The growing demand for insulin/medicine-delivery devices has stimulated intensive research on the development of wearable, micropump-based electronic devices [1]. Electroosmotic (EO) pumps, also known as electrokinetic pumps, are attracting increasing attention in drug-delivery systems owing to their fascinating features, such as simplicity, non-pulsation flow, high output pressure, low power consumption, and adjustable flow proportional to the applied voltage [1–5]. Over the years, Pt has been the preferred electrode material for EO pumps [5,6]. However, the critical limitation of consumable reactive metal electrodes, such as Pt-based electrodes, is that they must undergo electrolysis in an electrolyte, thereby generating gas bubbles and pH-altering species [1,7–10]. Such undesirable products can cause an inconsistent flow rate and a significant deterioration in the performance of the EO pump. Conducting polymers (CPs) have been employed as alternative electrode materials [4,9,11]. Unlike reactive metal electrodes, no polymer is released into the electrolyte because an electron moves through the conjugated molecule. To enhance the electrochemical performance and stability of polyaniline (PANI), organic sulfonic acids, such as poly(styrene sulfonic acid) (PSSA), are used as the stabilizer and doping agent [12]. In addition, the formation of PANI with negatively charged PSSA segments can enhance the redox activity of the CPs [13]. The CP, PANI, has been broadly employed in a variety of devices, including supercapacitors, electrochromic devices, sensors, solar cells, and fuel cells, because of its low cost, ease of synthesis, high electrical conductivity, and high active surface [14–20]. Besides PANI, numerous other redox polymers, such as poly(anthraquinone), polyquinone, PEDOT, and others, have been reported for EOP applications in the literature [9,21,22]. Nevertheless, few studies have investigated its application as an electrode material in EO pumps.

Portable and wearable electronic devices have recently become invaluable in various fields, particularly in the medical and healthcare fields. Consequently, they are attracting significant research interest [23]. Unlike other types of pumps, the EO pump is well suited to these applications because it operates without any mechanical moving parts and can be easily miniaturized to a size that enables its use in applications, such as insulin- or pain-medication delivery. The EO pump consists of two electroactive electrodes and a porous membrane, and it can achieve a sufficient pumping curve by transporting fluid without noise. However, one of the major factors limiting the flexibility of EO pumps is the rigid characteristics of the electrode material. The presence of supporting materials is inevitable in the fabrication of an electrochemically active electrode. The most common supporting material for electrode materials in EO pumps is carbon paper [7,10,24,25]. Although carbon paper is advantageous, it has critical shortcomings. For example, it is difficult to handle and collect during electrode fabrication and manufacturing processes or pump assembly as it can be easily torn or broken. Therefore, the nature of the supporting material is the key prerequisite for constructing a flexible electrode for EO pumps that must satisfy the multiple requirements for practical usage, including high material flexibility, mechanical strength, electrical conductivity, porosity, and electrochemical stability [26].

In this study, we developed a highly efficient and flexible PANI:PSS/carbon cloth (CC) electrode for EO pumps using an electrodeposition method. PANI:PSS was electrochemically synthesized on the CC substrate in an acidic aqueous medium via a cyclic voltammetry (CV) technique. Thereafter, the product was characterized by a variety of physicochemical methods, including field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy, electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The overall performance of the EO pumps incorporating the PANI:PSS/CC electrode was evaluated in a microfluidic system and compared with that of the pump incorporating the bare CC to validate the effects of the PANI:PSS coating. The performance and stability of the developed PANI:PSS/CC electrode were further demonstrated using an infusion device system.

Fig. 1 shows a typical FESEM image and the corresponding EDX result of the pristine CC used as the supporting material for the EO pump. As shown in Fig. 1a, the pristine CC had well-woven carbon fibers, with an average diameter of ca. 8.0 µm and a very smooth surface. As shown in the EDX result in Fig. 1b, the pristine CC predominantly consists of carbon with a small portion of oxygen, and no other impurities were observed.

PANI:PSS was fabricated on the CC via an electrodeposition method using a three-electrode system. A CC sheet as the WE was mounted in an electrode holder, along with Ag/AgCl as the RE and Pt wire as the counter electrode. Fig. 2 shows the typical cyclic voltammograms for the bare CC WE at a scan rate of 50 mV s^–1 in a 0.5 M H₂SO₄ solution with and without 0.5 M aniline and 4 mM PSSA.

