Abstract
In this work, sensitivity and response times of PDMS-based capacitive force sensors are enhanced via the modifications of the PDMS layer. Two modifying approaches are proposed; (i) change PDMS's (elastomer:curing agent) ratio and (ii) adding conductive polymer PEDOT:PSS into the PDMS layer. The change of PDMS (elastomer:curing agent) ratio from (10:1) to (30:1) increases the sensitivity from 0.4 ± 0.08 to 0.72 ± 0.23 kPa−1 (+80%) but it does not significantly affect the response/recovery times. In addition, by adding 1% wt. of PEDOT:PSS to PDMS (30:1), the further increment of sensitivity from 0.72 ± 0.23 to 1.44 ± 0.17 kPa−1 (+100%) and the shorter response time from 1.59 ± 0.02 to 0.45 ± 0.03 s (−72%) are observed. The mechanical and electrical studies reveal that the change of PDMS (elastomer:curing agent) ratio and the adding of PEDOT:PSS to PDMS layer result in the modification of PDMS's deformability and the increase of charge transportation, leading to the enhancement of sensing characteristics of the sensors.
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1. Introduction
Force-sensing devices are an electronic component that converts applying force to a change of physical output, e.g. resistance, voltage as well as capacitance. 1,2) Such sensing device is employed in several fields of applications, including tactile electronics, industrial control, and health care monitoring. 3–5) The force sensors with excellent sensing characteristics, e.g. high sensitivity, rapid response/recovery time, and high durability, are required to achieve the best performance in selective applications. 5–7) Therefore, the enhancement in sensing characteristics of force sensors becomes an impact research issue.
Among several categories of force-sensing devices, a capacitive force sensor, that detects an exerting force through a change in capacitance, is an interesting candidate due to several advantages in both sensing and operating characteristics, e.g. providing linear correlation between an applying force and a change in capacitance and exhibiting less hysteresis behavior during an operation. 8) One of the materials famously used in such sensors is polydimethylsiloxane (PDMS) owing to an excellence in mechanical properties, less toxicity, and biocompatibility as well as being simple to fabricate. 9–11) However, capacitive force sensor based on PDMS encounters limitations in low sensitivity and slow response/recovery time. 12) Since a simple structure of capacitor, a PDMS layer sandwiched by two electrodes, is employed to fabricate a conventional capacitive force sensor, the characteristics of sensors strongly depended on the properties of the PDMS layer. Hence, the modification of the PDMS active layer is a major approach to improve the sensitivity and response/recovery time of such sensors. 13,14) The sensitivity (s) of the capacitive force sensor, the change of capacitance induced by the exerting load, is related to several parameters as shown in Eq. (1).
where C0 and ∆C are the initial capacitance and the change of capacitance during applying load, respectively. ∆Lcompressive is a change in compressive load during applying load. ε is the dielectric constant of an active layer and A is the sensing area of the sensor. d0 and ∆d are initial thickness and the change of thickness taken place during applying load. 15,16) The further detail related to the derivation of Eq. (1) is explained in supplementary data. Equation (1) is suggested that, with the constant exerting load, the higher deformation of the active layer, the higher sensitivity can be obtained. Therefore, the modification of the active layer's mechanical properties is an approaching strategy to enhance the sensitivity of the capacitive force sensor. Moreover, the change in the dielectric constant of the active layer during applying load can be another key to further enhance the sensitivity of such sensor. Several ideas have been proposed to modify an active layer, i.e. introducing porous structure into the active layer to increase deformability and adding conducting material into the active layer to modify the dielectric constant. 17) However, the above-mentioned approaches could end up with consequential drawbacks in uniformity and durability of the sensor. Since PDMS is a cross-linked polymer, the mechanical properties of the PDMS layer depend on the crosslink density in the layer. Therefore, control the ratio of elastomer to curing agent used to prepare PDMS layer is an alternative approach to enhance the sensitivity of PDMS-based sensor. The less curing agent fraction is used to prepare the PDMS layer, the better deformability of the PDMS layer is obtained because fewer cross-linked domains are formed in the PDMS layer. 18,19) Moreover, adding a conductive material, e.g. metal oxide, carbon-based materials as well as polymers, into the PDMS layer is a common way to play with the change in dielectric constant that can consequently enhance the sensitivity of PDMS-based capacitive force sensor. 15,18,19) However, too rigid additive materials possibly lead to the decrease of sensor's sensitivity due to the poor deformation of the active layer. Then, the conductive polymer is an interesting candidate for use as an additive to modify the electrical properties of the PDMS active layer. One of the famous conductive polymers used in several electronic applications is [Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; PEDOT:PSS]. In the application related to a capacitive force sensor, PEDOT:PSS is only employed as an electrode. However, none of the literature has reported the utilization of PEDOT:PSS as an additive to enhance the performance of force sensing devices. Therefore, the investigation of the possibility to use PEDOT:PSS as an additive to modify the PDMS active layer could propose an alternative additive material for sensitivity enhancement of capacitive force sensor.
