brain and heart. With further development, it is also hoped that this silk-based fiber optic sugar sensor could be used in implantable medical devices and treatment strategies in biomedical applications."

Consistent, accurate readings

To test the repeatability and stability of the sensor over time, the researchers used it to measure solutions with unknown concentrations of fructose, sucrose or glucose sugars at room temperature. The measurements were each repeated 10 times at 5-minute intervals.

To quantitatively determine the performance of the silk-based fiber optic sensor, the researchers compared the light intensity spectra produced by the sensor with refractive index measurements acquired with a commercial refractometer. The sensor was able to both identify the type of sugar in the solution and provide a readout of the concentration.

"The measurement precision and sensing sensitivity we achieved suggests that the sensor can accurately estimate the concentration of an unknown sugar solution," said Liu. "Moreover, the sensing sensitivity for our proposed sensor completely encompasses the range of sugar concentrations found in human blood."

Before the sensor can be used for real-time measurements in a clinic or home-use device it will be necessary to improve its accuracy and enhance its stability under environmental changes so that it can be used for longer periods of time.

The researchers are also working on software that would allow the sensor to be used with mobile devices for point-of-care readings. They also want to extend the sensor's functionality so that it could be used to measure different biochemical components in human blood such as lactose and fat.