Repurposing PVA-based slime to address electrolyte challenges in portable electrochemical devices

Abstract

Application-oriented materials are being developed at a rapid pace, each providing enhanced properties compared to the existing options. While it is evident that these materials are proven to contribute positively to the intended application, they are often not translated to commercial utilization due to multiple reasons. Some of the reasons are the lack of ready availability of raw materials, their high cost or the requirement of multistep procedures of synthesis. In some cases, the reported shelf life of the material was evaluated only for a short span or has been studied under simulated accelerated conditions, which may vary in real-life scenarios. All these factors pose significant hurdles to their commercialization. Under such circumstances, repurposing appropriate commercially available materials, which exhibit enhanced shelf life and cost-effectiveness for novel applications, presents a feasible route to reduce the time required for commercial deployment. This perspective paper focuses on the scope of repurposing poly vinyl alcohol (PVA)-based slime as an electrolyte for portable electrochemical devices. An electrochemical measurement necessitates the presence of adequate salts for ionic conductivity. The conventional liquid electrolytes cause corrosion when spilled or seeped into the underlying surface. A spill-proof electrolyte with adequate flow properties is crucial for the development of in situ electrochemical probes. The gel electrolytes, which are currently being explored widely, have their own challenges. Originally marketed as a sensory toy, due to its low cost, inert, biocompatible nature, strippability, and long shelf life, slime has the potential to be used as an electrolyte because of the fact that borax in it provides sufficient ionic conductivity. Thus, PVA-based slime with its unique combination of characteristics is a potential material to be explored as another alternative to liquid electrolyte and conventional polymeric gels. However, to date, the medium has only been engineered from the perspective of improving mechanical properties to suit applications. This work analyzes the perspective of engineering slime medium for enhanced flow properties to achieve high ionic conductivity and conformability while retaining its strippability and spill-proof nature.

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Anu Renjith

Dr Anu Renjith completed her PhD at the Raman Research Institute, focusing on electrochemistry in soft ionically conducting media, particularly deep eutectic solvents and ionic gels. She pursued post-doctoral research at the Indian Institute of Science, developing portable analyzers for rapid food quality assessment. She also had a brief stint working with polymeric materials towards painless injections. Currently, at National Aerospace Laboratories, she works on developing portable corrosion sensors for aircraft surface monitoring using alternative electrolytes.

V. Lakshminarayanan

Prof. V. Lakshminarayanan is a material scientist who was associated with the Soft Condensed Matter group of the Raman Research Institute, Bangalore, India until he retired as a Senior Professor in 2014. Before joining the Institute in 1981, he was at the National Aerospace Laboratory working in the area of metallic corrosion of aircraft materials. His scientific contributions span electrochemistry of metallic corrosion, metal finishing, molecular self-assembly on surfaces and soft matter electrochemistry.

Harish C. Barshilia

Dr Harish C. Barshilia is a Chief Scientist at CSIR-National Aerospace Laboratories, Bangalore. He obtained his M. Tech. and PhD in Physics from the Indian Institute of Technology, Delhi, and pursued postdoctoral research at the University of Missouri, St. Louis, and the City University of New York, USA. His research focuses on nanoscience, nanotechnology, thin films, and surface engineering. He has published over 270 journal papers, 7 book chapters, and holds 17 patents.

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Introduction

With the increasing necessity for surface monitoring in industries such as construction, semiconductors, aerospace, and automotive manufacturing, regular assessments play a crucial role in extending the lifespan of critical components. For decades, electrochemical techniques have been used under controlled laboratory conditions for characterizing material targeted for applications in catalysis, energy storage, fuel cells, corrosion resistance, etc.¹ Even today, these techniques are the first choice when it comes to validating the efficiency of coatings and corrosion resistance of materials. Electrochemical techniques involve applying an external signal, typically in the form of either electrical current or voltage (potential), to a system and observing its response. Metal surface changes, such as oxidation or reduction, involve electrochemical transformations, which are directly reflected in the response signals such as current, potential, charge density, resistance (or impedance), capacitance, or inductance, depending on the specific electroanalytical method used. Since both the input and output signals are electrical in nature, the technique can be easily integrated with electronic circuits and devices. Because of these reasons, although the laboratory-based electrochemical instrumentation is often expensive and complex, by identifying the specific parameters and the range of input signal that brings about the characteristic change, components can be minimized and made cost effective. Recently, there have been reports of portable electrochemical units with dedicated software that can be connected to mobile phones via Bluetooth (Fig. 1).² Artificial Intelligence (AI)-based strategies are also being increasingly used to facilitate faster data acquisition and analysis.³ Such advancements in instrumentation and related software make the technique more user-friendly and cost-effective and eliminate the need for separate control and display units while maintaining effectiveness.

Fig. 1 Schematic of the challenges in developing portable electrochemical devices and their solutions (smartphone-based dual-readout detection image reproduced with permission from Elsevier).⁶

As mentioned above, the miniaturization of electronics and instrumentation has enabled compact electrochemical probes and sensors in recent times. However, most of the commercially available miniaturized electrochemical sensors at present are widely employed in medical diagnostics, pH measurement, and food analysis. In such applications, the probe does not often require an electrolyte, as the sample analyte by itself serves as the electrolyte, and the sensor requires only electrode packaging.⁴ However, in the case of electrochemical probes/sensors for monitoring the surface, while the probe electrode and the analyte surface itself take up the role of a counter/reference electrode and a working electrode, respectively, an appropriate electrolyte is needed. The criteria for any electrolyte to be used for an electrochemical measurement are inertness to the analyte, electrochemical stability within the studied potential range for the experimental duration, and appropriate ionic conductivity.⁵ Aqueous and organic solvents have been the conventional choice for electrolytes and are most commonly used in laboratory experiments. However, when it comes to field applications, the volatility and flammability of these electrolytes raise concerns about their use. Additionally, when used as electrolytes for surface studies, these liquids, due to their very low viscosity, are associated with issues such as spillage and permeation within surface coatings.

Ionic liquids were designed and developed to have low volatility and negligible vapour pressure compared to the conventional electrolytes, even at high temperatures. Ionic liquids are salts made from a combination of weakly coordinating anions and cations that remain in a liquid state without the need for a solvent to disperse them. The liquid-like properties of these salts at room temperature are due to the bulky size of the ions, which prevents them from fitting into lattice sites properly.⁷ Ionic liquids have been extensively explored as electrolytes in laboratory-based electroanalytical studies, with ions being carefully chosen based on their reactivity and selectivity to the analyte under study.⁸ However, some of the drawbacks of ionic liquids are high cost, poor shelf life and moisture sensitivity, which make them not suitable for commercial applications. Furthermore, ionic liquids are associated with the possibility of spillage, as their viscosity is in the range of normal viscous liquids, and hence, they may not be feasible for use as electrolytes in in situ surface studies. Deep eutectic solvents based on hydrogen-bonded complexes are also being explored as cost-effective alternatives to ionic liquids.⁹

Due to the rapid development of energy-related applications, especially in batteries, novel conducting electrolytes had to be developed. These electrolytes needed to be non-flammable and spill-proof to meet the intrinsic safety requirements of the battery. Besides this, dendrite formation, a major issue associated with some commonly used liquid electrolytes in batteries, also needed to be addressed. To meet all these criteria, solid electrolytes made from a mixture of solid polymers, such as polyethylene oxide (PEO), and salts were explored initially. Despite demonstrating exceptional mechanical strength, these materials exhibit a low ionic conductivity of about 10⁻⁷ mS cm⁻¹ at ambient temperature.¹⁰ Plasticizers such as ethylene carbonate were added to the solid electrolyte to make the materials more amorphous, thus enhancing the ionic conductivity.¹¹ Other polymers, including PMMA, PAN, and PVC, have also been well explored as polymer electrolytes, a few of which are commercially available (Table 1).