The response/recovery times of the capacitive force sensor, the time period that the sensor requires to rise the capacitance from the initial value to the final one, are mainly involved with not only the mechanical properties but also the electrical properties of the active layer. The rapid response behavior could be obtained from the active layer with high elasticity and high charge transportation. However, the active layer with high elasticity leads to a decrease in sensitivity because the lower deformation could be taken place. Hence, it is interesting to find a way to improve the response/recovery time without the deterioration of the sensitivity sensor. One of the possibilities to enhance charge transportation of PDMS is to introduce a conducting additive into the PDMS layer. 15,20,21) Nevertheless, the poor dispersive additive could form the major phase separation leading to the degradation of sensing performance of the sensor. Therefore, the conductive polymers become a suitable choice to use as conducting additive to enhance charge transportation in the PDMS layer.
In this work, the sensing characteristics, e.g. sensitivity and response/recovery times, of PDMS-based capacitive force sensors are enhanced via modification of the PDMS active layer. Two modifying approaches are proposed to adjust the specific properties of the PDMS layer; (i) change the ratio of elastomer to curing agent used to prepare PDMS active layer and (ii) adding the conductive polymers [Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; PEDOT:PSS] into the cross-linked PDMS active layer. In addition, the mechanical and electrical characteristics of PDMS-based active layers are investigated to reveal the origin of the sensing enhancement. Moreover, the durability and reliability tests with over 100 loading and unloading cycles of the compressive load were applied to capacitive force sensors using modified PDMS as the active layer. Finally, the possibility of applying the developed force sensor in this work to the application of artificial skin is demonstrated.
2. Experimental methods
2.1. Fabrication of PDMS-based capacitive force sensors
In this work, the simple structure of the capacitor as shown in Fig. 1, consisting of a dielectric active layer sandwiched by two electrodes, is used to fabricate PDMS-based capacitive force sensors. The fabrication of the sensor was divided into two parts as followed.
Fig. 1. (Color online) Schematic representation of materials and device's structure used to fabricate PDMS-based capacitive force sensors and the list of PDMS active layers modified by (i) varying PDMS (elastomer:curing agent) ratio, and (ii) adding PEDOT:PSS into PDMS layer.
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Standard image High-resolution image2.1.1. Preparation of sensor's electrodes
In this research, PEDOT:PSS (PH 1000, Ossila Ltd.) coated on polyester (PE) film is used as electrodes for capacitive force sensors. Firstly, the PE films were cleaned by ethanol to eliminate organic contaminants. Then, 10 min of oxygen plasma process was applied to the PE films in order to make a hydrophilic surface. PEDOT:PSS (2% solid content dispersed in Di water) was coated on the PE substrate by rapid convective deposition 22,23) to form PEDOT:PSS film with the thickness of 200 nm and the sheet resistance of 60 ohm/square. The electrodes were annealed at 85 °C to form the PEDOT:PSS films and remove the undesirable solvent from the electrodes.
2.1.2. Preparation of PDMS active layers and assembly of capacitive force sensors
PDMS (Sylgard 184 silicone elastomer, Dow Inc.), a cross-linked polymer, was used as an active layer of the capacitive force sensor. Two approaches are applied to modify the PDMS active layer; (i) change the ratio of PDMS elastomer to curing agent, and (ii) adding PEDOT:PSS into PDMS layer. To prepared the PDMS layers on PEDOT:PSS electrodes, firstly, the mixture of PDMS elastomer and curing agent was mixed together with the different ratios of 10:1, 20:1, and 30:1 by weight. Moreover, 1% by weight of PEDOT:PSS was added to the PDMS mixture with a ratio of 30:1. All conditions of PDMS mixtures were coated on PEDOT:PSS electrodes by spin coating technique to form the active layer of the force sensors. Then, all the samples were fully cured in the temperature-controlled oven at 85 °C for 4 h. The average values of the thickness of the PDMS layer prepared with differently modified conditions were evaluated via cross-sectional micrographs as shown in Fig. S1 (see supplementary data available online at stacks.iop.org/JJAP/60/SCCE09/mmedia). The average thickness of all samples is approximately in the range of 80 μm. Therefore, the effect on the modulus and initial capacitance of the sensors attributed to the difference of the initial thickness can be neglected. Finally, the top electrode was assembled with the sensor after finishing the curing process. The four capacitive force sensors with the active area of 1 × 1 cm2, prepared by different PDMS active layers were obtained.
