3D Printed Carbon Nanotubes Reinforced Polydimethylsiloxane Flexible Sensors for Tactile Sensing

Technology is constantly evolving, and chronic health issues are on the rise. It is essential to have affordable and easy access to remote biomedical measurements. This makes flexible sensors a more attractive choice owing to their high sensitivity and flexibility along with low cost and ease of use. As an additional advantage, 3D printing has become increasingly popular in areas such as biomedicine, environment, and industry. This study demonstrates 3D-printed flexible sensors for tactile sensing. A biocompatible silicone elastomer such as polydimethylsiloxane (PDMS) with low elastic modulus and high stretchability makes an excellent wearable sensor material. Incorporating CNTs at varying concentrations (0.5, 1, 2)wt% enhances the sensor’s mechanical strength, conductivity, and responsiveness to mechanical strain. In addition to enhancing the thermal stability of the composite by 44%, multi-walled carbon nanotubes (MWCNTs) also enhanced the breaking strength by 57% with a 2 wt% CNT loading. Moreover, the contact angle values improved by 15%, making it a biomedical-grade hydrophobic surface. The electrical characteristics of these sensors reveal excellent strain sensitivity, making them perfect for monitoring finger movements and biomedical measurements. Overall, 2 wt% CNT-PDMS sensors exhibit optimal performance, paving the way for advanced tactile sensing in biomedical and industrial settings.