Table 1 Modifications attempted on polymeric gels to tailor ionic conductivity and rheological properties for specific application needs¹²

Polymeric gels	Description	Application
1-Ethyl-3-methylimidazolium trifluoromethane sulfonate + PEO + lithium trifluoromethanesulfonate	An ionic conductivity of ∼ 3 × 10^−4 S cm^−1 was achieved with 40 wt% of ionic liquid	Electrolyte for energy storage and conversion devices
Cholinium lactate methacrylate + cholinium lactate + ethylene glycol dimethacrylate	With 60% ionic liquid plasticizer, ionic conductivity increases from 10^−8 to 10^−3 S cm^−1, while the elasticity is lost and the gel tends to flow	Electrolyte for ECG- and EEG-based diagnostics
Polyurethane/tetrahydrofuran + 1,2-dimethyl-3-ethoxyethyl-imidazolium bis(trifluoromethane sulfonyl)imide/ethanol	High sensitivity to a wide range of strains (0.1–300%) and pressures (0.1–20 kPa). Ionic conductivity of the ion gels could reach 1.20 mS cm^−1	Ultra-durable ionic skins
Poly(tetrafluoroethylene) + 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide	Flexible film with high ionic conductivity of 1.01 mS cm^−1	Flexible supercapacitors
Choline chloride + ethylene glycol + gelatin	The prepared deep eutectic solvent gel electrolyte had large stretchability, exceeding 300% strain, together with a relatively high room temperature ionic conductivity of 2.5 mS cm^−1	Ionic skin
1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide + poly(ethylene oxide) + lithium 4,5-dicyano-2-(trifluoromethyl)imidazole	The addition of ionic liquid decreased the viscosity of the medium and increased the number of charge carriers (6.37 × 10^−5 S cm^−1 at ambient temperature; 1.78 × 10^−4 S cm^−1 at 60 °C)	Electrolytes for Li-ion batteries
Polymer electrolytes based on polyethylene glycol (PEG) complexed with potassium nitrate (KNO3)	Increased amorphousness and mobile charge carriers caused an ionic conductivity increase by more than two orders (8.24 × 10^−6 S cm^−1 at RT)	Electrochemical cells

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At present, gel polymer electrolytes (GPE) and blends of polymer electrolytes and ionic liquids are the most explored media due to their improved balance of electrochemical and rheological characteristics. Another approach to maintain a good balance is to gelate liquid electrolytes using gelator molecules.¹³ In both cases, the gel structure was particularly favorable for batteries because it slowed the discharge process, allowing batteries to retain adequate charge for longer periods. Fig. 2 shows a comparison of various physical and electrochemical characteristics of liquid, all-solid electrolytes and GPE.

Fig. 2 Comparison of wide-temperature performance between liquid, all-solid electrolytes and GPE (reproduced with permission from the Royal Society of Chemistry).¹⁴

Some of these hydrogels and other gel-based electrolytes have been used for electrochemical sensors too. Hydrogels have been found to undergo significant changes in their optical, mass, and ionic properties in response to selective analyte or surface reactions and aid in improving the sensitivity of electrochemical sensors.¹⁵ A transparent, self-supporting, non-corrosive, spill-proof electrolyte based on polymethyl methacrylate (PMMA) was patented for electrochromic devices.¹⁶ Portable electrochemical probes using gel-based electrolytes to monitor the progress of corrosion of carbon and galvanized steel have also been reported.¹⁷ A pectin-based gel was used as an electrolyte in a portable electrochemical device for the determination of cadmium and lead. A ready-to-use low-cost electrochemical sensor for the onsite analysis of paraquat, fabricated using a PVA-based gel electrolyte, has also been reported.¹⁸

Polymer-based gel electrolytes are definitely associated with spill-proof characteristics and work well on surfaces with a smooth profile. Due to their elastic nature, gel electrolytes are used in the form of films or sheets as shown in Fig. 3 in various applications. Hence, to ensure proper adhesion and surface contact with the analyte surface under study, the gel electrolyte is either kept under pressure or heated to a liquid phase and applied on the surface with the help of a brush.¹⁹ Soft gels are often smeared over the surface, leading to difficulty in confining them to a defined area and necessitating time-consuming post-measurement cleaning procedures. These methods are often not feasible for all types of surfaces, especially surfaces with specific geometries of small dimensions or surfaces with heat-sensitive coatings. Additionally, some ionic gels liquefy at room temperature or under field conditions, lessening the advantages of their gel state. Incorporating salts/ionic liquids into the gel is an established way of achieving the desirable ionic conductivity.²⁰ However, the added salts can result in instability during storage over a period of time. Though agar-based gels have been explored for many in situ applications, studies have shown that the gel dehydrates rapidly and has a short shelf life of about a week, making them unsuitable as a ready-to-use electrolyte.²¹

Fig. 3 Cell design scheme (a), picture of the cell with the gelled electrolyte from the side in contact with the WE (b) and picture of the cell/sample connection setup (c) (reproduced with permission from Elsevier).²²

It is evident that, to date, spill-proof characteristics, which are essential for portable probes, have been achieved mainly by improving mechanical properties. However, this often compromises adequate surface contact, good wettability, and efficient ion transport, which are crucial for electrolytes used in in situ electrochemical surface studies. While gels with a fine balance of liquid and solid properties have been achieved by research in recent times, due to their physical nature, a gel cannot simultaneously flow as a liquid and also take a solid shape at any specific temperature. In this context, exploring non-Newtonian fluids, particularly those based on polyvinyl alcohol (PVA) and borax (sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O), presents a promising direction for future research in portable in situ electrochemical surface probes.²³ These liquids, commonly called slime, possess the non-Newtonian characteristics of being able to flow, while remaining in a gel state (Fig. 4).

Fig. 4 Photograph of the PVA–borax based slime (reproduced with permission from John Wiley and Sons).²⁴

PVA–borax-based non-Newtonian electrolyte

Preparation

Since PVA–borax slime has primarily been used as a sensory toy or to demonstrate polymer experiments for educational purposes, it is often packaged as a ‘Do It Yourself’ kit.²⁵ Though the basic components of the slime are PVA and borax dissolved in aqueous medium, they are often prepared via do it yourself procedures using house hold products such as cleaning solutions, contact lens solution, school glue, etc.²⁶ This underscores that the non-Newtonian behaviour of the slime is not restricted to precise compositional ratios. The classic PVA slime is prepared from 4% aqueous solutions of borax and PVA. These aqueous solutions are usually prepared by dissolving 40 g each of the components, PVA or borax, separately in 960 g of distilled water. Since PVA does not dissolve readily, the polymer solution is dissolved under heating, while borax dissolves readily.^24,26,27 The preparation procedure involves vigorous mixing of aqueous solutions of PVA and borax in a relative ratio of 4 : 1 or 5 : 1 at room temperature.