2.2. Characterization of PDMS-based capacitive force sensors
Sensing characteristics of PDMS-based capacitive force sensors, e.g. sensitivity, response/recovery times, as well as 100-cycles reliability test, were measured by Precision LCR Meter (E4980A, Agilent Technologies Inc.). To investigate the sensor's sensitivity, the change of relative capacitance was precisely monitored while a compressive load in the range of 0–23 kPa was continuously applied on the sensor. For response and recovery time, the compressive load of 0.75 kPa was placed on the sensors and then removed after 30 s. The response and recovery times were extracted from the period of time that the sensor requires to rise (or fall) the output signal from 10% to 90% (90% to 10% in recovery time) of the full scale. The viscoelastic properties of all PDMS-based active layers were investigated through strain sweep measurement (HAAKE MARS rheometer, Thermo Fisher Scientific Inc.) at the compressive load of 1 kPa to reveal the elastic modulus (G'), loss modulus (G''), and tan(δ), the ratio of G'' to G'. Dielectric constants of PDMS layers prepared by different modifications were investigated using a Precision LCR Meter (E4980A, Agilent Technologies Inc.) in the frequency range of 1000 to 1000 000 Hz. Potentiostat (Autolab PGSTAT302N, Metrohm AG Ltd.) was employed to investigate the natural charge transport behavior of the sensors via charge–discharge characteristics obtained from the applying current and the switching time of 1 nA and 10 s, respectively. Dielectric constants of PDMS films were investigated using Precision LCR Meter (E4980A, Agilent Technologies Inc.) in the range of frequency from 1000 to 1000 000 Hz. The durability and reliability tests of the capacitive force sensors were investigated by measuring the characteristics of capacitance taken place during loading and unloading compressive load of 1.13 kPa to the sensors over 100 cycles.
3. Results and discussion
3.1. Effect of active layer modification on the sensitivity of PDMS-based capacitive force sensors
Figure 2(a) shows the relative change in capacitance, induced by the compressive load in the range of 0–23 kPa, obtained from the capacitive force sensors using differently modified conditions of PDMS active layers. By applying compressive load, all the sensors exhibit the change in capacitance similarly to the common mechanical behavior of elastomer, indicating that the change in capacitance observed in all sensors majorly results from the deformation of the PDMS active layer. When the external force is applied to the capacitive force sensor, it can induce a change in PDMS thickness. The more change in the PDMS thickness, the larger change in capacitances across the PDMS layer. Then, the better sensitivity of the capacitive force sensors could be achieved. According to Eq. (1), the sensitivities of capacitive force sensors are determined by extracting the slope in Fig. 2. It is clearly seen that all the sensors show two regimes of sensitivity; (i) 0–1.5 kPa and (ii) 1.5–23 kPa. The presence of two characteristic regimes observed in PDMS-based capacitive force sensors can be explained by the ability of PDMS deformation as a function of applying compressive stress. Since PDMS is an amorphous crosslinking polymer, it initially contains lots of free space between adjacent polymer chains. Hence, at the regime involving the small applying compressive load, the PDMS layer can be effectively deformed by moving polymer chains closer to each other. The better deformation of PDMS leads to the steeper change in capacitance of the capacitive force sensor in such a regime. On the other hand, the further applying stress to PDMS, the polymer chains will become closer until they reach the limitation. As consequence, the poor deformation of PDMS occurs in this regime even if the higher applied stress is applied. Hence, the gradual change in capacitance induced by applying compressive load in this regime is observed. In this work, the sensing characteristics of the sensors in low compressive load (0–1.5 kPa), as shown in Fig. 2(b), are of interest owing to not only the significant enhancement in sensor's sensitivity but also the suitable range of compressive load required for the artificial skin applications (<1 kPa). 24)
Fig. 2. (Color online) Relative change of capacitance (∆C/C0) induced by applying compressive load in the range of (a) 0–23 kPa, and (b) 0.45–1.45 kPa, obtained from capacitive force sensors with variously modified PDMS as the active layer.
