The first part of the thesis deals with the design and manufacture of a self-connecting flexible strain sensor. MWCNTs are deposited on the bottom surface of the strain sensor except on the micropillar. The last part of the thesis concerns the sensing behavior of a self-attached flexible strain sensor.
Adhesion strengths of SCM sensors with different pillar stem diameters (Ds, 15 µm and 20 µm) and spacing ratio (SR, 1 and 2). Adhesion behavior of the strain sensor. a) Adhesion strengths of SCM sensors with Ds 15 µm post and SR 1 to different substrates (preload: 100 kPa, pulling speed: 1 mm s-1). Schematic illustration showing the principle of operation of the self-connecting strain sensor under bending load.
Deformation sensor property sensitive to strain and insensitive to pressure. a) (i) Schematic illustration showing the working principle of the planar PDMS (CP) sensor coated with MWCNTs. Schematic illustration showing the pressure-insensitive working principle of MWCNT-selectively coated micropillar strain sensors (SCM). b) Relative resistance changes measured by CP and SCM sensors as a function of pressure (applied load = 80%).
Introduction
Research backgrounds
- Flexible tactile sensor - Existing problem and motivation
Easy differentiation of the different mechanical stimuli of tensile stress and normal pressure is also a critical requirement for the practical application of flexible tactile sensors [33]. Although previous flexible tactile sensors have shown a high sensitivity to voltage and pressure, electrical output signals responding to these input signals are similar and indistinguishable from each other [43]. Consequently, the decoupling of strain and pressure is highly challenging with most of the previously reported flexible tactile sensors.
In general, despite recent developments, self-attachable flexible strain sensors with excellent sensing performance and strong adhesion strengths, as well as the ability to decouple pressure and strain, are rarely explored (Table 1). For example, previous studies have reported strain sensors that can exhibit pressure (or stretch) insensitivity. On the other hand, strain sensors with improved adhesion strength showed a limited GF or strain range [47,48].
Research concept & outline
Design & fabrication of the self-attachable flexible strain sensor
- Materials and Methods
- Fabrication of the pressure-insensitive self-attachable flexible strain sensor
- Surface analysis
- Evaluation of adhesion behavior of the self-attachable flexible strain sensor
- Characterization of the piezoresistive sensing behavior of the pressure-insensitive flexible
- Design and fabrication of the pressure-insensitive self-attachable flexible strain sensor
High-resolution SEM images of the microstructures and MWCNT percolation networks were obtained using a microscope (S-4800, Hitachi). The square samples (1 × 1 cm2) were fixed on the surface of the motorized part with the microstructure side of the samples down. Then, an out-of-plane shear was applied vertically (pull speed of 1.0 mm s-1) until the detachment of the samples from the substrates occurs.
The sheet resistance of the deposited MWCNT percolation networks was evaluated using a four-point probe method with a resistivity meter (CMT-SR1000N, AIT). Two opposite sides of the rectangular samples (initial length of 2 cm and thickness of 1 mm) were fixed by mechanical clamping and connected to electrodes (copper wire) using a silver paste to reduce contact resistance. The MWCNTs were deposited on the bottom surface of the strain sensor, except for the micropillars (Figure 2a-ii).
On the other hand, the application of normal pressure does not significantly change the MWCNT percolation network because the MWCNT layer is very thin (thickness of ∼200 nm), and thus the deformation of the layer under pressure is highly limited. Design of Pressure Insensitive Self-Attachable Flexible Strain Sensor. a) (i, ii) Schematic illustration showing the pressure-insensitive strain sensor with active MWCNT layer and bio-inspired adhesive micropillar layer. iii) Strain-sensitive and pressure-insensitive properties of the sensor. Although the deposited MWCNT layer acts as an active component of the sensor, it hinders the conformal adhesion of the sensor to the target substrate [51].
We utilized the nature-inspired micropillar structure that includes protruding ends in our sensor design to endow the sensor with a strong self-attachment capability. The MWCNTs deposited over the tips of micropillars were selectively removed by using an adhesive tape, as the MWCNTs on the tips would inhibit the adhesion of the micropillar array. MWCNTs were selectively deposited on the bottom of the sensor, apart from the micropills, forming percolation networks.
