Electrical Resistivities of PVA/Activated Carbon-Based Hydrogels

This study investigates the physical and electrical properties of the polyvinyl alcohol (PVA) hydrogel incorporated with edible Activated Carbon (AC). Three samples, namely PVA, PVA/AC 0.5, and PVA/AC 1.0 were prepared using the freeze-thaw method. The samples underwent six freeze-thaw cycles, each consisting of freezing at –25°C for 20 hours and subsequent exposure to room temperature for 4 hours. The porous network of hydrogel is attributed to the hydroxyl groups of PVA, resulting from the intermolecular cross-linking of PVA chains. The edible AC was uniformly dispersed within the hydrogel network, leading to a reduction in hydrogel pores. As a result, the electrical resistivity of PVA, PVA/AC 0.5, and PVA/AC 1.0 hydrogels measured 1052.9 ± 165.0 Ω.cm, 403.1 ± 29.2 Ω.cm, and 59.9 ± 4.7 Ω.cm, respectively. The incorporation of edible AC significantly decreased the resistivity of the hydrogel. So, this hydrogel is promising for biomedical and edible electronics applications.


Introduction
Hydrogels, categorized as polymer-based substances, possess a unique 3D structure with interconnected networks and are notable for their remarkable capacity to absorb and retain large amounts of water [1].They can be produced from various polymers, both natural and synthetic.Hydrogels have broad applications in the biomedical, food industry, and agriculture.However, the biomedical field stands out due to diverse uses such as wound healing, drug delivery, antibacterial devices, and tissue engineering, making it an intriguing area of exploration [2][3][4][5][6].Additionally, hydrogels show promise in edible electronic applications like implantable and wearable devices, as well as electrodes.These applications require good conductivity, tissue-like behaviour, responsiveness, and biodegradability.The conductivity of hydrogels is crucial for monitoring health conditions in the digestive tract by detecting electronic signals from body fluids and transmitting these signals for diagnosis [7][8][9][10].
Polyvinyl alcohol (PVA) is a synthetic polymer endowed with hydrophilic properties due to the presence of hydroxyl groups (-OH).This hydroxyl group renders PVA highly suitable as a matrix for manufacturing hydrogels.Moreover, PVA boasts non-toxic attributes, excellent mechanical properties, and biocompatibility, making it a favoured choice in biomedical applications [11][12][13][14].However, the resistivity of PVA hydrogel remains relatively high, necessitating the incorporation of supplementary materials like activated carbon to enhance its electrical characteristics.
Activated carbon (AC), a carbon-based solid possessing a well-developed porous structure with abundant active sites and diverse pores, proves particularly valuable [15].Edible-AC emerges as a practical substance for tracking fluids in the digestive tract due to its non-toxic nature, relatively high conductivity, and expansive surface area [16].Hydrogels with improved electrical properties can be manufactured by integrating activated carbon into the PVA matrix.This combination reduces the resistivity of PVA-based hydrogels, rendering them viable for applications in edible electronics.
This study concentrates on preparing hydrogels using the freeze-thaw method and evaluating their physical and electrical attributes employing PVA as the matrix.The addition of edible activated carbon was explored to decrease the hydrogel's resistivity.This research aims to establish hydrogel properties conducive to advancement in biomedical and edible electronics applications through morphological and resistivity assessments.

Materials
Polyvinyl alcohol (PVA) 99+% hydrolyzed (MW = 89,000 -98,000 Da) was purchased from Sigma Aldrich-Singapore, edible Activated Carbon (AC) was provided by PT.Puri Rempah-Indonesia, and demineralized water was obtained locally.There is no further purification process for all these materials.

Preparation of PVA/AC hydrogels
PVA was dissolved in demineralized water at a concentration of 10 wt% and stirred at 110°C for 4.5 hours using a magnetic stirrer.Once a uniform solution was achieved, the PVA solution was gradually cooled to room temperature under continuous stirring.Following this, activated carbon (AC) was added into the PVA solution and stirred at room temperature for 1 hour to ensure complete dispersion of the AC.The resulting solution was allowed to undergo a degassing process for 1 hour.
Subsequently, the blended solution was carefully poured into a 5x2.5 cm template.The final step involved employing the freeze-thaw method to produce the PVA/AC-based hydrogel.In the freezing phase, all samples were subjected to -25°C for 20 hours, followed by a thawing process wherein the samples were left at room temperature for 4 hours.The freeze-thaw process was repeated six times (6 cycles).For a visual representation of the hydrogel fabrication process and the variations in sample composition, refer to Figure 1 and Table 1, respectively.

Characterization
The surface morphology of PVA/AC-based hydrogels was observed using a JEOL JSM-6510LA Scanning Electron Microscope (SEM).The images were captured at a magnification of 5000 times.Three distinct samples were examined: PVA, PVA/AC 0.5, and PVA/AC 1.0.Before analysis, the hydrogels underwent a brief drying process to eliminate surface water molecules.Additionally, the electrical properties were evaluated by measuring the resistance of the hydrogels using the four-point probe method with a Keysight 34465A 6½ digit Multimeter.The hydrogel's electrical resistivity value was calculated using equation (1) as follows: where ρ is the electrical resistivity (Ω.cm), R is the electrical resistance of hydrogel (Ω), A is the surface area of hydrogel (cm 2 ), l is the width of hydrogel (cm), t is the thickness of hydrogel (cm), and d is the distance of sense probes (cm).