Structure and role of components

PVA chains form the backbone of the gel network, while borax plays the role of cross-linking the polymer molecules to form a network. Applications to date have been focussed on the strippable elastic properties of slime, hence the preparation of conventional slime has a higher percentage of PVA in the slime medium to achieve good mechanical properties. Slime contains approximately 96% of water, which exists either as free or immobilized water within the cross-linked network. The high percentage of water contributes to the flow properties of the medium. The percentage of the core components and the molecular weight of the polymer control the mechanical integrity of the system. The composition of commercially available slime usually has a high percentage of PVA, as its intended applications so far have been focused on mechanical properties. The chemical structure and the mechanism of formation of the PVA-based non-Newtonian liquid are given in Fig. 5.

Fig. 5 (A) Dissociation of sodium tetraborate decahydrate (borax) in water. (B) Physical crosslinking process and equilibrium of complexation between PVA chains and borate ions (reproduced with permission from Elsevier).²⁸

When dissolved in water, borax yields boric acid and borate, with a buffer pH of about 9. Boric acid can undergo only mono-diol complexation and hence cannot bring about cross-linking, whereas the borate ion, with its tetrafunctional nature, can form inter/intra cross-links between the PVA molecules.²⁹ The chemical reactions leading to the formation of the slime provide insights into the dynamics of the liquid–solid transition and also the ionic conductivity. The facile liquid–solid transition of the slime is attributed to the reversible nature of the cross-links (breaking and re-forming of cross-links).

There are two possibilities regarding the nature of bonding between the borate and the hydroxyl groups: (i) formation of an ester bond or (ii) hydrogen bonding interaction. Casassa et al. proposed that ester bond formation is energetically driven and proceeds in the presence of a suitable catalyst.³⁰ However, the ester bonds formed by the condensation may not undergo rapid reversible equilibrium of bond formation and disruption. The dynamic transition of the gel–liquid clearly points to the possible hydrogen bonding, as hydrogen bonding is only a secondary interaction and can easily be disrupted with mild shear stress. The bonds can be re-formed by the close proximity of the borate ions and hydroxyl groups of PVA. This has been confirmed by IR-based studies as well.³¹

Another explanation for slime formation is reversible chemical cross-linking, where borate ions initially condense with a cis-diol group on one polymer chain to form a monoborate ester.³² In the subsequent steps, the –OH groups of the monoborate ester react with another cis-diol group on the same or a different polymer chain, resulting in the formation of a didiol cross-link. The polymer chain, associated with borate ions and the sodium counter cations, behaves as a polyelectrolyte. Thus, there is extensive charge repulsion, which keeps the polymer chains distant and expanded.³³

Simultaneously, PVA molecules also interact with the solvent water molecules through hydrogen bonding interactions. Thus, while the hydrogen-bonded borate–PVA network keeps the PVA molecules together, the interaction of PVA with water tends to disperse the PVA in water. In addition to the charge repulsion within the polymer chain, this balance also contributes to the gel-like properties of the gel. In systems where the percentage of water is too high, the excess water can disrupt the formation of the hydrogen bonding network, thus preventing gel formation. Since borax is a cross-linking agent, an increase in the percentage of borax increases the density of cross-linking, leading to a stable polymeric dispersion. However, such an increase would make the polymeric chain highly negative, leading to charge repulsions within the polymer chain. The counter cations of sodium remain in proximity to the negatively charged network of the chain to neutralize the excess negative charge.

Is slime a hydrogel?

As mentioned in the previous section, the facile reversible chemical crosslinking, hydrogen bond formation and reorganization, and even the repulsive interactions within the polymeric chain all contribute to the physical transition of slime under stress, allowing it to remain a viscous and elastic material, simultaneously. A viscoelastic medium is associated with a specific storage modulus (G′) and loss modulus (G′′), quantifying the solid and liquid characteristics of the medium, respectively. The material is characterized by oscillatory shear measurements as both the moduli depend on the frequency of the oscillatory shear deformation applied to it. At low frequencies of oscillatory shear, the polymer chains get more time to disentangle and move easily, thus making the flow properties (loss modulus (G′′)) more dominant.²⁷ In contrast, at high frequencies, the material has less time to disentangle and rearrange, leading to a more elastic response, and the storage modulus (G′) becomes more significant. Based on the fine balance of the elasticity and flow properties, the trend reverses at a particular frequency, known as the crossover frequency. The crossover frequency gives direct information on the relaxation time of the material.³⁴where τ is the relaxation time, and ω_c is the crossover frequency.

Typical gels are usually associated with extended relaxation times, as they take a long time to relax to their previous state, whereas liquids exhibit negligible relaxation times.²⁷ Notably, despite the gel-like characteristics of slime, the relaxation time of slime is finite and much shorter than that of a typical gel. Consequently, slime cannot be rheologically classified as a gel.³⁵ Instead, these viscoelastic materials are better described as high-viscosity polymeric dispersions.

Functional benefits of slime as a portable electrolyte

In addition to the basic requirements of an electrochemical medium, a portable in situ electrochemical probe for field conditions requires certain additional characteristics such as spill resistance, good conformability, and ease of pre- and post-measurement procedures.³⁶ The flow property of slime surpasses that of a typical gel and provides the former better conformability, self-healing ability and even ionic conductivity, making it a promising choice for portable electrochemical probes.³⁷ The shelf life of slime is also evident from the wide availability of slime in the market. Optical stability of PVA slime over a period of two months was earlier reported by Alesanco et al.²⁴ A comparison of the beneficial traits of slime vs. gel and liquid electrolyte is given schematically in Fig. 6. A detailed evaluation of the suitability of the medium in portable electrochemical applications is presented in the subsequent sections.

Fig. 6 Illustration of the characteristics of the slime medium that are ideal for portable applications.

Spill-proof nature

Electrochemical response is measured from the area of the analyte that is exposed to the electrolyte. For this reason, securely containing electrolytes within the desired geometric area of the analyte surface and preventing spillage is extremely important. This is particularly challenging for in situ measurements on fielded structures, especially when using liquid electrolytes, which tend to spread uncontrollably over the analyte surface during measurements. Added to this, the electrolyte penetrates into the coatings through pores and minute defects. This may not be observed in the case of polymeric hydrogel-based electrolytes. Electrolyte penetration into the underlying metal can itself cause corrosion, as the ionic species are often trapped within the coating after the measurement.^27,38 The corrosion studies carried out using 3.5% NaCl were reported to induce pitting corrosion on thermal spray coatings of stainless steel due to penetration of the electrolyte within the surface defects during measurement.³⁹ The inherent elasticity of the slime makes it spill-proof, preventing uncontrolled spreading over the analyte surface. This ensures effective containment of the electrolyte in maintaining the desired geometric area during in situ measurements. Furthermore, these electrolytes on the analyte surface are likely to be unaffected by the random environmental fluctuations during measurements, unlike the liquid electrolytes.