Recent years have seen the use of three-dimensional (3D) printing in a variety of professional and academic settings.This potential technology can create geometrically complex pieces quickly compared to traditional production techniques. 1 Due to its ease of use, rapid manufacturing capabilities, and accurate and controlled deposition of multifunctional elements into bespoke 3D printed structures, and fabrics, it is now extensively used for wearable sensors, 2 smart bandages, 3 and textiles. 46][7] These advancements have catalyzed a paradigm shift in sensing methodologies with seamless data aggregation, refining accuracy and predictive prowess and unprecedented sensitivity in detecting biomolecular interactions.Furthermore, materials like MXene, with their exceptional electrical conductivity and mechanical properties, alongside polyaniline's electrochemical sensitivity, have enabled sensors to detect subtle changes in their surroundings with remarkable precision and reliability. 8,9n developing wearable sensors various factors need to be considered including material, biocompatibility, flexibility, scalability, and cost.It's important to choose an appropriate substance for any device that comes into contact with the skin.A silicone elastomer like PDMS is a desirable choice because of its excellent stretchability, low elastic modulus, and biocompatibility. 10owever, PDMS's poor electrical properties pose a big challenge for developing piezoresistive sensors.PDMS composites can be significantly improved by adding an appropriate nanofiller. 11CNT-Si ink can be used for wearable sensing applications, especially motion recognition applications.Shar et al. prepared CNT-Silicone ink by mixing amino-functionalized MWCNT, RTV Silicone, and PDMS at 2000 rpm for 5 min, followed by drying.The formulated inks with different CNT concentrations are 3D printed using a nozzle diameter of 200 μm to 1.60 mm.The prepared electrode showed considerable changes in current due to strain and compression.Electrodes with 5 wt% CNT showed optimum performance because of excellent conductivity as well as stretchability. 12Ma et al. 13 performed a comparative study on the performance of Carbon Black-CNT-Silicone and Carbon Black-Graphene-Silicone inks for piezoresistive sensing applications.Using Direct Ink Writing (DIW), prepared inks with different CNT/Graphene concentrations are printed on a PDMS substrate with a nozzle diameter of 1.4 mm, speed (60-100) mm/min, and pressure of 0.3 MPa.The prepared sensors were tested by cyclic loading and unloading at 20 mm min −1 , 1000 cycles of strain (0.1), and 2000 cycles of pressure (10 kPa), and they showed good sensitivity.Moreover, the printing speed and concentration of CNT/Graphene greatly affected the sensitivity.Further, dry electrodes can also be prepared by casting.Attaching a metal cup and silver wire to a cast CNT/ PDMS biopotential measurements such as Electroencephalogram (EEG), and Electromyogram (ECG) can be realised.However, the noise of the obtained signals poses a significant challenge in their commercial applications.One solution to noise reduction can be incorporating a band stop or bandpass filter. 14oreover, the infill percentage can greatly affect the piezoresistive properties of the sensor.Since the change in resistance is caused by mechanical stress and physical contact with the material, incorporating gaps in the structure may change the properties.Nassar et al. 15 showed that MWCNT/Thermoplastic polyurethane sensors 3D printed with 50% infill showed optimum performance compared to original sensors, 30% and 100% infill sensors.Moreover, printing needle diameter and curing temperature can change the morphology, thus the piezoresistive properties of sensors.MWCNT/PDMS sensors when cured for longer durations (more than 12 h.),MWCNT/PDMS sensors start losing their alignment and degrading their properties.Curing at 120 C has shown the most promising results.In addition, a small needle diameter is necessary to avoid wider patterns and ensure the formation of a conductive network. 16Poompiew et al. analyzed the effect of incorporating CNT and polypyrrole in flexible polyurethane (FPU) electrodes.MWCNT and polypyrrole significantly improved the electrical and z E-mail: bhavyajain516@gmail.com; krishnakant_p@mt.iitr.ac.inECS Sensors Plus, 2024 3 020606 mechanical properties of the sensors.Added 1 wt% of MWCNT doubled the electrical conductivity of FPU resins Addition of 5 wt% polypyrrole in CNT/FPU ink resulted in 5 times higher conductivity, which can be attributed to the improved electron localization efficiency between CNT molecules due to polypyrrole.However, polypyrrole significantly reduces the flexibility and stretchability of sensors and thus is not ideal for piezoresistive applications. 17The addition of carbon nanofillers such as CNT or graphene can significantly improve the thermal, mechanical, and electrical characteristics of the polymer matrix.It is, therefore, worth finding the most efficient carbon nanofiller to get the optimum performance of the polymer sensor.Gnanasekaran et al. 18 showed that CNT is a more effective nanofiller than graphene due to better conductivity and thermo-oxidative behavior.Besides 3D printing, there are various techniques used for fabricating sensors.In this regard, Liu et al. 19 utilised a wet spinning technique for preparing CNT   ECS Sensors Plus, 2024 3 020606 polyurethane strain sensors.The prepared sensors showed good cyclic stability and repeatability and could recognise various human motions including wrist movement and eye blinking.Furthermore, Yu et al. 20 reviewed various physiological monitoring applications for wearable sensors.
In this work, a 3D-printed piezoresistive sensor is prepared using CNT (0.5, 1, 2) wt% and PDMS.A CNT-PDMS layer is 3D printed on a PDMS substrate to ensure biocompatibility and stretchability.The proposed sensors are tested on various grounds including their morphology, mechanical properties, thermal properties, wettability, and electrical properties.Benefitting from excellent sensing characteristics, the CNT-PDMS sensors are further used for tactile sensing.