As shown in Fig. 3d, tight adhesion of the fabricated flexible strain sensor to the curved surface of the syringe occurred without the use of additional adhesive tapes due to the intrinsic adhesive nature of the micropillar array. a) SEM images of the fabricated strain sensor with (b) MWCNT layer and (c) micropillar layer.
Adhesion behavior of the self-attachable flexible strain sensor
Adhesion behavior of the strain sensor. a) Measured adhesion strengths of planar PDMS (P), MWCNT-coated planar PDMS (CP), fully MWCNT-coated PDMS micropillar (ECM), and selectively MWCNT-coated PDMS micropillar (SCM) (prestress: 100 kPa, pulling rate : 1.0 mm s-1). Adhesion strength of ECM and SCM sensors as a function of MWCNT coating dose (preload: 100 kPa, pulling speed: 1.0 mm s-1). Based on the measured resistance of the sheet and the thickness of the MWCNT layer in relation to the dosage of the MWCNT coating, we estimated the conductivity (= 1/sheet resistance × 1/thickness) of the MWCNT layer.
According to a previous study, the conductivity of CNT layers formed by spray coating becomes almost independent of the thickness if the penetration network is sufficiently formed [60]. a) Sheet resistance, MWCNT layer thickness, and (b) conductivity of self-assembled flexible strain sensors in relation to MWCNTs coating dose. However, D's less than 15 μm can deteriorate the structural stability of the pillars while requiring much higher production costs. As expected, the set of micropillars with Ds of 15 μm and SR of 1 exhibited the highest adhesion strength because it has the highest pillar density among the different samples.
All the samples showed increased adhesion strengths with an increase in preload and exhibited adhesion saturation at a preload of ∼100 kPa (Figure 7). In addition to the glass substrate, the SCM sensor exhibited strong self-adhesion to a wide variety of substrates, including Si, Au, Ag, Al, Cu, and ITO (Figure 8a). These results demonstrate that the flexible strain sensor with a selectively coated active CNT layer can adhere to various target substrates and closely interface with them, enabling precise detection of the mechanical deformations of the substrates.
Error bars in (a) represent the standard deviation, and each test was repeated ten times. b) Adhesion durability of the SCM sensor after repeated cycles of attachment and detachment.
Sensing behavior of the self-attachable flexible strain sensor
When a relatively high tensile strain (>60%) was applied to the sensor, a slight overshoot was observed, followed by a decrease in relative resistance over time. When a tensile stress is applied to the sensor, the stress is transferred to the PDMS and MWCNT layers, causing MWCNTs to rearrange. This results in a gentle restoration of the conduction paths between the MWCNTs, resulting in a temporal decrease in resistance.
The strain sensing behavior of the proposed strain sensor was very robust and durable (Figure 14). In contrast, the SCM sensors adhered firmly to the substrate and formed due to the strong self-attachment to the bend of the PET substrate. Photographs showing the adhesion and bending behavior of the fully MWCNT-coated micropillar (ECM) and selectively MWCNT-coated micropillar (SCM) strain sensors attached to a PET film under bending.
Although the uncoated planar back side of the SCM sensors could be attached to the PET surface, they were also easily peeled off when bent (Figure 17). Attachment behavior of selectively MWCNT-coated micropillars (SCM) strain sensors. a) Schematic illustration showing different methods of attaching SCM sensors to a PET substrate using (i) planar backside and (ii) frontside of micropillars selectively coated with MWCNTs. Photograph showing the different adhesion behaviors of SCM sensors attached to a flexible PET substrate using back and front faces under bending load.
Based on the bioinspired adhesive microstructures, the SCM strain sensor can be firmly attached to the skin of the wrist. The SCM sensor showed stable and reproducible electrical behavior during repeated wrist flexion. The minimally deformable thin configuration (thickness: ~200 nm) of the MWCNT coated layer minimizes changes in percolation networks and electrical resistance.
Time-lapse relative resistance measurements further demonstrated that the SCM sensor is insensitive to pressure and strain (Figure 21b). Deformation sensor property sensitive to strain and insensitive to pressure. a) (i) Schematic illustration showing the working principle of the planar PDMS (CP) sensor coated with MWCNTs. ii) Schematic illustration showing the pressure-insensitive working principle of MWCNT-selectively coated micropillar (SCM) strain sensors. Deformation sensor property sensitive to strain and insensitive to pressure. a) Relative resistance changes measured with the SCM sensor as a function of pressure for different strains (0–80%).
Conclusion and Discussion
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