Conformability to the surface

Conformability is the ability of the medium to take up the profile of the surface on which the medium is contained. While conventional liquids can flow easily into the roughness of the solid electrolyte, it is not the case for gels. Poor conformability leading to low interface contact may result in loss of information on electrochemical response, especially in the case of highly rough samples or surfaces with pits or crevices on the surface.⁴⁰ The mechanical toughness of the gel-based electrolytes delivers poor conformability, which necessitates additional ways to ensure good interface contact. This has been reported earlier in studies with agar-based gels for surface cleaning applications.^19,41 The slime medium can provide superior contact with the analyte surface due to its conformability and wettability on both rough and smooth surfaces, as evident from its widespread use in cleaning applications. The wettability of the slime medium has been confirmed earlier by contact angle measurements.⁴²

Gong et al. studied the conformability of PVA-based slime over different sand papers of grades ranging between 400 and 3000 mesh size.⁴³ A very low concentration of 0.1 ppm level contaminants was drop-cast on the sandpapers. In subsequent steps, slime mixed with silver nanoparticles (AgNPs) was stamped on the contaminated surface. Surface Enhanced Raman Spectroscopy (SERS) studies carried out on the AgNP-incorporated slime samples peeled out from the contaminated sandpapers could record signals corresponding to even 1 picomol of the contaminant from a mesh size of 400. The study was also carried out on an extremely smooth surface, such as that of glass, with a contaminant loading of 1.6 ng per 3 cm².

Strippability/peelability

While most gels possess wettability to adhere onto the surface and adsorb the contaminants on the surface, PVA-based slime has an added advantage as being simultaneously wettable and strippable. The strippability of the slime is attributed to the intrinsic elastic modulus of the slime, which is independent of frequency. The intrinsic elastic modulus required for peeling a medium from a surface is 400 Pa.⁴⁴ The intrinsic elastic modulus of slime can be engineered by increasing the cross-linking density, as per the equation:where ρ_e is the cross-linking density, k_B is the Boltzmann constant and T is the temperature.

It has been reported that an increase in the composition of PVA from 2% to 3% in the presence of acetone as cosolvent resulted in an increase in cross-linking density. This was attributed to the fact that the hydrogen bonding interactions between borax and PVA increased in the presence of acetone. Due to the good adhesion and wettability, slime can capture contaminants by entrapping them between the fine pores developed via crosslinking networks.⁴⁵

The adhesive yet strippable property of slime was utilized by Mahrous et al. to selectively adsorb the radioisotope molybdate (⁹⁹Mo) and decontaminate various surfaces.⁴⁶ In their study, the composition of PVA : borax was initially optimized based on the strippability from the surface. Aluminium hydroxide was then incorporated into the media to impart selectivity to the Mo species. The decontamination of ⁹⁹Mo could be achieved by applying a thin layer of the optimized composition of slime for a duration of 120 min. The phenomenon was explained by the electrostatic attraction and ionic exchange between MoO₄²⁻ and Al(OH)₃. The adhesion of MoO₄²⁻ ions to the hydrogel or their diffusion into the hydrogel via small capillaries also contributed to the same. Banerjee et al. developed strippable gels based on PVA and various plasticizers such as glycerol, diethylene glycol, polyethylene glycol and polypropylene glycol for decontaminating various surface platforms used in nuclear and other chemical industries.⁴⁷ A small concentration of oxalic acid (0.1 M) was used as a decontaminating agent for various radionuclides.

The adhesive nature of the slime would be beneficial to increase the sensitivity of electrochemical surface measurement, as the contaminant would be bound to the thin layer of the electrolyte medium. The strippable nature brings about effective removal of the contaminants from the surface while ensuring that no gel remnants are left behind post measurement (Fig. 7). This contributes to significantly reducing the post-measurement cleaning procedures.

Fig. 7 Stripping the PVA/borax slime film from the surface of (a) stainless steel and (b) wood (reproduced from ACS Omega under the terms of the CC-BY-NC-ND 4.0 license).⁴⁸

Ion transport properties and electrochemical stability

The ability of a medium to facilitate the transport of ions is measured by its conductivity. The flow properties of the medium are as critical as the presence of ions in facilitating efficient ion transport. Good ionic conductivity minimizes the resistance contributed by the electrolyte to the overall electrochemical response.⁴⁹ This would ensure that the response is mainly from the analyte surface.

For monitoring the effectiveness of coatings, the total impedance (resistance) of the electrolyte must be lower than the impedance of the coating at low frequencies.³⁸ A typical defect-free protective coating has an impedance of 1 GΩ cm⁻², and hence, the ionic conductivity of the slime must be tailored to achieve a much lower impedance. Liu et al. determined the ionic conductivity of PVA–borax slime to be approximately 0.9 mS cm⁻¹.⁵⁰ Conventional polymeric gels and liquid electrolytes require the addition of salts for ionic conductivity to achieve this criterion. The ionic conductivity of the most commonly used polymeric hydrogels is in the order of mS cm⁻¹, even with the addition of salts, as given in Table 2.

Table 2 Ionic conductivity of a few polymer–salt complexes reported in the literature⁵¹

Gel electrolyte	Ionic conductivity
Agar gel/17% LiClO4	0.065 mS cm^−1
PVA/LiNO3/PyrrNO3/formamide	33 mS cm^−1
P(VP-co-VAc)/KI/TPAI/MPII	4.09 mS cm^−1
PUA/TBAI/I2	0.188 mS cm^−1
PVA/H3PO4/[EMIM]BF4	40 mS cm^−1
PVAPB/0.9 M KCl	1.02 mS cm^−1
PVA/H3PO4/PySH	22.57 mS cm^−1
PVDF–HFP	0.60 mS cm^−1
PMMA/NaClO4/PC/FEC	6.2 mS cm^−1
PVA–borax slime	0.9 mS cm^−1

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The workable potential window is another important parameter for an electrochemical medium. The electrochemical potential window refers to the potential range of the electrolyte medium, where the electrolyte is stable and does not undergo any redox change. In the case of aqueous medium, the electrochemical potential window is 1.23 V. This narrow range is attributed to the electrochemical splitting of water, leading to hydrogen and oxygen evolution observed beyond the range.⁵² Thus, ionic liquids devoid of any solvent are associated with a very broad potential window (3–4 V). Though hydrogels are associated with a large amount of water, the potential window is found to be wider than that of aqueous medium as the water is bound within the polymer network.⁵³

Alesanco et al. tried out PVA-based slime as an electrolyte for electrochromic devices.²⁴ The electrochemical potential window reported ranged from −2.8 V to +2.8 V. However, the in situ surface studies focussed in this paper are mostly intended for monitoring fielded or assembled structures and would require non-destructive techniques involving minimum perturbation in terms of both applied potential and non-corrosive/reactive electrolyte. Hence, in situ electrochemical surface probing studies on fielded structures, in most cases, will be carried out without an applied potential (at open circuit potential).

Sensitivity of slime to chemical species in the surface

Gels have been used in sensors due to their ability to trap ions or other chemical species through capillary absorption via the pores in their network.⁵⁴ This property enables supramolecular hydrogen-bonded gels to respond quickly and selectively to NO₂ and NH₃ gases at room temperature. When gases are trapped in the hydrogel, ion movement is restricted, leading to an increase in resistance. In gels with a viscoelastic nature, the switching behaviour of the medium, when the gas moves in and out of the medium, is relatively fast. Similarly, PVA-based slime containing pores within its network has the potential to selectively adsorb chemical species from surfaces, which would contribute to the sensitivity of the electrochemical response when used as an electrolyte.