Materials & Fabrication
Materials.-MWCNTs(outer diameter 10-12 nm, density 1.8 g cm −3 , length 8-12 μm, and purity >95%) used in this study are procured from C-Nano Technology Limited, Beijing, China.PDMS base elastomer and curing agent procured from Sylgard 184 Silicone Elastomer from Dow Chemical Company, USA, all other chemicals procured from local sources and used without any further purification.
Preparation of composite ink.-MWCNTs (0.5, 1, and 2) wt% ultrasonicated using a probe sonicator (20 kHZ, 750 W, PCI Analytics, India), for 45 mins using a 5-3-5 sec on-off-on cycle.Further, 3 ml of PDMS base elastomer is added to the solution and sonicated again for 30 min.As soon as the solution is prepared, it is kept on a magnetic stirrer at 55 °C, 400 rpm overnight to evaporate the acetone, then the PDMS curing agent is added in a 1.5:10 ratio to the solution and mixed well.This solution is used as ink for 3D printing tactile sensors over a separately cured PDMS substrate.A 5 ml plastic syringe diameter with a nozzle diameter of 0.41 mm is used for 3D printing.
3D printing of sensors.-Firstly, a 0.9 mm thick PDMS substrate is prepared by casting it in an aluminium mold at 75 °C for 4 hr.Next, a Cellink Bio-X 3D printer is used to print the CNT-PDMS layer using a speed of 25-30 mm sec −1 , pressure 40 kPa, and nozzle diameter 0.41 mm.Afterwards, the pattern is cured at 120 °C for 15 min.Further to avoid contamination a thin layer of PDMS is applied to the prepared sensor.Figure 1a presents the 2 wt% CNT-PDMS flexible sensor and from Figs. 1b-1c it is evident that the sensor is highly flexible and stretchable.Figure 1d presents the schematic representation of the key components of the sensor.A CNT-reinforced PDMS ink is 3D printed on a PDMS layer, with a very thin PDMS capping layer.The sensor is attached to the epidermis using a commercially available skin adhesive.
Characterization and testing.-Theelectrodes are characterised using a range of methods to determine their morphology, conductivity, strength, and thermal behavior in response to concentrations of CNTs.The morphological analysis of the samples was done using Carl Zeiss Ultra Plus FESEM in the voltage range of 5kV-7kV.The samples were priorly Au coated for 60 secs.To assess the effect of MWCNT addition on the crystallinity of PDMS matrix, X-ray diffraction was done using Rigaku Smartlab 3 kV diffractometer.All the samples were analysed in the range of 2θ = 5°to 80°, with scan rate of 1°/min at the step size of 0.2, in Cu-Kα radiation (λ = 1.5401A°).The electrical conductivity is measured by a digital multimeter and Keithley 2450 Source meter.The thermal stability is analysed by Thermogravimetric Analysis (TGA) using Netzsch STA 449F3 Jupiter, and effect of MWCNT addition on thermal properties was examined by Differential Scanning Calorimetry (DSC) using Netzsch DSC 204F1 Phoenix.The mechanical strength is interpreted by tensile modulus using a "Low load mechanical tester (BOSE, USA, model number 3230, Series III)."The wettability behavior is assessed using "Kruss Drop Shape Analyser DSA25," measuring the water contact angle on the 3D printed specimens.Moreover, to understand the possibility of electrode incorporation in biomedical applications, tactile sensing is performed by finger movement.

