In this thesis several advances are made to the emerging field of 3D printed mechanical sensors. Techniques and processes were developed to enable the integration of highly conductive, and capacitive and piezoresistive sensing features embedded within 3D printed elastomeric materials. With these technologies we enabled the fabrication of fully 3D printed piezoresistive and capacitive sensors for normal force, shear forces and angular bending sensing.

Integrating highly electrically conductive flexible silver films (<125 µ⠊ cm), printed by Direct Ink Writing (DIW), into a flexible thermoplastic polyurethane (TPU) printed by Fused Deposition Modelling (FDM) for low-power mechanical sensors was not possible due to thermal constraint. Using a cost-effective LED laser, we developed an in-situ laser sintering technique to in-situ sinter printed silver film. In this fashion, we achieved sub 100 µm thickness features with resistivity values as lows as 58.2±7.7 µ⠊ cm. This technique was exploited to create fully 3D printed flexible capacitive normal pressure sensors (4.47±0.12 fF/N), conductors with low resistance (<2⠊), and a piezoresistive bending sensor. In this way we enabled the fabrication of fully 3D printed flexible electronic devices, with a significant conductivity increase.

As FDM is limited in terms of compatible materials, due to a need for thermoplasticity, we developed stable silicone DIW printable material systems. A structurally self-supporting material system composed of Crystalline Nanocellulose (CNC) reinforced silicone (CNC-PDMS) was realised with ideal shear thinning behaviour required for DIW printing. By integrating printed strain gauges into the CNC-PDMS made from a piezoresistive carbon black (CB) silicone ink (CB-PDMS), piezoresistive normal force sensors were DIW printed with a linear normal force sensitivity of 14.9±0.4⠊/kPa (0.22±0.07%/kPa) up to a load of 1000 kPa. Dynamic normal force sensing was demonstrated for these pressures up to a frequency of at least 2 Hz. Using 3D printing, a shear force sensor was printed that could sense shear forces and their direction up to 15 N. The digital nature of this technology enables the fabrication of fully 3D printable personalised wearables with embedded sensors able to sense gait.

Lastly, we created sensors with 3D topology, for which we developed a fumed silica reinforced DIW printable UV curable silicone ink. Using the ability of the ink to be shear-thinning, a novel capacitive bending sensor design was conceptualised with angular faces. By stacking layers of the silicone ink strategically, angular faces were generated with angles up to 41.2±2.3° on which conductive silver plates were printed. Through this method, the bending sensor was realised with slanted plates which increased in capacitance when bend towards each other and decreased when bending away from each other, which allow for a sensor with an asymmetric sensing behaviour. With a sensitivity of 2.50±0.04 fF/° (0.11±0.00 %F/°), the sensor was able to accurately track the bending of a limb. Using the developed sensing mechanism and printing approach, we demonstrated the fabrication of a novel human motion monitoring sensor.

The results presented in this PhD thesis could be seen as a stepping stone towards the development of fully 3D printed personalised wearables with integrated sensing. By exploiting the digital nature of 3D printing it was shown that novel and custom tailorable devices can be enabled.