Ferrofluid-infiltrated optical fibers for shear-sensing smart pads

Shear-sensing probes such as flexible smart pads are indispensable for use in the health sector. Sensing elements are embedded in prosthetic modules, wheelchairs, and rehabilitation beds to monitor acute or repetitive shear stress, which predominantly contributes to the development of pressure ulcers. However, real-time, accurate, high-dynamic-range monitoring of shear stresses occurring in artificial media or human tissues has emerged as a significant technological challenge in several application sectors, including soft robotics,¹ rehabilitation, and smart fabrics and clothes.² Photonic technologies have recently been used in developing sensing smart pads that measure pressure or microshear by employing fiber Bragg gratings³ and vertical-cavity surface-emitting lasers,⁴ respectively. Nevertheless, a flexible and reliable optical fiber technology has yet to be demonstrated that can sense shear stress without interference from pressure cross-talk while exhibiting high-dynamic-range operation.

We have recently presented several photonic devices based on ferrofluid-infiltrated microstructured optical fibers (MOFs), either pristine or inscribed with Bragg gratings.^{5, 6} The magnetofluidic approach applied in those photonic devices allows for easy manipulation of the spectral behavior of the MOF Bragg reflector with external magnetic fields. The magnetic field induces either translation of the ferrofluid along the length of the grating or changes to the magnetoviscosity refractive index in the ferrofluid. We have realized miniaturized ‘in-fiber’ magnetometers,^{7, 8} A/C magnetic field modulators,⁹ and spectral trimmers^{10, 11} based on this magnetofluidic MOF design.

We exploited this hybrid fusion of ferrofluids and MOF Bragg reflectors to develop smart pads that can precisely and reliably measure shear stress displacement (ranging from hundreds of microns to several millimeters in a single sensing element) in rehabilitation, soft robotics, and bionic exoskeleton applications. Simultaneously, the sensing pad shows low sensitivity to vertical pressure loads applied along the top surface. Figure 1 shows the operating principle of the pad.

Figure 1. Shear-sensing concept. The fundamental operation of the hybrid magnetofluidic Bragg grating-based microstructured optical fiber (MOF) when (a) no shear stress is applied (No-Shear) or (b) shear stress is applied (Shear State). FBG: Fiber Bragg grating.

Fabrication proceeds by infiltrating a ferrofluid inside a short segment of a strong, uniform MOF Bragg grating. The ferrofluid segment is much shorter than the length of the grating and acts as a strong phase and loss defect, forming a Fabry-Perot cavity between the two opposite sections of the divided grating.¹¹ The cavity induces a parasitic notch mode that can be measured in the grating reflection spectrum: see Figure 1(a). Previous studies showed that the parasitic notch undergoes significant visibility changes accompanied by a slight wavelength shift when the ferrofluidic defect is translated along the length of the uniform grating. These changes correlate with the spatial translation changes of the ferrofluidic defect induced by an external magnetic stimulus.

To build the sensing pad, we embedded both the small magnetic stimulus and the infiltrated MOF Bragg gratings into a flexible polydimethylsiloxane (PDMS) block (see Figure 1). The ferrofluid is spatially immobilized by the strong magnetic field, and the MOF Bragg grating moves relative to the defect on shear stress. The result is an integrated transducing system that exhibits sensitivity to shear stress changes applied parallel to the MOF Bragg grating axis while remaining almost unaffected by other compressive or strain forces.

Figure 2. A fiber pigtailed, shear-sensing pad designed for operating in 1D mode, using a ferrofluid-infiltrated MOF Bragg grating. The sensing pad, with lateral dimensions of approximately 3.5×2cm, is enclosed within the red-dashed line.

To demonstrate this, we infiltrated a 22mm Bragg reflector inscribed in a grapefruit geometry MOF ‘drawn’ by Acreo using approximately 2mm of commercial ferrofluid and embedded in a 5mm-thick, 3×2cm PDMS pad. We integrated a small (3mm-diameter) spherical magnet into the flexible pad and aligned it with the ferrofluidic defect to construct the actuation system (see Figure 2). We tested the hybrid shear-sensing smart pad using standard micromechanical actuation stages to study its range of operation. The primary quantity monitored was the shear translation between the lower and the upper part of the sensing pad.

Figure 3. (a) Spectral measurements of the Bragg reflected signal under different shear stress displacements. The notch wavelength vicinity is λ₁and the reference plateau is λ₂. (b) Corresponding parasitic notch visibility versus shear displacement and force with respect to the spectral data in (a).

Figure 3(a) and (b) presents, respectively, the spectral changes of the ferrofluidic MOF Bragg grating sensing element and the correlation between shear displacement, force, and parasitic notch visibility. Using the PDMS sensing pad, we measured shear displacements between 500μm and 4.5mm, corresponding to shear forces up to approximately 4N, for a shear modulus G=7.1KPa. Other sensing pads fabricated in our labs can measure microshear displacements from 200μm. These sensing pads can also measure the direction parity of the shear stress applied and provide fully reversible operation without creep effects by tailoring the effective length of the grating and spatial positioning of the ferrofluidic defect in the MOF reflector. We obtained results of similar accuracy while applying a vertical pressure load of 10KPa to the smart pad.

We are working toward further improvement of this hybrid technology to develop an MOF-based smart pad designed to simultaneously measure and identify shear stress, pressure, and temperature, with the ability to tailor the technology to soft robotic and rehabilitation applications. Recently, we developed a 2D shear stress sensing pad that uses two cross-positioned MOF Bragg gratings capable of measuring shear stress, with low cross-talk between the lateral x and y axes, and diagonal shear stresses and their direction and parity. Our future plans include realization of a hypersensing smart pad that implements both biochemical and mechanical sensing capabilities.

This work was partially supported by the European Commission project SP4-Capacities IASIS under contract 232479.