Results & Discussion
Morphological analysis.-Figure2a shows the entanglement of high aspect ratio MWCNTs.It is evident from Fig. 2b that MWCNTs are sparsely distributed in the PDMS matrix in the case of 0.5 wt% CNT-PDMS 3D printed composite.On the other hand, in the case of 2 wt% CNT-PDMS composite, a uniform distribution of MWCNTs can be observed, as indicated in Figs.2c-2d.So, it can be concluded that there is almost no agglomeration of CNTs in the PDMS matrix.Moreover, Table 1 presents a comparative analysis between the proposed and reported sensors based on the fabrication technique, nanofiller loading, and properties.
X-ray diffraction analysis.-TheX-ray diffraction is performed to study the effect of CNT addition in the PDMS matrix.All the samples are analyzed in Rigaku SmartLab 3 kW diffractometer, in the range of 2θ = 5°to 80°, with a scan rate of 1°/min at the step size of 0.2, in Cu-Kα radiation (λ = 1.5401A°).The X-ray diffraction patterns of synthesized samples are depicted in Fig. 3.A prominent peak at approximately 2θ = 12°is visible for all the samples, indicating the prominent amorphous phase.Peak sharpening upon the addition of MWCNTs is evident from the diffraction patterns.All the spectra have a broader peak at around 2θ = 26.5°,which further confirms the presence of PDMS matrix. 21,22In the case of 2 wt% CNT/PDMS composite, two additional peaks appear in the diffraction pattern at 2θ = 27.9°and31°, which are indicative of the presence of MWCNTs in the PDMS matrix.It is somewhat difficult to detect the presence of MWCNTs in the 0.5 wt% CNT/PDMS and 1 wt% CNT/PDMS composites as no prominent peaks are detected in the diffraction pattern.The scarce amount of MWCNTs and the presence of attached amorphous carbon groups would be the possible reasons for the absence of the peaks but a sharpening of the peak at 2θ = 12°confirms the presence of MWCNTs. 23ermal behavior.-Tounderstand the thermal behavior of the CNT-PDMS composite, TGA and DSC tests are conducted, and the findings are depicted in Fig. 4. DSC and TGA are performed at a heating rate of 10 °C min −1 with nitrogen purging.Both heat flow experiments are performed at room temperature as a reference.From the thermograms, it is observed that all the samples exhibited thermal stability up to 250 °C.Beyond that, the PDMS polymer began to undergo weight loss, whereas the other composites displayed thermal stability up to 360 °C.This degradation temperature is defined as the point of inflexion on the TGA curve.Further, it can be observed that major weight loss of about 30%-35% occurred between (360-600)°C.In addition to facilitating heat conduction, conductive nanofillers also prevent the emission of combustion products during decomposition. 24From Fig. 4a it is evident that after the incorporation of CNT, both the onset and maximum degradation temperature showed significantly higher values, which justifies the addition of a nanofiller for achieving higher thermal stability.Figure 4b shows that the addition of CNTs does not cause any kind of thermal transition in the PDMS.
Mechanical testing.-Themechanical strength of the 3D printed sensors is evaluated by mechanical loading and unloading as shown in Fig. 4c, and the stress-strain response is illustrated in Fig. 4d.The stiffness of sensors increased with CNT loading. 25Along with an increase in the loading proportion of the CNTs, a slight increase in tensile strength can be observed.This can be attributed to the uniform dispersion of MWCNTs in the PDMS matrix due to ultrasonic mixing.Uniformly distributed MWCNTs act as a bridged network across the matrix and facilitate load distribution throughout the volume of the matrix.The high aspect ratio of MWCNTs also plays a crucial role in the uniform distribution of load.It is also important to note that CNTs may act as second-phase particles resulting in deflection of cracks, thus delaying crack propagation in the matrix, imparting better mechanical properties.The tensile strength of the sensor is significantly higher than that of printed composite ink.The yield strength of the sensor is approximately 10 times that of the 2 wt% CNT/PDMS sensor.Among the sensors, all samples have similar yield strength values, but the breaking strength for 2 wt% CNT/PDMS composite is 57% higher than that of other sensors.This can be explained by differences in the curing durations of PDMS substrate and PDMS-CNT sensor.Prolonged cooling facilitates better curing of PDMS substrate by providing sufficient time for cross-linking of the hydrocarbon and silicon molecules.Sensors, on the other hand, are cured rapidly at higher temperatures as compared to those of a substrate.This provides a very short time for crosslinking and thus lower mechanical properties are observed.
Wettability studies.-Given the usage of the proposed sensors for human motion detection, it is vital to understand how well the composite interacts with liquid, particularly water.The wettability is depicted in Figs.5a-5d regarding the contact angle, and Fig. 5e compares PDMS and sensors at various CNT concentrations.It is evident from Fig. 5e that with an increase in CNT loading, the contact angle increases and is greater than 90°.The addition of 2 wt % MWCNTs in the PDMS matrix led to a 15% rise in the water contact angle values.This shows that the surface is hydrophobic and is suitable for biomedical applications.
Electrical properties & tactile response.-Thepiezoresistive characteristics of nanotubes make CNT-PDMS sensors extremely sensitive to mechanical deformation, strain, temperature pressure, or other physical changes.This can be due to a change in spacing between the nanotubes, affecting the overall electrical conductivity and resistance.Mathematically, the change in resistance (ΔR) can be represented by the equation, ΔR = S⋅R⋅ϵ, where ΔR is the change in resistance, S is the gauge factor, which indicates how sensitive the sensor is to changes in strain, R is the initial resistance of the sensor, ϵ is the mechanical strain applied to the sensor.The 3D-printed CNT-PDMS sensor is connected to the Keithley 2450 Source Meter, establishing a complete electrical circuit.The Source Meter applies a known voltage (Vexc) across the sensor, serving as the stimulus for current flow through the sensor.As the voltage is applied, the resulting current passing through the sensor is measured by the Source Meter.This current value varies depending on the resistance of the sensor, which, in turn, changes with applied strain.Using Ohm's Law (V = IR), where V is voltage, I is current, and R is resistance, we calculated the resistance of the sensor at any given moment during the measurement.Therefore, the change in current reflects the change in resistance (ΔR) due to the applied strain.Essentially, Relative Resistance Change = (R−Ro)/Ro X 100%, where Ro is the initial electrical resistance, and R is the immediate electrical resistance measured at a specific moment.
Figures 6a-6c present the relative current over time, here "ΔI" is the change in current, and "Io" is the initial current value.For CNT (0.5, 1, 2) wt% -PDMS sensors the relative current is measured while increasing the strain from (15-50)%.A significant increase in current is observed with a rise in strain.For the same amount of strain, the relative current is 3x times in CNT 1 wt% sensors than in 0.5 wt%.This can be due to enhanced contact and reduced interfacial spacing in the nanotubes.Moreover, with an increase in CNT concentration, the conductivity of the sensors further increases, and the maximum current value is observed for 2 wt% CNT-PDMS sensors, with a 50% strain.The formation of a dense bridged   7 shows the ability of the proposed sensors to recognize finger movement and bending.For the first 10 sec, a to-and-fro 45°finger movement is performed, followed by complete 90°bending.The sensor recognizes the finger movement through the increment/decrement of strain and reflects in terms of resistance in the multimeter as shown in Fig. 7b.Again, with an increase in the strain, the resistance increased significantly, in a predicted manner, thus making the CNT-PDMS sensors a reliable choice for biomedical measurements.