Saeed et al. studied the interaction of PVA–borax slime with transition metals using ionic conductivity-based measurements and observed that transition metals complex with the slime.⁵⁵ PVA-based slime was used as a non-invasive medium for a portable NMR spectrometer to adsorb metal corrosion products and other contaminants from surfaces of varying porosity.^45,56 The NMR signals demonstrated that PVA-based slime could trap ions such as Cu²⁺, Fe²⁺ and Zn²⁺ within its cross-linked network, and that an increase in the concentration of boron ions could enhance the efficiency of contaminant removal.

Studies carried out in aqueous dispersions of PVA showed that the presence of salts such as sodium sulphate alters the arrangement of water molecules on the PVA chain and affects the diffusion of PVA.⁵⁷ The hydrodynamic radii of PVA in the dispersion (∼48 nm) were found to decrease to 37 and 41 nm, in the presence of 0.02 mol per dm³ NaCl and Na₂SO₄, respectively. The decreased values clearly show that these salts facilitate the removal of water molecules from the hydration sphere of PVA. Salts with cations from group 1 and group 2 of the periodic table, along with aluminium and ammonium, have also been reported to have a salting-out effect on PVA in aqueous solution.⁵⁸ While electrochemical measurements were not carried out in the study, changes in the hydration sphere of ions brought about by such salts will definitely cause changes at the electrochemical interface, which will be reflected in the electrochemical response. As mentioned earlier, PVA-based slimes have not yet been explored as a medium for electrochemical sensors. However, since slimes fall under the category of polymeric dispersions, the available data on the sensitivity of aqueous PVA dispersions to salt-based products are likely applicable to slime.

Engineering prospects of slime

The inherent non-Newtonian behaviour of slime meets a broad range of criteria required for a functional medium. Additionally, slime can be engineered to fine-tune its ionic-rheological characteristics for specific applications through two primary approaches: incorporating functional additives to modify its properties and adjusting the ratios of its core components. Both methods provide flexible control over its conductivity, stability, and mechanical behaviour.

Additive-based engineering of slime

The presence of polymeric chains imparts an inert nature to the medium, facilitating the incorporation of various additives without affecting the stability and non-Newtonian characteristics of the medium. This is particularly beneficial in improving the sensitivity and selectivity of the medium in analytical contexts. Various research groups have modified the rheological properties of slime by incorporating additives, which has been confirmed by concomitant improvement in the respective parameters (Table 3). Riedo et al. added polyethylene oxide to PVA-based slime to increase the strippability and conformability of the slime to clean deteriorated wall paintings.²⁷ It was observed that, although the PEO molecules did not participate in the cross-linking, PEO stabilized the medium by increasing the spacing between the cross-links (network mesh size). The compatibility of the slime with organic solvents also improved to a considerable extent. Incorporating organic co-solvent along with water into PVA–borax slime has also been demonstrated to stabilize the cross-linked network and improve mechanical properties.³²

Table 3 Representative examples of slime medium explored with various additives⁵⁹

Additives	Effect of addition	Scope of study
Clay	Increased stiffness (Young's modulus > 224 MPa); reduced flexibility	Influence of household products on slime
Magnetic particles (NdFeB)	Imparts magnetic behavior to slime	Soft robotics for drug delivery and foreign object removal
Sodium perborate (NaBO3)	In situ generation of hydrogen peroxide for detoxification	Decontamination of chemical warfare agents
Agar gel	Improved shape stability and mechanical strength	Surface cleaning of artworks
PEO	Enlarges pore size of the system	Surface cleaning of artworks
Agarose + EDTA + PST	Binds metal ion impurities and aids removal	Surface cleaning of artworks
Ethyl viologen dibromide + 1 : 1 mix of potassium ferrocyanide & ferricyanide	Enhances charge transfer and electrochromic properties	Electrolyte for electrochromic devices
Adsorbents (e.g., PB, bentonite, S-CHA)	Removes radioactive cesium	Surface decontamination
Aluminum hydroxide	Selective removal of ^99Mo from contaminated surfaces	Surface decontamination

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To date, incorporation of additives was mainly targeted for improving rheological characteristics, as electrochemical applications of slime have not been explored much. The literature reported to date primarily focuses on customizing the composition and/or adding reagents to enhance its mechanical properties or cleaning properties.^27,60 Although additives can be used to modify the properties of slime, optimizing the composition of its core components offers a simpler, more cost-effective, and practical way to achieve the desired properties while preserving shelf life. Furthermore, for the targeted electrochemical portable application discussed here, slime only requires minimal elasticity to ensure that it remains spill-proof and strippable. Research focused on this aspect would improve the flow properties, which in turn can enhance ionic transport within the medium.

Composition-based engineering of slime

Slime is formed by combining two chemically distinct components: PVA and borax. These components differ significantly in both molecular weight and ionic characteristics. Among the two, borax serves as the sole source of mobile ions and is therefore solely responsible for the ionic conductivity of the medium. However, as discussed in the context of slime formation, borax also acts as a crosslinking agent, increasing the number of crosslinks within the PVA matrix. While this enhanced crosslinking imparts greater elasticity to the medium, it can simultaneously reduce ionic conductivity too. Thus, the overall ionic conductivity depends on a delicate balance between the mobile ion contribution from borax and the structural rigidity introduced by crosslinking. Determining the threshold borax composition is crucial for achieving an optimal balance between ionic conductivity and flow properties.

As mentioned earlier, elasticity is not a requirement for electrolytes intended for in situ surface monitoring. However, a certain degree of elasticity, if present, can help the medium remain spill-proof and easily removable (strippable). However, there are no reported studies on the rheological and ionic conductivity characteristics of slime formulations containing borax concentrations higher than the standard ratio of 1 : 4 (PVA : borax). Hence, a few preliminary experiments in this perspective were carried out by the authors, and a patent has been filed.⁶¹ The study involved compositions with varying borax : PVA ratios of 1 : 0.5, 1 : 1, 1 : 2, 1 : 3, 1 : 4, 1 : 5, and 1 : 6, as shown in Table 4. Visual inspection showed that the slime characteristics, though minimal, could be observed only from an equal composition of borax and PVA. The medium behaved like a typical viscous liquid when the borax content was more than that of PVA (1 : 0.5).

Table 4 Ionic conductivity of varying relative compositions of borax : PVA⁶¹

Composition of borax : PVA	Ionic conductivity (mS cm^−1)
1 : 1	8.35
1 : 2	5.64
1 : 3	4.44
1 : 4	3.60
1 : 5	2.78
1 : 6	2.33

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As shown in Table 4, ionic conductivity increased with increasing borax content, consistent with earlier studies.^50,62,63 However, while increased polymer content resulted in higher elasticity, a similar trend was not observed with increasing borax content, which was expected to enhance cross-linking. This can be explained by the increased repulsive interactions among the negatively charged borate species, which hinder further crosslinking at very high concentrations of borax.⁶⁴ These findings suggest that the repulsive effects of excess borate ions can be strategically utilised to tailor the flow behaviour and ionic conductivity of the slime medium.

