From fingertip pulse oximeters providing continuous blood oxygen monitoring to laparoscopic imagers supplying visualization for minimally invasive surgery, small, capable sensors improve medical care. A team of Spanish researchers has now developed an ultrashort lab-in-fiber (LIF) sensor intended to extend that legacy by simultaneously measuring both compressive stress and refractive index.1
The new device is designed to be sheathed in a standard 400-µm-diameter surgical needle where the compressive stress sensor detects contact with the target tissue. Simultaneously, the sensor detects refractive index, which indicates medically relevant characteristics of the local environment. These measurements can be performed with other devices, but compact size and simultaneous measurement offer a unique, rapid, and minimally invasive capability.
Small and capable
The power of lab-on-chip devices has been extended to LIF designs, where functionality is added directly in an optical fiber, with its intrinsic ability to transmit light to interrogate and read out sensor status. Professor Pablo Roldán-Varona of the University of Cantabria (Santander, Spain) and his colleagues looked at previous LIF incarnations and realized their 4- to 5-mm lengths were too long to serve as robust biomedical sensors. They set out to design a much shorter sensor, incorporating a Fabry-Perot interferometer and a Bragg grating in a 300 µm length of a 125 µm optical fiber. The Fabry-Perot cavity is a “bubble” inserted in the fiber, and the Bragg grating is created with a periodically varying refractive index.
The bubble is formed with a femtosecond laser that inscribes an indentation in a fiber end face, which is then spliced to the cleaved face of an identical fiber. The initial indentation and the duration and magnitude of the splicing current determine the bubble size. In use, the bubble is illuminated with a broadband light source, which interrogates the free spectral range (FSR) of the Fabry-Perot cavity created by the two opposed fiber faces. A larger bubble is more sensitive to compression-induced changes in cavity length, but also leads to reduced coupling between the two fiber cores. Simulation indicated that a diameter of about 70 µm represented an acceptable tradeoff. The researchers identified fabrication parameters that allowed them to repeatably create 70-µm bubbles with about 5% variation in diameter.
The fiber grating was also fabricated with femtosecond laser pulses, delivering about 1 µJ to induce a refractive-index variation of about 0.005 at a spatial period of 1.61 µm. To ensure that the grating and cavity measurements remain separable, the grating begins 18 µm beyond the cavity and extends for 200 µm to the fiber end. The end of the fiber is tapered to a diameter of 75 µm, which increases the interaction between the fiber and the surrounding medium while still retaining some measure of robustness. The refractive index of the surrounding medium modifies the effective index of the grating, changing the wavelength of the Bragg reflection.
The final step in the fabrication is to bond the fiber inside the surgical needle, with only the final 200 µm extending beyond the needle’s lip.
Roldán-Varona’s team numerically simulated the electromagnetic field propagation of various designs to determine the optimum tradeoffs, then developed manufacturing methods to produce the desired features. The key final step remained: confirm their predictions.
Dual capabilities
To verify performance, the fiber was coupled to a 1300–1600 nm broadband input source, and the reflection was detected with a spectrum analyzer. The researchers induced various levels of axial strain and immersed the fiber in sugar water of various concentrations. The FSR measurement provides a strain sensitivity of 6.7 pm per 10-6 change in strain. Contact with typical tissues induces strain changes 200–400X larger, making tissue contact easily detectable. Grating reflectance also shifts with strain, but that can be corrected for with the independent measurement of FSR. The reflectance shift with refractive index is 11.5 nm per refractive-index unit over the physiologically relevant range of 1.33 to 1.35 (around the index of water).
Roldán-Varona’s team is evaluating other design approaches, such as placing the cavity downstream from the grating and inserting a through-hole for infusing the surrounding medium. “Our device represents an advance in a clinical procedure,” he says, “since it allows the needle to be inserted automatically into the desired tissue, but the great advantage of the sensor, unlike the vast majority, is that it detects in reflection, so it is minimally invasive.” He also noted that integration with a surgical needle “allows the advantages of optical fiber—small size and immunity to electrical interference—to be leveraged in a medical environment.” For example, the sensor could indicate when the needle enters an organ, such as the liver, and then measure index variations due to abnormal conditions.
REFERENCE
1. P. Roldán-Varona et al., Opt. Lett., 45, 18 (2020); doi:10.1364/ol.399407.