PVA/AC-based hydrogels morphological analysis
The PVA/AC-based hydrogels produced by the freeze-thaw method for six cycles are shown in Figure 2. The PVA hydrogel is visually recognizable by its solid white hue (Figure 2a).As the AC content within the hydrogel rises, the PVA/AC hydrogel transitions to a deep black shade (provided in Figure 2b and Figure 2c).Adding AC content at PVA/AC 0.5 or PVA/AC 1.0 does not significantly alter the hydrogel's appearance.All the hydrogels exhibit a dense structure.The solidification of the hydrogel is attributed to the creation of crosslinks among polymer chains linked to PVA hydroxyl groups, driven by water molecules during freezing.Subsequently, these crosslinks formed and organized further during the thawing process [17,18].The opacity of the PVA/AC hydrogel increases with an elevation in freeze-thaw cycles [6,19].Figure 3 illustrates the surface morphology of PVA and PVA/AC hydrogels, which were produced using the freeze-thaw method for six cycles.In Figure 3(a), the PVA hydrogel displays a collapsed surface with unevenly distributed pores.This outcome arises from the retention of water molecules within the pores and on the hydrogel's surface.Nonetheless, the porous architecture of the hydrogel remains discernible, attributed to the separation of liquid and solid phases.This process involves freezing the liquid phase into ice crystals, gradually expanding and intersecting with each other [20].The solid phase corresponds to the crystalline region, resulting from intermolecular cross-linking facilitated by the PVA chain's hydroxyl functional groups (-OH) [21].
Observing the PVA/AC hydrogels, specifically the PVA/AC 0.5, reveals scattered pores across the surface, albeit smaller than those in the pure PVA hydrogel.Conversely, the PVA/AC 1.0 hydrogel indicates surface collapse due to water molecules confined within the pores and the hydrogel's surface.This collapse is further exacerbated by an increased content of AC within the PVA matrix.Notably, even the pore diameter, in this case, is tiny.Figure 3 substantiates the trend of diminishing pore size attributed to the escalating dispersion of AC particles within the PVA hydrogel matrix.

Electrical resistivity of PVA/AC hydrogels
An essential aspect of utilizing hydrogels in edible electronics is their electrical conductivity, a crucial property.Electrical conductivity, functioning inversely to electrical resistivity, governs the capacity to transmit electrical signals.This attribute becomes particularly relevant when hydrogels are employed for monitoring and diagnosing human body fluids.The measurement of electrical resistance in PVA/AC hydrogels was carried out using the four-point probe method.The values encompassing electrical resistance, along with the hydrogel's thickness, width, and the distance between the sensing probes, are meticulously outlined in Table 2.By substituting the values from Table 2 into equation ( 1), the electrical resistivity of the PVA/AC hydrogel is calculated.The resulting electrical resistivity value is presented in Figure 4.The electrical resistivity of hydrogels is 1052.9 ± 165.0 Ω.cm, 403.1 ± 29.2 Ω.cm, and 59.9 ± 4.7 Ω.cm for PVA, PVA/AC 0.5, and PVA/AC 1.0, respectively.Incorporating edible AC into PVA hydrogel significantly decreased the resistivity of the hydrogel.A low electrical resistivity value signifies heightened electrical conductivity within a material.This characteristic implies that the PVA/AC hydrogel exhibits superior conductivity properties compared to the pure PVA hydrogel.Reducing the hydrogel's electrical resistivity value could render it more suitable for applications in edible electronics.

Conclusion
In this study, we have successfully synthesized PVA/AC-based hydrogels incorporating varying levels of AC content.The investigation encompassed an analysis of the physical and electrical attributes of the PVA/AC hydrogels, prepared using the freeze-thaw method for six cycles.The initial PVA-based hydrogel exhibited discernible pores attributed to intermolecular crosslinking facilitated by the hydroxyl groups of the PVA chain.Nevertheless, the distribution of these pores exhibited unevenness due to the entrapment of water molecules within the pores and on the hydrogel's surface.Adding AC into the PVA hydrogel matrix yielded hydrogels with reduced pore diameters.This incorporation aimed at enhancing the hydrogel's electrical properties, as evidenced by decreased electrical resistivity values: 1052.9 ± 165.0 Ω.cm for PVA, 403.1 ± 29.2 Ω.cm for PVA/AC 0.5, and 59.9 ± 4.7 Ω.cm for PVA/AC 1.0 hydrogels.These outcomes highlight the potential of PVA/AC-based hydrogels for biomedical applications and emerging fields like edible electronics.

Table 1 .
The sample composition variations of PVA/AC-based hydrogels.

Table 2 .
The data of electrical resistance, width, and thickness of PVA/AC hydrogels and distance of the sensing probes.