The rheological characterization (Fig. 8) confirmed that the material exhibited both loss modulus and storage modulus, though the latter was lower. It can be seen that crossover of the loss/storage modulus due to liquid–gel transition usually observed for PVA slimes is not found for the [1 : 1] slime.²⁴ The loss modulus was consistently higher than the storage modulus across all the amplitudes, indicating that the material's flow behaviour dominates over its elastic behaviour.⁶⁵ However, it is also apparent that while the loss modulus dominates over the storage modulus, the difference is not drastic, unlike in simple viscous liquids. This confirms that the composition retains the viscoelastic spill-proof nature, which is an important factor considered during composition optimization. While rheological characterization and visual inspection indicate that the [1 : 1] slime is both strippable and spill-proof, its ability to retain shape may be limited, which is not a major concern for electrolytes used in portable applications.

Fig. 8 Amplitude sweep measurements showing storage modulus G′ and loss modulus G′′ of the PVA : borax slime of 1 : 1 composition.⁶¹

To confirm whether a composition with minimal elasticity and high ionic conductivity meets the desired characteristics of a portable electrolyte, several factors were evaluated. These include adequate flow properties, conformability, strippability, and spill-proof nature. The flow properties of slime are evident in Fig. 9(a). Slime was poured into custom-made aluminium sheets containing circular or rectangular through-holes. The soft material could easily be stripped from the surface using a spatula, as shown in Fig. 9(b). The slime could easily be contained within the rectangular holes of the sheet, even filling the minor deviations along the edges of the intended shape while remaining spill-proof (Fig. 9(c)).

Fig. 9 Photographs of the as-prepared [1 : 1] composition PVA : borax slime showing (a) flow behavior, (b) strippability, and (c) spill-proof nature.

In addition to incorporating additives and modifying the relative composition of PVA and borax, controlling the dilution of the slime medium is another effective strategy for manipulating the flow properties. This is because, in a concentrated polymer solution, the extent of interchain cross-linking increases, resulting in a better-networked structure. This leads to an increased gel stiffness.⁶⁴ The pH of the medium also has a significant role in engineering the elasticity of the medium. When the pH of the PVA–borax slime is shifted to an alkaline side, borax dissociates to two units of borate ions, which doubles the cross-linking. The increase in cross-linking improves the micro rheological properties.

Besides the increased cross-linking, the presence of NaOH is also found to increase the lifetime of the networked structure of the didiol complex,²⁸ which also increases the elasticity of the slime. Previous studies involving the addition of baking soda, which is mildly alkaline, also reported an increase in elastic modulus of the PVA–borax slime.²⁶ It is to be noted that the ionization equilibria and the sol–gel transition are sensitive to pH. A transformation in the physical nature of the electrolyte will cause a concomitant change in ionic conductivity and other electrochemical parameters, which will also contribute to the enhancement of the sensitivity of the electrochemical measurements.

Other emerging electrochemical applications of PVA-based slime

Recently, slime has started gaining increased attention as a tunable medium for applications ranging from battery to electroplating technologies. Wang et al. utilized the concept of PVA based slime in making a novel medium based on poly acrylic acid–borax medium for lithium ion batteries.⁶⁶ The media helped in retaining 91.2% of the capacity even after 50 cycles. The cycle stability was found to extend beyond 500 cycles. The PVA-based medium was also used as a medium in an electrochromic device (ECD) with potassium ferro/ferricyanide and ethyl viologen as redox and electrochromic species, respectively.²⁴ It was found that the use of slime simplified the fabrication process of the ECD and also made it a spill-proof system. The ECD exhibited an optical contrast (>65% at 550 nm), fast switching time (<5 s, estimated for 90% of the total transmittance change at 550 nm), and good cyclability (8000–10 000 cycles). The optical stability of the PVA–borax slime was studied for a period of two months and was found to be stable, indicating the good shelf life of the material.

The good wettability and high flexibility of PVA–borax slime were utilized for electroplating highly reactive potassium metal (K metal) with a copper current collector.⁴² The medium exhibited stable cycling lifetime for 700 h at 0.5 mA cm⁻² and 500 h at 1 mA cm⁻² at 10% depth of discharge (DOD). Dendrite formation, which is a serious challenge in the plating of K metal, was not observed when slime was used as the medium. While most of these applications dealt with utilizing the ionic conductivity of the slime medium, Feng et al., added carbon black into the slime medium to impart electrical conductivity to the medium (e-slime) and explore its potential in biosignal sensing. The electrical conductivity of the e-slime was found to be 0.33 mS m⁻¹.⁶⁷ E-slime electrodes had better skin contact, which contributed to better ECG signal quality compared to commercial electrodes. PVA–borax slime incorporated with silk fibroin has been used as a sensing platform to monitor human body motion for applications in health management, soft robotics, and human–machine interfaces.⁶⁰ The use of slime in these applications demonstrates excellent ion transport and electrochemical stability across a wide potential range, multiple cycles, and the addition of various species. Its physical properties, along with its inert nature, are well established through its widespread use as a commercial surface cleaning agent with effective decontamination properties.

Summary and recommendations

PVA–borax-based slime has traditionally been studied as a model system for non-Newtonian fluids, with rheological properties primarily explored for cleaning and sensory applications. This work presents a new perspective by exploring the inherent ionic conductivity of the medium, imparted by the borax component. The unique combination of non-Newtonian properties and inherent ionic conductivity offers significant advantages over conventional gel and liquid electrolytes. These include strippability, conformability, and spill-proof characteristics—features absent in state-of-the-art electrolytes. Additionally, the cost-effective nature and prolonged shelf life of the slime have the potential to address existing gaps in utilizing electrochemical probes for portable field applications.

Unlike previous studies that emphasized mechanical properties, this work prioritizes customizing the flow behaviour and ionic conductivity of the slime for targeted electrochemical applications. While additives such as metal salts and ionic liquids have enhanced the properties, composition engineering can minimize the number of components, simplify the preparation procedure, and reduce costs. Since such electrochemical and rheological data were not previously available, experimental studies were conducted to determine whether composition engineering can improve ionic properties while retaining spill-proof and strippable characteristics.

The results demonstrate that systematic variation of PVA, borax, and water ratios can achieve favourable flow characteristics and superior ionic conductivity, making these formulations promising candidates for electrochemical studies. Notably, the ionic conductivity of the slime significantly exceeded that of soft gel media, even without additives. This research opens new avenues for developing cost-effective, spill-resistant, and strippable electrolytes with extended shelf life, achieved through compositional optimization rather than reliance on additives. These findings aid in the development of electrolytes suitable for in situ electrochemical studies on field structures, while also reducing manufacturing complexity and environmental impact.

Data availability

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the CSIR-Senior Research Associateship (13/9256-A/2023). We thank the Soft Condensed Matter Group at the Raman Research Institute, Bangalore, India, for assistance with characterization studies.