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Standard image High-resolution imageThe sensitivities of all sensors are shown in Table I. The modification of PDMS active layer by means of varying the ratio of PDMS elastomer to curing agent from (10:1) to (30:1) leads to the increase in sensor's sensitivity from 0.4 ± 0.08 to 0.72 ± 0.23 kPa−1 (+80%). To understand the origin of the sensing enhancement owing to the change of elastomer to curing agent ratio, the mechanical properties of modified PDMS layers are investigated. Because PDMS is a cross-linked polymer, the viscoelastic parameters, G', G'', and tan δ, are employed to present the mechanical behaviors of modified PDMS layers. Figure 3(a) Illustrates elastic modulus (G') measured from each modified PDMS layer. Noting that elastic modulus (G') represents an ability of viscoelastic materials to resist the deformation induced by an external force. 25,26) The lower value of G' means that the better deformation can be taken place in viscoelastic materials. By changing the ratio of PDMS (elastomer:curing agent) ratio from (10:1) to (30:1), the decrease in G' is clearly observed, indicating that the higher ratio provides PDMS layer higher deformability than the lower one. 27) As a consequence, the sensitivity of the PDMS (30:1) sensor is enhanced. 28) Therefore, the modification of the PDMS active layer by means of the change in PDMS (elastomer:curing agent) ratio can successfully enhance the sensor sensitivity through the control of deformability of the PDMS active layer. However, the further increase in the ratio over (30:1) leads to severe drawbacks in both uniformity and durability of the sensors. Such a limitation could be overcome by the second modification approach.
Table I. The sensing characteristics, e.g. sensitivity, response, and recovery times obtained from capacitive force sensors prepared by differently modified PDMS as the active layer.
| Type of active layer | Sensitivity (kPa−1) | Response time (s) | Recovery time (s) |
|---|---|---|---|
| PDMS 10:1 | 0.40 ± 0.08 | 1.76 ± 0.03 | 1.43 ± 0.04 |
| PDMS 20:1 | 0.49 ± 0.08 | 1.74 ± 0.03 | 1.21 ± 0.01 |
| PDMS 30:1 | 0.72 ± 0.23 | 1.59 ± 0.02 | 1.12 ± 0.01 |
| PDMS 30:1 (PEDOT:PSS) | 1.44 ± 0.17 | 0.45 ± 0.03 | 0.37 ± 0.04 |
Fig. 3. (Color online) Rheological parameters; (a) elastic modulus (G') and (b) tan δ, the ratio of loss modulus (G') to elastic modulus (G'), measured by strain sweep measurements with applying a compressive load of 1 kPa to variously modified PDMS films.
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Standard image High-resolution imageInterestingly, the sensitivity of the PDMS-based capacitive force sensor can be further improved by introducing the PEDOT:PSS additive into the PDMS active layer. Adding 1% of PEDOT:PSS into PDMS (30:1) active layer can remarkably increase the sensor's sensitivity from 0.72 ± 0.23 to 1.44 ± 0.17 kPa−1 (+100%) as clearly presented by the steeper slope in Fig. 2(b). Nevertheless, in this case, the sensitivity enhancement cannot be explained by the increment in deformability of the modified PDMS layer because the G' value of PEDOT:PSS-mixed PDMS (30:1) film is larger than that of PDMS (30:1) film, as shown in Fig. 3(a). As reported in the literature, the young's modulus of PDMS and PEDOTPSS are ∼1 MPa and ∼3 GPa, respectively. 27,29) The above information indicates that the existence of PEDOT:PSS in PDMS strengthens the composited active layer and should possibly reduce the sensitivity of the sensor due to the decrease in deformability of the active layer. However, the sensitivity of the sensor using the PEDOT:PSS-composited PDMS as the active layer does not follow the mechanical prediction. Therefore, it should have another synergetic factor that enhances the sensor's sensitivity. As stated by Eq. (1), the change in capacitance could be induced by several factors, including the change in dielectric constant (∆ε) with/without applying