Conclusions
This work presents a high-sensitivity 3D-printed CNT-PDMS sensor.The proposed sensor is analysed at various CNT (0.5, 1, 2) wt% concentrations.Moreover, the sensors are evaluated based on their electrical, mechanical, and thermal properties.By incorporating nanofillers into the PDMS, the properties of the composite ink have been improved significantly, and the characteristics have improved substantially when CNT concentration is increased.Overall, 2 wt% CNT-PDMS samples demonstrated the best performance with maximum strength, thermal stability, conductivity, lower response times, and no agglomeration.It is due to MWCNT's excellent conductivity and high strength that this performance has been achieved.Further, the sensors exhibit appreciable electrical properties.Electrical properties are determined by comparing relative resistance and current.Changes in strain/pressure are visualized as changes in resistance/ current.Additionally, the sensors successfully track finger movement at various angles, making them suitable for both biomedical and industrial use.In future studies, one possible direction could be understanding the biocompatibility of the sensors with cells, in addition to assessing cardiac activities for chronic disorders.

Figure 1 .
Figure 1.Design and characteristics of flexible sensor.(a) Zoomed-in image of 3D Printed CNT-PDMS Sensor, photographic images of sensor subjected to (b) bending, (c) twisting (d) Exploded view schematic illustration of the sensor.

Figure 3 .
Figure 3. X-ray diffraction patterns of all samples.

Figure 4 .
Figure 4. Thermal and mechanical behavior: (a) TGA, (b) DSC curve, (c) Tensile testing of prepared samples, (d) Stress vs strain curve for proposed sensor, and CNT-PDMS composites.

Figure 7 .
Figure 7. Tactile sensing: (a) Photographic image of on-body measurement, (b) Resistance plot for on-body testing.

Table I .
Comparison of performance and properties of reported flexible sensors.CNTs throughout the PDMS matrix with an increase in the loading proportion of MWCNTs explains the rise in relative current.The repeatability of 3D-printed sensors is evaluated by applying multiple stretch-release cycles.As presented in Figs.6d-6ethe sensors showed excellent reproducibility and resistance recoverability.For every stretch/release motion, a corresponding change in resistance is observed.Figure