References

1. (a) C. Fan, Y. Liu, X. Yin, J. Shi and K. Dilger, ACS Omega, 2021, 6, 20331; (b) B. Qiu, J. Wang, Y. Xia, Z. Wei, S. Han and Z. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 9185; (c) R. C. Bacon, J. J. Smith and F. M. Rugg, Ind. Eng. Chem., 1948, 40, 161; (d) F. Lu, B. Song, P. He, Z. Wang and J. Wang, RSC Adv., 2017, 7, 13742.
2. (a) J. O. Baiyokvichit, Z. Li and J. T. Huang, Measurement, 2025, 251, 117233; (b) H. Fu, H. Chow, M. Lew, S. Menon, C. Scratchley and M. A. Parameswaran, arXiv, 2015, preprint, arXiv:1509.08591,  DOI:10.48550/arXiv.1509.08591; (c) J. Xu, J. Shen, B. Zhang, Y. Zhang, X. Lv and G. Zhu, Electrochim. Acta, 2024, 481, 143952; (d) A. Ainla, M. P. S. Mousavi, M. Tsaloglou, J. Redston, J. G. Bell, M. T. Abedul and G. M. Whitesides, Anal. Chem., 2018, 90, 6240; (e) S. Zhang, S. Wang, B. Sun, S. Chen, Q. Ma, K. Han, C. Yin, X. Wang and H. Jiang, Sens. Actuators, B, 2025, 425, 137000.
3. (a) A. Mistry, A. A. Franco, S. J. Cooper, S. A. Roberts and V. Viswanathan, ACS Energy Lett., 2021, 6, 1422; (b) D. Doonyapisut, P. Kannan, B. Kim, J. K. Kim, E. Lee and C. Chung, Adv. Intell. Syst., 2023, 5, 2300085; (c) B. Chang, Electrochim. Acta, 2023, 462, 142741.
4. (a) M. Wang, Y. Yang and J. Min, Nat. Biomed. Eng., 2022, 6, 1225; (b) Q. He, B. Wang, J. Liang, J. Liu, B. Liang, G. Li, Y. Long, G. Zhang and H. Liu, Mater. Today Adv., 2023, 17, 100340.
5. D. Pletcher, R. Greff, R. Peat, L. M. Peter and J. Robinson, 11 - The design of electrochemical experiments, Instrumental Methods in Electrochemistry, ed. D. Pletcher, R. Greff, R. Peat, L. M. Peter and J. Robinson, Woodhead Publishing, 2010, pp. 356–387,  DOI:10.1533/9781782420545.356.
6. S. Zhang, S. Wang, B. Sun, S. Chen, Q. Ma, K. Han, C. Yin, X. Wang and H. Jiang, Sens. Actuators, B, 2025, 425, 137000.
7. I. Krossing, J. M. Slattery, C. Daguenet, P. J. Dyson, A. Oleinikova and H. Weingärtner, J. Am. Chem. Soc., 2007, 129, 36.
8. (a) V. V. Singh, A. K. Nigam, A. Batra, M. Boopathi, B. Singh and R. Vijayaraghavan, Int. J. Electrochem., 2012, 165683; (b) L. Yu, Y. Huang, X. Jin, A. J. Mason and X. Zeng, Sens. Actuators, B, 2009, 140, 363; (c) D. S. Silvester, Analyst, 2011, 136, 4871.
9. A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and R. K. Rasheed, J. Am. Chem. Soc., 2004, 126, 9142.
10. A. Magistris and K. Singh, Polym. Int., 1992, 28, 277.
11. X. Qian, N. Gu, Z. Cheng, X. Yang, E. Wang and S. Dong, Mater. Chem. Phys., 2002, 74, 98.
12. (a) Y. Kumar, S. A. Hashmi and G. P. Pandey, Solid State Ionics, 2011, 201, 73; (b) M. Isik, T. Lonjaret, H. Sardon, R. Marcilla, T. Herve, G. G. Malliaras, E. Ismailova and D. Mecerreyes, J. Mater. Chem. C, 2015, 3, 8942; (c) T. Li, Y. Wang, S. Li, X. Liu and J. Sun, Adv. Mater., 2020, 32, 2002706; (d) X. M. Zhang, M. Kar, T. C. Mendes, Y. T. Wu and D. R. MacFarlane, Adv. Energy Mater., 2018, 8, 1702702; (e) H. Qin, R. E. Owyeung, S. R. Sonkusale and M. J. Panzer, J. Mater. Chem. C, 2019, 7, 601; (f) A. R. Polu, A. A. Kareem and H. K. Rasheed, J. Solid State Electrochem., 2023, 27, 409; (g) A. R. Polu, A. A. Kareem, K. Kim, D. Kim, M. Venkanna, H. Kh. Rasheed and K. U. Kumar, Korean J. Chem. Eng., 2023, 40, 2975.
13. (a) K. Hanabusa, D. Inoue, M. Suzuki, M. Kimura and H. Shirai, Polymer, 1999, 31, 1159; (b) K. Hanabusa, K. Hiratsuka, M. Kimura and H. Shirai, Chem. Mater., 1999, 11, 649.
14. X. Zhou, Y. Zhou, L. Yu, L. Qi, K. Oh, P. Hu, S. Lee and C. Chen, Chem. Soc. Rev., 2024, 53, 5291.
15. K. Kaniewska and M. Karbarz, Electrochem. Sci. Adv., 2022, 2, e2100172.
16. S. L. Wunder, H. P. Wang and Y. K. Yarovoy, Gel electrolytes for electrochromic and electrochemical devices, US Pat., US6232019B1, 2001.
17. G. Monrrabal, A. Bautista and F. Velasco, Corrosion, 2019, 75, 1502.
18. K. Charoenkitamorn, C. Chotsuwan, S. Chaiyo, W. Siangproh and O. Chailapakul, Sens. Actuators, B, 2020, 315, 128089.
19. E. Al-Emam, H. Soenen, J. Caen and K. Janssens, Herit. Sci., 2020, 8, 106.
20. (a) V. Gregorio, N. García and P. Tiemblo, ACS Appl. Polym. Mater., 2022, 4, 2860; (b) N. Ming, S. Ramesh and K. Ramesh, Sci. Rep., 2016, 6, 27630; (c) K. L. Chai, M. M. Aung, I. M. Noor, H. N. Lim and L. C. Abdullah, Sci. Rep., 2022, 12, 124.
21. L. Ntombela, N. Chetty and B. Adeleye, Results Opt., 2023, 12, 100442.
22. B. R. Barat and E. Cano, Electrochim. Acta, 2015, 182, 751.
23. (a) O. R. Keisar, V. Nahum, L. Yehezkel, I. Marcovitch, I. Columbus, G. Fridkin and R. Chen, ACS Omega, 2021, 6, 5359; (b) S. S. Mahrous and M. S. Mansy, Appl. Radiat. Isot., 2024, 213, 111493.
24. Y. Alesanco, J. Palenzuela, A. Viñuales, G. Cabañero, H. J. Grande and I. Odriozola, Angew. Chem., Int. Ed., 2015, 2, 218.
25. J. M. Ting, R. G. Ricarte, D. K. Schneiderman, S. A. Saba, Y. Jiang, M. A. Hillmyer, F. S. Bates, T. M. Reineke, C. W. Macosko and T. P. Lodge, J. Chem. Educ., 2017, 94(11), 1629.
26. J. M Ashraf, L. Nayfeh and A. Nayfeh, Sci. Rep., 2022, 12, 3953.
27. C. Riedo, F. Caldera, T. Poli and O. Chiantore, Herit. Sci., 2015, 3, 23.