The potential was scanned for 30 cycles between –0.4 V and +0.8 V (Ag/AgCl), as PANI was the redox active. Note that the upper potential limit was set to +0.8 V because the half-oxidized emeraldine state of PANI can undergo full oxidation to polypemigraniline above +0.8 V. The peak at less than +0.8 V is attributed to the oxidation of PANI from the fully reduced form of PANI, leucoemeraldine, to the half-oxidized emeraldine state [28–31]. After 30 cycles, the color of the CC electrode turned dark green, as shown in the inset in Fig. 2. The CV curves show the current plateau in the sulfuric acid electrolyte, indicating electrochemical inactivity. Conversely, distinct redox peaks corresponding to the PANI oxidation (+0.5 V) and reduction (–0.2 V) states in the mixed solution of aniline, polystyrene sulfuric acid, and sulfuric acid were observed.

The morphology of the as-prepared CC and PANI:PSS/CC are presented in Fig. 3. Fig. 3a shows the featureless morphology of the bare CC with a very smooth surface. Fig. 3b show the images of the PANI:PSS composite directly electrodeposited on the CC, where the surface of the CC was well covered by rough and densely packed nanoparticles. The morphology of PANI:PSS/CC clearly indicates that the nucleation and growth of PANI:PSS primarily occurred on the carbon fibers of the CC. The carbon fibers in the CC can provide highly accessible active sites for electrochemical polymerization of aniline monomer and, thus, result in a relatively uniform coating of PANI on the CC. The elemental composition was analyzed by EDX, as shown in Fig. 1, and the data are listed in Table 1. For the CC, no apparent content of nitrogen and sulfur was detected, as shown in Fig. 1b. For PANI:PSS/CC, the nitrogen and sulfur contents were 6.69% and 3.27%, respectively, indicating the presence of PANI and PSS in this sample. The absence of any other peak reveals that there is no significant impurity in this sample. From the FESEM and EDX results, it can be concluded that PANI:PSS was well coated onto the CC and that the electropolymerization occurred mostly on the surface of the carbon fibers. To achieve homogeneous and uniform PANI:PSS attached to the surface of the carbon fibers, several electrodeposition parameters including monomer concentration, electrolyte composition, the number of cycles, potential range, scan rate, bath temperature have been optimized.

Fig. 4 shows the 2D elemental mapping images of the PANI:PSS/CC sample obtained by EPMA. The elemental mapping images indicate the spatial distribution of the corresponding elements, i.e., C, N, and S, present in the PANI:PSS/CC. It was observed that the density distribution of nitrogen matched well with that of carbon, and the nitrogen coexisted with sulfur. Nearly uniform and well-dispersed nitrogen and sulfur elements were observed along the carbon fibers, revealing that PANI:PSS was relatively evenly distributed over the surface of the carbon fibers.

To further investigate the surface chemical composition of the as-prepared electrodes, XPS analyses were performed, and the results were compared with the EDX results in Table 1. Fig. 5 shows the survey XPS spectra of CC and PANI:PSS/CC. The survey spectrum of PANI:PSS/CC is similar to that of the CC, except for the existence of sulfur and the higher nitrogen content. For the CC, two high-intensity peaks at –284.3 eV and 532.3 eV and a very low-intensity peak at –400.8 eV were observed, corresponding to C 1s (81.55 at.%), O 1s (17.29 at.%), and N 1s (1.16 at.%), respectively. The binding energy peaks of carbon, oxygen, nitrogen, and sulfur were observed on the XPS spectrum of PANI:PSS/CC. Based on the results obtained from XPS, the existence of sulfur and relatively higher nitrogen content after electropolymerization demonstrates that PANI:PSS was successfully synthesized on the surface of CC via CV, which is in good agreement with the EDX and EPMA results. Table 1 summarizes the elemental composition of PANI:PSS/CC obtained by the EDX and XPS measurements. From the EDX analysis of PANI:PSS/CC, the nitrogen and sulfur contents were determined to be 6.69 at.% and 3.27 at.%, respectively. From the XPS analysis, the nitrogen and sulfur contents in the same electrode were determined to be 5.94 at.% and 4.12 at.%, respectively. The observed differences between the EDX and XPS results might be due to the differences between the surface and bulk chemistry of the coated material. According to the XPS results, sulfur appears to be slightly more abundant on the surface of PANI:PSS/CC, in contrast with the EDX results. However, we believe that the differences between the nitrogen and sulfur contents in the electropolymerized PANI:PSS/CC electrode obtained by the two methods are not significant [32].