load to the sensor. Figure 4 presents the comparison of the dielectric constants obtained from the sensors using PDMS (30.1) and PEDOT:PSS-composited PDMS (30:1) as the active layers. The compressive load of 1.13 kPa is applied to the sensor to reveal the change in dielectric constant attributed to the deformation of modified-PDMS active layer. The significant change in the dielectric constant (+4.40%) induced by the applying compressive load is observed in the sensor using PEDOT:PSS-composited PDMS (30:1) as the active layer. The increase in the dielectric constant caused by the deformation is explained by the relative dielectric constant of composited material as expressed in Eq. (2). 15,30)
where εr, εPDMS, and εPEDOT:PSS are relative dielectric constant of PEDOT:PSS-PDMS composited materials, PDMS and PEDOT:PSS, respectively. VPDMS and VPEDOT:PSS refer to the volume fraction of PDMS and PEDOT:PSS in the active layer, respectively. From the literature, the εPDMS and εPEDOT:PSS were reported as 3.0 and 2450, respectively. 30–32) Moreover, young's modulus of PEDOT:PSS is significantly larger than that of PDMS (∼three orders of magnitudes). Therefore, the deformation in PEDOT:PSS-composited PDMS layer is rather taken place in the PDMS instead of PEDOT:PSS domains. When the external force was applied to the sensors, the volume fraction of the PDMS (low ε) is decreased while the volume fraction of PEDOT:PSS (high ε) is relatively increased. Then, the relative dielectric constant of composited layer tends to increase when the compressive load is applied to the sensor. As a consequence, the sensitivity of the sensor using PEDOT:PSS-mixed PDMS as the active layer is additionally enhanced by the change in dielectric constant (∆ε) as expressed in Eq. (1). Furthermore, the sensitivity and operating range of the capacitive force sensor using PEDOT:PSS-mixed PDMS (30:1) as the active layer are compared with those previously reported in the literature as shown in Table II. 4,12,33,34) The results indicate that the enhancement of PDMS-based capacitive force sensor by introducing PEDOT:PSS into the active layer can provide comparable sensitivity to other enhancement approaches, i.e. introducing porous-like structure to the PDMS layer and adding the conductive additive into the PDMS layer. Therefore, the modification of the PDMS active layer by means of adding PEDOT:PSS into the PDMS layer can be considered as an alternative approach to enhance the sensitivity of the PDMS-based capacitive force sensor through the change in the relative dielectric constant of the active layer. Noting that a cycle test of loading and unloading compressive load in the range of 0–23 kPa was carried out to evaluate the hysteresis behavior of the capacitive force sensors using differently modified PDMS as the active layer (see Fig. S2 in supplementary data). The hysteresis is significantly observed only in the compressive load over 2.5 kPa (low sensitivity regime). However, in the compressive range lower than 2.5 kPa, which is in the interesting regime due to the suitability of artificial skin applications, the hysteresis behavior is not significantly observed. Therefore, it could be suggested that the force sensors developed in this work have enough potential to use as the sensing component in artificial skin applications.
Fig. 4. (Color online) The comparison of the change in dielectric constants, measured with/without applying a compressive load of 1.13 kPa, obtained from capacitive force sensors using PDMS (30:1) and PEDOT:PSS-mixed PDMS (30:1) as the active layer. The number in the blanket representing the ratio of PDMS elastomer to curing agent.
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Standard image High-resolution imageTable II. Comparison of sensitivity and range of measurement obtained from capacitive force sensor using PEDOT:PSS-mixed PDMS (30:1) as the active layer to the others previously reported in the literature.
