Examples of 3D printed sensors

There have been some developments on the 3D printing field, and the sensors field is no excep-tion. In the later years, some research papers have been published, showcasing 3D printed capacitive sensors, as well as, resistive sensors, showing big promises to the sensors future although using different 3D printing techniques. Therefore, the following subchapters will report some research examples that have been developed regarding the 3D printed force and strain sensors, along with an example of 3D printed EMG sensor.

To the author’s knowledge, there were no developments regarding the contact resistance modu-lation sensors and the piezoelectric and triboelectric sensors 3D printed.

1.5.1 3D printed capacitive sensors

In the work of M. Schouten, R. Sanders, and G. Krijnen et al., a flexible capacitor was designed to be 3D printed with the FDM printing technique, combining regular and conductive thermoplastic poly-urethane (TPU) materials to produce a force sensor, Figure 1.6 example a. With the chosen dimen-sions for the electrodes and dielectric, it is possible to do a parallel plate approximation, which allows the application of the Equation 2.2. With this approximation, it is possible to establish a relationship between the variance of the measured capacitance and the applied force [47].

S. J. Leigh et al. were also able to produce a capacitive sensing device for a customized 3D printed Human-Interface-Devices (HIDs) to be printed with FDM printing techniques, Figure 1.6 exam-ple b. The primary purpose of their work was to develop a conductive thermoplastic composite, which they labelled as ‘carbomorph’ and demonstrate how it can be applied, by a low-cost 3D printer (FDM technique), to print electronic sensors able to sense mechanical flexing and capacitive changes [48].

Although these two studies showed excellent results and applicability, the presented sensors are only able to measure pressure variations. Ideally, a sensor that could measure pressure and strain simul-taneously would be more complete and better to quantify the interaction forces between the user and exoskeleton.

With this idea in mind, Laszczak et al. developed a capacitance-based sensor to be used as an interface between a stump-socket and a lower-limb amputation with the goal of measuring the pressure and shear stress simultaneously, Figure 1.6 example c. The sensor was produced with a hybrid approach, where only the dielectric is 3D printed, the electrodes were made out of copper [53].

The sensor was constituted by three separated electrodes on the top, Ex, Ey and Ez and one single electrode on the bottom that was common to the other three, Ecom. To separate the top and bottom elec-trodes a 3x3 array filleted pillars was designed and 3D printed. With this arrangement, three capacitors are formed. The electrode Ez is completely overlapped by the Ecom, in opposition to Ex and Ey that are only partially overlapped. If the force applied in the electrodes is perpendicular to the sensor, there is only a shift in the distance between plates, d, so every capacitor will register the same variation of capacitance (Equation 2.2), however, if the force is not perpendicular, the overlapping area, A, between Ex or Ey with Ecom will change, so every capacitor will register different changes in capacitance, with the three different values is possible to know the direction and the magnitude of the applied force (Equa-tion 2.2).

Although it is a promising sensor design, only the dielectric was 3D printed, so maybe it is possible to apply the M. Schouten et al. and S. J. Leigh et al. printing technique, a multi-material printing technique using FDM technique, to their sensor design.

1.5.2 3D printed resistive sensors

When S. J. Leigh et al. developed the conductive thermoplastic composite, the ‘carbomorph’, they also test it for mechanical flexing. By using the piezoresistive nature of the ‘carbomorph’, they were able to measure the flexion of the fingers. To do it, they embedded the sensors, made from the conductive thermoplastic material, into an ‘exo-glove’ interface, above the fingers’ articulations, and register the resistance variations measured in the sensors, Figure 1.7 example a. All the ‘exo-glove’

structure was 3D printed, using a multi-material printing technique [48]. A. Dijkshoorn et al. also referred 3D printed sensors with the same base of principle and construction.

Figure 1.6: Examples of 3D printed capacitive sensors (a) M. Schouten et al.’s sensor [47]; (b) S. J. Leigh et al.’s sensor [48]; (c) P. Laszczak et al.’s sensor [53]

(a) (b) (c)

Using a polyject technique, B. Shih et al. designed two different sensors, a uniaxial strain sensor and a multilayer integrated strain and pressure sensor, Figure 1.7 example b. In contrast to the previous example, in this study, the resistivity was considered constant, limiting the output variations to be only due to the changes in the sensor’s geometry. However, it wasn’t completely successful since there were some changes in the resistivity, leading to the important to conclusion: when the material is conductive because it has carbon particles incorporated, the variations in resistivity can’t be negligible [54].

A more complete sensor was developed by S. Harada et al., a three-axis tactile force sensor, Figure 1.7 example c. In this sensor, an arrange of electrodes were specially designed to incorporate four strain sensors, two temperature sensors and a fingerprint structure, per cell. The fingerprint was applied in the top of a membrane and the four strain sensors were uniformly placed around it, allowing the determination of the touch and slip/friction force applied, [55]. To print these sensors, they used a screen printing technique on a flexible substrate, which allowed them to print electrodes with a height inferior to 30µm, which is not possible with nowadays FDM printing techniques, however, maybe ap-plying the same principle as the resistive S. J. Leigh et al.’s sensors to this sensor design, could produce a 3D printed resistance-based pressure-like force and strain sensor.

1.5.3 3D printed EMG sensors

To the author’s knowledge, there is only one study where an EMG sensor was developed by using an FDM technique, the G. Wolterink, R. Sanders, and G. Krijnen 3D-printed sEMG electrode. To produce these sensors, they 3D printed, in one go, two conductive parts, with a conductive TPU, con-nected by a non-conductive TPU material. The conductive parts are the transducers of the sensor allow-ing the application of the bipolar measurement principle. The ground was printed separately. With the new sensors, they were able to have the same results as the ones achieved with the commercial ones [44], which shows excellent prospects to the application of 3D printing techniques to the EMG field.

(a) (b) (c)

Figure 1.7: Examples of 3D printed resistive sensors (a) S. J. Leigh et al.’s sensor [48]; (b) B. Shih et al.’s sensor [54]; (c) S. Harada et al.’s sensor [55]

Figure 1.8: G. Wolterink, R. Sanders, and G. Krijnen 3D-printed sEMG sensor [44]