The structure of the as-prepared electrodes was studied by XRD, as shown in Fig. 6. The XRD pattern of PANI:PSS/CC is similar to that of CC, considering the peak overlap between the carbon and emeraldine form of semicrystalline PANI. It has been reported that the diffraction peaks for the emeraldine form of PANI can be found at approximately 2θ = 19°–22° and 25°–27°, corresponding to the (020) and (200) planes of PANI, which have parallel and perpendicular periodicity chains, respectively [33–38]. PANI is known to exhibit some crystallinity, which can be ascribed to the repetition of benzenoid and quinoid rings in the PANI chains [12]. The XRD patterns of CC and PANI:PSS/CC exhibit a strong broad diffraction peak centered at ca. 2θ = 25°, corresponding to the (002) plane of graphite. Another peak with a relatively low intensity appears at ca. 2θ = 43°, corresponding to the (100) plane of graphite according to JCPDS card No. 75-1621 [33–38]. The XRD results indicate that no major structural changes, evidenced by peak shifts and the appearance of other peaks related to impurity phases, occur in the electropolymerized PANI:PSS/CC.

The FTIR spectra of CC and PANI:PSS/CC are shown in Fig. 7. The samples were analyzed in the wavelength range of 4000–500 cm⁻¹. Compared with the CC spectrum, the PANI:PSS/CC spectrum displays some characteristic peaks. For PANI:PSS/CC, the adsorption peak at approximately 1121 cm⁻¹ is assigned to the stretching vibration of the –SO₃⁻ group in PSS, revealing that the prepared PANI were doped with PSS [39–41]. The peaks located at –1305 cm⁻¹ and 1379 cm⁻¹ correspond to the π-electron delocalization induced in the polymer through protonation or C–N–C stretching vibration and the C–N= stretching vibration between benzenoid and quinoid units, respectively [40–45]. The peaks appearing at –1486 and 1559 cm⁻¹ are attributed to the stretching vibrations of the C=C benzenoid (N–B–N) and quinoid (N=Q=N) rings, respectively [39–45]. The peak at 1627 cm⁻¹ can be assigned to the stretching vibrations of the C–C bonds of both tri- and tetrasubstituted aromatic groups [42–45]. Thus, the FTIR results provide further evidence of the successful electropolymerization of PANI and PSS on the CC.

The application of the developed PANI:PSS/CC was demonstrated in a low-voltage EO pump, and the schematics of the assembled pump with the net reactions are presented in Fig. 8a. The EO pump was configured as shown in Fig. 8b, where a single silica nanopore membrane was symmetrically sandwiched with each of the flow-through electrodes, contact SUS strips, and PC cases. Note that adequate flushing was carried out using a syringe pump to remove air bubbles from the fluidic pathway to prevent their detrimental effects on the pump’s performance [46]. The details of the pump assembly and preactivation process have been reported in our previous publication [10]. The pump’s performances were evaluated by operating at a constant potential under different flow-opposing backpressures. The process that occurs on the PANI:PSS/CC electrodes of the EO pump can be described by the following reactions [47]:

where H⁺ and e⁻ indicate the proton and electron, respectively.