| Active layers | Sensitivity (kPa −1 ) | Linear range (kPa) | References |
|---|---|---|---|
| pure PDMS | 0.0014 | 0–10 | 33 |
| PDMS with porous structure | 0.26 | 0–0.02 | 34 |
| PDMS with Pyramidal structure | 0.55 | 0–1.2 | 35 |
| PDMS with Ag-NPs additive | 0.0072 | 0–10 | 33 |
| PDMS with Ag-NWs additive and Pyramidal structure | 0.831 | 0.–0.3 | 4 |
| PEDOT:PSS-mixed PDMS (30:1) | 1.44 | 0–1.5 | This work |
3.2. Effect of active layer modification on response/recovery times of PDMS-based capacitive force sensors
Two major factors that involve with the time-response behavior of the capacitive force sensor are (i) mechanical response time, the interval that the dielectric layer requires to deform to the final thickness, and (ii) electrical response time, the interval that the active layer requires to reach to its final capacitance. The former is related to the mechanical properties of the active layer while the latter is correlated to the charge transportation in the active layer. The rapid response/recovery time can be obtained from the capacitive force sensor equipped with high elasticity and fast charge transportation. 35,36) To investigate the effect of active layer modification on the time-dependent behavior of PDMS-based capacitive force sensors, the compressive load of 0.75 kPa was applied to the sensors and held for 30 s. Then, the compressive load was rapidly removed from the sensor. Figure 5 shows the response/recovery behavior obtained from PDMS-based capacitive force sensors prepared by differently modified PDMS active layers. The response and recovery times of each sensor extracted from Fig. 5 are shown in Table I. The modification of PDMS active layers by varying the ratio of elastomer to curing agent does not exhibit the dominant change in both response/recovery times. In this modification approach, the major factor that determines the response/recovery times of the sensors should be the mechanical behavior of the PDMS active layer. Figure 3(b) shows the value of tan δ, the ratio of loss modus (G'') to elastic modulus (G'), extracted from the differently modified PDMS active layers. It is worth noting that the low value of tan δ implies the high elastic-dominant material, which consequently provides the rapid time-response behavior. According to Fig. 3(b), the low values of tan δ indicate that all PDMS films modified by varying the ratio of elastomer to curing agent show the majorly elastic-dominant films. However, tan δ values obtained from all modified PDMS films are indistinguishable (less than 0.05). This result suggests that the mechanical time-response of all PDMS films should be indifferent. Therefore, the response/recovery time of the sensors using PDMS (10:1), (20:1), and (30.1) as the active layer does not show significant improvement.
Fig. 5. (Color online) Response and recovery characteristics, induced by applying a compressive load of 0.75 kPa, obtained from capacitive force sensors using differently modified PDMS as the active layer.
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Standard image High-resolution imageThe drastic shortening in response time (−72%) and recovery times (−67%) of the sensor using PEDOT:PSS-mixed PDMS (30:1) as the active layer is shown in Fig. 5 and Table I. However, the value of tan δ obtained from the composited PDMS film, shown in Fig. 3(b), is not much different compared to the others. Therefore, it should have another key parameter that affects the time-response behavior of the sensor using the composited PDMS as the active layer. Charge–discharge characteristics of all modified PDMS layers were investigated to reveal the ability of charge transportation in the active layers. The charge–discharge characteristic curves of all modified PDMS layers are presented in Fig. 6 and the charging and discharging times are shown in Table III. Figure 6 clearly indicates that the PEDOT:PSS-mixed PDMS (30:1) film exhibits the fastest in both charging and discharging times while the others modified PDMS films show similar charge–discharge behaviors. The shortening in charge–discharge behavior suggests that charge transportation in the composited PDMS film is faster than that of other modified PDMS films. Therefore, the reduction in response/recovery times of the sensor using PEDOT:PSS-mixed PDMS as the active layer is attributed to the improvement in charge transportation in the sensor that sequentially allows the capacitive force sensor reach to the final capacitance within a shorter interval time.
Fig. 6. (Color online) Normalized charge and discharge characteristics obtained from capacitive force sensors using differently modified PDMS as the active layer. The applying current and charge–discharge period were 1 nA and 10 s, respectively.
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Standard image High-resolution imageTable III. Charging–discharging times obtained from capacitive force sensors using differently modified PDMS as the active layer. The applying current and charging–discharging period are fixed at 1 nA and 10 s, respectively.
| Type of active layer | Charging time (s) | Discharging time (s) |
|---|---|---|
| PDMS 10:1 | 6.74 | 6.43 |
| PDMS 20:1 | 6.71 | 6.45 |
| PDMS 30:1 | 6.65 | 6.66 |
| PDMS 30:1 (PEDOT:PSS) | 4.18 | 4.08 |
3.3. Durability and reliability test of capacitive force sensors using modified PDMS as the active layer
To reveal the durability and reliability of capacitive force sensors developed in this work, the characteristic of capacitance caused by loading-unloading cycles with the compressive load was carried out. The constant compressive load of 1.13 KPa was repeatedly applied to the sensors over 100 cycles. Figure 7 presents the change of capacitance over the loading-unloading cycles of each capacitive force sensor using differently modified PDMS as the active layer. The change in the initial capacitance is used as a parameter to reveal the durability of the sensors while the change in capacitance induced by the applying load is employed to evaluate the reliability of the sensors.
