Flexible miniature shear sensors for prosthetics

The medical community is increasingly demanding sensors that can be used on a patient without disturbing normal body functions. These so-called unobtrusive sensors are made from very flexible or ideally stretchable materials so that their presence is not noticed. More specifically, there is a need for tactile sensors that can measure pressure and friction (‘shear’) stresses. In limb prostheses, for example, excessive interfacial stresses (shear stress being the most critical) between the stump and the socket can cause serious injuries and limit a patient's mobility.¹ Active or ‘smart’ prostheses could significantly improve the comfort of the user by incorporating a control system that detects the locations where high stresses exist and subsequently relieves them using an actuator.

A number of the sensors available today are based on electrical sensing principles, that is, they use changing capacitance or resistance for sensing. In addition, they lack mechanical flexibility, protrude out of the prosthetic device, or are simply too large for this application. Electrical sensors pick up electromagnetic interference caused by the surroundings but also by the human body. Consequently, we are working on optical tactile sensors that can be embedded in very thin foils to obtain the flexibility required for a prosthetic socket.^2,3

Our mechanically flexible shear sensor relies on the changing coupling between a light source and a detector (see Figure 1). When the source is being displaced laterally, more or less light is captured by the detector. This principle is used for shear sensing by employing a deformable transducer layer that converts mechanical stresses into lateral displacements. To achieve a highly accurate miniature sensor, we used a vertical-cavity surface-emitting laser (VCSEL) chip as the light source and a photodiode chip as the detector.

Figure 1. Principle of the shear sensor. The changing coupling of a light source and detector depends on the lateral displacement of both components. PDMS: Polydimethylsiloxane. VCSEL: Vertical-cavity surface-emitting laser.

The device consists of two thin, flexible substrates (one substrate for the VCSEL and the other for the photodiode) with a deformable sensing layer in between (see Figure 1). To achieve this optoelectronic packaging foil, bare die chips were thinned down to 20μm and subsequently embedded in polymer layers, yielding a total foil thickness of only 40μm.⁴ On top of both substrates, we spin-coated a 90μm-thick layer of silicone (Sylgard^® 184, Dow Corning).⁵ After thermally curing this layer, we treated it with an air plasma to activate the surface. Finally, we aligned the substrates, and the two silicone layers were brought into contact to create a permanent bond. The resulting stack with the silicone transducer layer is shown schematically in Figure 1. Figure 2 shows a fabricated prototype.

Figure 2. Mechanical flexibility of the fabricated prototype. A 1×4VCSEL array was used on one substrate and a 1×4photodiode array on the other substrate. Two separate wires are used for each component in the array. More specifically, we employed bare die chips with total dimensions of 1mm ×350μm for the array with four VCSELs and chips with 1mm × 450μm for the one with four photodiodes (both on a 250μm pitch).

We also constructed an analytical simulation model based on the characteristics of the specific VCSEL source. For this purpose, the VCSEL beam was considered to be ideal, that is, the transverse electric field and intensity were taken to be Gaussian functions. In addition, the detector was assumed uniform: in the model, the same output signal is generated regardless of the location of the light spot on the detector's surface. The most important simulation in terms of the sensor design shows the influence of the vertical VCSEL-to-photodiode distance (the only parameter that can easily be changed) on the sensor response: see Figure 3. Based on the results obtained with this model, this distance was chosen to be 200μm. Since there is a 10μm polymer layer on top of both the VCSEL and the photodiode (for protection), the corresponding thickness of the transducer layer is 180μm. Note that Figure 3 only shows the response to lateral displacement. For the feedback to the actual shear stress the mechanical properties of the transducer layer had to be considered. This means that the sensitivity with respect to shear stress is highly tunable by selecting the appropriate type of transducer material. For the current prototype, we used a silicone layer with a shear modulus of about 400kPa.

Figure 3. Simulated influence of the vertical distance between VCSEL and photodiode on the sensor response.

Finally, we verified the simulated response on the fabricated prototype (see Figure 2), which was mounted on a rigid substrate to facilitate the measurement. We applied a lateral displacement on top of the sensor and measured its response (i.e., the photodiode current) and the resulting shear force (N). The former was compared with the analytically simulated values (see Figure 4). In the linear part of the range between 2 and 5.5N, the sensitivity was −350μA/N. Depending on the application, this can be tuned by selecting a transducer material with the desired shear modulus.

Figure 4. Comparison of the measured and simulated sensor response (photodiode current) with respect to the applied shear force.

In summary, we fabricated an optical shear sensor using thin and flexible polymer-embedding foils, making it suitable for curved or moving surfaces such as a limb prosthesis or the human body. The sensing principle is based on the changing coupling between a VCSEL and a photodiode, separated by a deformable transducer layer. The sensor can be used for several applications requiring different sensitivity and working range by selecting the appropriate type of transducer material. In the future, we will work on placing several shear sensors into a raster (grid) to obtain sensing points distributed over a larger area. In particular, this will require developing a system that combines the information of each of these sensors.

This work is partially conducted within the framework of the FAOS (Flexible Artificial Optical Skin, funded by the Institute for the Promotion of Innovation by Science and Technology, Belgium) and PHOSFOS (Photonic Skins for Optical Sensing, funded within the European Commission's Seventh Framework Programme) projects. Jeroen Missinne is supported by a PhD fellowship from the Research Foundation of Flanders. The author also acknowledges the support of his colleagues Bram Van Hoe, Erwin Bosman, Sandeep Kalathimekkad, Geert Van Steenberge, Peter Van Daele, and Jan Vanfleteren.