The number of electrons on the PANI backbone does not change during the reversible redox reactions, but only the generated hydrogen ions (H⁺) can freely move toward the opposite side of the electrode through the surface-charged porous media, which can induce concerted flow of the bulk fluid. In the PANI:PSS composite structure, PANI could act as a conductive medium, and the PSS could promote hydrophilicity and proton swapping. Fig. 8c shows the dependence of the peak current (I_peak) on the applied constant potential (V_app). The applied potential is varied from 1 to 4 V. The peak current shows a linear dependence on the applied potential. There was a linear relationship with a regression coefficient of R² ≥ 0.99 between the applied potential and the peak current, highlighting the conductive nature of the electrode material. The peak current of the EO pump built with PANI:PSS/CC was higher than that of the pump with the bare CC, indicating that the PANI:PSS/CC electrode is more electrochemically active than the bare CC. Almost no gassing was observed in the EO pump incorporating the PANI:PSS/CC electrode even with continuous operation at applied potentials of up to 4 V, whereas gas bubbles were observed for the CC case, particularly at relatively high potentials (>3 V). Such gas bubbles, which evolved during the pumping, could inhibit parts of the surface pores of the electrode and/or membrane, causing an unequal distribution of the fluid flow and the decay of the EO flow [4,7–9,46]. To study the effects of the PANI:PSS coating on the EO pumping performances, the pressures and flow rates of the EO pumps built with CC and PANI:PSS/CC electrodes were measured using a microfluidic system, which consists of digital flow and pressure sensors, as well as pressure generator. Fig. 8d shows the pump-performance (Q–P) curves for the CC and PANI:PSS/CC electrodes obtained at 2 V using DI water as the working fluid. In the presence of backpressure (P), the flow rates (Q) were measured sequentially from the smallest to largest using a digital microfluidic flow sensor. As shown in Fig. 8d, the volumetric flow rate linearly decreases as the flow-opposing pressure increases, regardless of the type of electrode used. The measured maximum pressures (P_max) for the EO pumps with CC and PANI:PSS/CC were 91 and 32 kPa at 2 V, respectively. The maximum flow rates (Q_max) at 2 V for the EO pumps with CC and PANI:PSS/CC were 13.9 and 52.1 μL min⁻¹, respectively. Note that P_max is obtained at a zero net flow rate, whereas Q_max is measured under a zero-backpressure condition. The obtained maximum fluxes are 7.0 and 26.1 μL min⁻¹ V⁻¹ cm⁻² for the EO pumps with CC and PANI:PSS/CC, respectively. These values are considerably higher than those previously reported, particularly for metal-electrode-based EO pumps. In the literature, experimental data of the normalized EO performances lie in the ranges of 0.1–8 kPa V⁻¹ and 0.1–25 μL min⁻¹ V⁻¹ cm⁻² for P_max and Q_max, respectively [1,7,47–52]. Many applications, such as drug delivery, have finite pressure loads. For example, a subcutaneous tissue pressure of approximately 7.4 ± 7.8 kPa at an injection rate of 1 mL min⁻¹ is required to mitigate back pressure and pain when using a drug-delivery pump [53]. Considering a typical pump operating in the middle point of the plot, the flow rate (Q) of the EO pump with PANI:PSS/CC delivered at 2 V was 38.4 μL min⁻¹ at the backpressure of 20 kPa, which was –7.9 times higher than that for the bare CC (4.7 μL min⁻¹). The EO pump with PANI:PSS/CC exhibited considerably better EO performances than that with the bare CC, highlighting the pronounced effects of PANI:PSS on the EO flow performance. The higher EO flux afforded by PANI:PSS/CC compared with that by the bare CC may be due to the relatively high number of exposed active sites for redox reactions, the comparatively high conductivity attributed to PANI, and the efficient hopping of protons attributed to PSS. The Q value can be calculated using the following relationship:

where Q is the flow rate and P is the flow-opposing pressure.

The calculated Q values at a backpressure of 20 kPa for the EO pumps with CC and PANI:PSS/CC are 5.3 and 40.5 μL min⁻¹, respectively, which are in good agreement with our experimental data.

The application of the developed PANI:PSS/CC electrode was demonstrated in an EO-pump-based infusion system. The system was designed to mimic an infusion device, such as an insulin patch, consisting of check valves and a 2-mL reservoir filled with DI water, as shown in Fig. 9a. Further, the pumped fluid was quantitatively measured in real time by a digital flow sensor at room temperature, as shown in the inset of Fig. 9b. The EO pump built with PANI:PSS/CC was continuously operated at ± 2 V until the reservoir was completely discharged or up to 6 days, stably delivering a total volume of approximately 1.6 mL of the reservoir solution at a mean flow rate of 13.34 µL min⁻¹, which is sufficient to infuse basal or bolus insulin doses for patients with diabetes. Note that the total volume of the reservoir solution to be used for testing is less than 2 mL, due to the priming procedure employed to fill the water in the fluidic pathways and push any air out of the tubing before the testing. The obtained Q value is lower than the Q_max of the pump without an infusion system, due to the finite system resistances exerted by the check valves and the reservoir. Moreover, the current was stably maintained for 6 days of continuous operation at 2 V, as shown in Fig. 9c. These results suggest that there was negligible degradation in the performance and life span of the EO pump incorporating the PANI:PSS/CC electrode. The utilization of a high-performance and nearly reversible electroactive PANI:PSS/CC electrode enables efficient and durable EO pumping at a very low potential.

In this work, we successfully demonstrated a high-performance and robust EO pump built with an electrochemically active polymer PANI:PSS/CC electrode. PANI:PSS was fabricated on a flexible, porous, and conductive CC via CV and characterized by a variety of physicochemical methods. The EO pump assembled with PANI:PSS/CC electrode presented a maximum pressure of 91 kPa at 2 V, with a maximum flux of 26.1 μL min⁻¹ V⁻¹ cm⁻². The EO pump with PANI:PSS/CC exhibited considerably better EO performances than that with the bare CC, indicating the pronounced effects of the PANI:PSS coating on the pump’s performance. The performance and stability of the developed PANI:PSS/CC electrode were further demonstrated using an infusion device system. The test results showed that the EO pump with the PANI:PSS/CC electrode stably delivered a total volume of ca. 1.6 mL of the reservoir water solution at a mean flow rate of 13.34 μL min⁻¹, which is sufficient for the infusion of basal–bolus insulin doses for patients with diabetes.