Fig. 7. (Color online) Durability and reliability test results of capacitive force sensors using different modified active layers; (a) PDMS (10:1), (b) PDMS (20:1), (c) PDMS (30:1), and (d) PEDOT:PSS-mixed PDMS (30:1) over 100 cycles with the compressive load of 1.13 kPa.
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Standard image High-resolution imageFrom a durability point of view, the rapid degradation in the initial capacitance (approx. 0.15%) is observed in all sensors at the first 10 cycles. However, the further applying cycles, only the sensors using PDMS (30:1) as an active layer exhibit the further degradation in the initial capacitance (approx. 0.23%) while the other sensors do not show any significant deterioration. Noting that, the continuing decrease in the initial capacitance over the test cycle observed in the sensor with PDMS(30:1) is in agreement with the mechanical result indicating the lowest elastic modulus of PDMS(30:1) compared to the others. Interestingly, by introducing PEDOT:PSS additive into PDMS(30:1), the durability of the sensor can be enhanced due to the increase in mechanical robustness of PEDOT:PSS-mixed PDMS as clearly seen in Fig. 3(a). From a reliability viewpoint, the small differences in the change of capacitance measured from the 1st to the 100th cycle are observed in all sensors (approx. 3.25%). The smallest fluctuation of 1.78% is observed in the sensor using PEDOT:PSS-mixed PDMS(30:1) as the active layer.
From the above-mentioned results involving sensitivity, response time, durability as well as reliability, the capacitive force sensors developed in this work shows sufficient potential to use as a sensing component in artificial skin applications. Especially, the sensors using PEDOT:PSS-mixed PDMS (30:1) as the active layer that provides excellent sensing characteristics. To demonstrate the possibility to use the developed sensor in the practical application, the capacitive force sensor using PEDOT:PSS-mixed PDMS(30:1) as the active layer was attached to the finger in order to sense the bending of a finger at various angles. The distinguishable real-time signals obtained from the different bending angles, as exhibited in Fig. 8, evidently demonstrates the possibility to apply the capacitive force sensor using PEDOT:PSS-mixed PDMS(30:1) as the active layer in artificial skin applications.
Fig. 8. (Color online) Sensing characteristic of the capacitive force sensor using PEDOT:PSS-mixed PDMS (30:1) as the active layer measured from the bending finger with different angles.
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4. Conclusions
In conclusion, the sensing characteristics, e.g. sensitivity and response/recovery times of PDMS-based capacitive force sensor are enhanced by means of modification of active layers. Two modification perspectives are proposed; (i) varying the ratio of PDMS elastomer to curing agent and (ii) introducing the PEDOT:PSS, a conducting polymer, to the PDMS layer. From the sensitivity point of view, the increase in PDMS elastomer fraction from (10:1) to (30:1) shows a significant enhancement in sensor sensitivity as high as 80% due to the deterioration of mechanical properties in the PDMS active layer. The further increment in sensitivity of 63% can be achieved by adding 1% wt of PEDOT:PSS into the PDMS (30:1) layer. The existence of PEDOT:PSS in the PDMS layer provides the change in dielectric constant during the applying compressive load to the sensor leading to the additional enhancement in the sensor's sensitivity. In the response/recovery time, the change in PDMS elastomer to curing agent ration does not have a significant influence on the time-response characteristics of the sensor. However, the modification of the active layer by adding PEDOT:PSS into the PDMS layer provides a drastic reduction in both response and recovery times, attributed to the enhancement in charge transportation took place in the composited active layer. The test of 100 cycles of loading-unloading compressive load in the range of 1.13 kPa indicates the sufficient durability and reliability to employ the modified capacitive force sensors developed in this work for practical applications. Finally, two modifying approaches proposed in this work could be applied to other enhancement perspectives, e.g. introducing the nanostructure to the active layer, to achieve the excellent sensing characteristics of PDMS-based capacitive force sensor.
Acknowledgments
The first author was supported by the master's degree scholarship from King Mongkut Institute of Technology Ladkrabang Research Fund [Grant No. KREF016326]. This research was supported by the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [Grant No. B05F630020]. The HAAKE MARS rheometer used in viscoelastic measurements was kindly supported by LMS Instruments Co., LTD. with the valuable suggestions from Dr. Preeyanuch Prasopnatra.