28. H. F. Mahjoub, M. Zammali, C. Abbes and T. Othman, J. Mol. Liq., 2019, 291, 111272.
29. S. W. Sinton, Macromolecules, 1987, 20, 2430.
30. E. Z. Casassa, A. M. Sarquis and C. H. Van Dyke, J. Chem. Educ., 1986, 63, 57.
31. (a) G. A. Hurst, M. Bella and C. G. Salzmann, J. Chem. Educ., 2015, 92(5), 940; (b) K. Bolewski and B. Rychły, Über, d. Struktur, d. Bindung von, Kolloid Z. Z. Polym., 1968, 228, 48.
32. L. V. Angelova, P. Terech, I. Natali, L. Dei, E. Carretti and R. G. Weiss, Langmuir, 2011, 27, 11671.
33. H. L. Lin, T. L. Yu and C. H. Cheng, Colloid Polym. Sci., 2000, 278, 187.
34. L. Piculell, M. Egermayer and J. Sjöström, Langmuir, 2003, 19, 3643.
35. K. Almdal, J. Dyre, S. Hvidt and O. Kramer, Polym. Gels Networks, 1993, 1, 5.
36. (a) B. J. Feldman and Z. L. Ouyang, Self-powered analyte sensor, European patent, EP3255154A1, 2022; (b) A. N. Jina, A. Parmar, B. Chua, J. Tamada, J. Lee, M. J. Tierney, N. Kaur, P. Magginetti and S. P. Desai, Devices, systems, methods and tools for continuous analyte monitoring, US Pat., US20100049021A1, 2010; (c) V. Krishnamani, S. Pirbadian, K. H. Pauley, S. P. Balashov and A. Devadoss, Glucose sensors and methods of manufacturing, US Pat., US20220151521A1, 2022.
37. (a) C. Iwakura, H. Wada, S. Nohara, N. Furukawa, H. Inoue and M. Moritab, Electrochem. Solid-State Lett., 2003, 6, A37; (b) G. Ruano, J. I. Iribarren, M. M. Pérez-Madrigal, J. Torras and C. Alemán, Polymers, 2021, 13, 1337.
38. A. H. England and T. L. Clare, Electroanalysis, 2014, 26, 1059.
39. P. Kutschmann, M. Grimm, T. Lindner, K. R. Ernst, O. Schwabe, C. Pluta and T. Lampke, Coatings, 2022, 12, 344.
40. S. L. Wunder, H. P. Wang and Y. K. Yarovoy, Gel electrolytes for electrochromic and electrochemical devices, US Pat., US6232019B1, 2001.
41. F. G. Torres and G. E. De-la-Torre, Carbohydr. Polym. Technol. Appl., 2021, 2, 100023.
42. S. Wang, Y. Yan, D. Xiong, G. Li, Y. Wang, F. Chen, S. Chen, B. Tian and Y. Shi, Angew. Chem., Int. Ed., 2021, 60, 25122.
43. Z. Gong, C. Wang, C. Wang, C. Tang, F. Cheng, H. Du, M. Fan and A. G. Brolo, Analyst, 2014, 139, 5283.
44. I. Natali, E. Carretti, L. Angelova, P. Baglioni, R. G. Weiss and L. Dei, Langmuir, 2011, 27, 13226.
45. V. Stagno, A. Ciccola, R. Curini, P. Postorino, G. Favero and S. Capuani, Gels, 2021, 7, 265.
46. S. S. Mahrous and M. S. Mansy, Appl. Radiat. Isot., 2024, 213, 111493.
47. D. Banerjee, U. Sandhya, S. A. Khot and C. Srinivas, Development of Strippable Gel for Surface Decontamination Applications, Bhabha Atomic Research Centre, Mumbai (India), 2015.
48. O. R. Keisar, V. Nahum, L. Yehezkel, I. Marcovitch, I. Columbus, G. Fridkin and R. Chen, ACS Omega, 2021, 6, 5359.
49. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons, New York, 2nd edn, 2001.
50. J. Liu, Q. He, M. Pan, K. Du, C. Gong and Q. Tang, J. Mater. Chem. A, 2022, 10, 25118.
51. (a) E. Raphael, C. O. Avellaneda, M. A. Aegerter, M. M. Silva and A. Pawlicka, Mol. Cryst. Liq. Cryst., 2012, 554, 264; (b) C. Ramasamy, P. Porion, L. Timperman and M. Anouti, Synth. Met., 2023, 299, 117469; (c) N. H. Ming, S. Ramesh and K. Ramesh, Sci. Rep., 2016, 6, 27630; (d) K. L. Chai, M. M. Aung, I. M. Noor, H. N. Lim and L. C. Abdullah, Sci. Rep., 2022, 12, 124; (e) S. Alipoori, S. Mazinani, S. H. Aboutalebi and F. Sharif, J. Energy Storage, 2020, 27, 101072; (f) M. Jiang, J. Zhu, C. Chen, Y. Lu, Y. Ge and X. Zhang, ACS Appl. Mater. Interfaces, 2016, 8, 3473; (g) K. Sun, M. Dong, E. Feng, H. Peng, G. Ma, G. Zhao and Z. Leib, RSC Adv., 2015, 5, 22419; (h) J. Zheng, W. Li, X. Liu, J. Zhang, X. Feng and W. Chen, Energy Environ. Mater., 2023, 6, e12422; (i) J. Liu, Q. He, M. Pan, K. Du, C. Gong and Q. Tang, J. Mater. Chem. A, 2022, 10, 25118.
52. H. Tomiyasu, H. Shikata, K. Takao, N. Asanuma, S. Taruta and Y. Park, Sci. Rep., 2017, 7, 45048.
53. (a) Y. Ouyang, X. Li, S. Li, Z. L. Wang and D. Wei, ACS Appl. Mater. Interfaces, 2024, 16, 18236; (b) Y. Liu, X. Liu, B. Duan, Z. Yu, T. Cheng, L. Yu, L. Liu and K. Liu, J. Phys. Chem. Lett., 2021, 12, 2587.
54. P. Zhou, Z. Zhang, F. Mo and Y. Wang, Adv. Sens. Res., 2024, 3, 2300021.
55. R. Saeed, S. Masood and Z. Abdeen, Int. J. Sci. Technol., 2013, 3, 132.
56. V. Stagno, C. Genova, N. Zoratto, G. Favero and S. Capuani, Molecules, 2021, 26, 3697.
57. B. Filova, L. Musilova, A. Mracek, M. L. Ramos, L. M. P. Veríssimo, A. J. M. Valente and A. C. F. Ribeiro, J. Mol. Liq., 2020, 304, 112728.
58. S. Konan and A. Banno, Process for separating polyvinyl alcohol from solutions, German patent, DE2557443C3, 1985.
59. (a) J. M. Ashraf, L. Nayfeh and A. Nayfeh, Sci. Rep., 2022, 12, 3953; (b) M. Sun, C. Tian, L. Mao, X. Meng, X. Shen, B. Hao, X. Wang, H. Xie and L. Zhang, Adv. Funct. Mater., 2022, 32, 2112508; (c) O. R. Keisar, V. Nahum, L. Yehezkel, I. Marcovitch, I. Columbus, G. Fridkin and R. Chen, ACS Omega, 2021, 6, 5359; (d) E. Al-Emam, H. Soenen, J. Caen and K. Janssens, Herit. Sci., 2020, 8, 106; (e) C. Riedo, F. Caldera, T. Poli and O. Chiantore, Herit. Sci., 2015, 3, 23; (f) Y. Alesanco, J. Palenzuela, A. Viñuales, G. Cabañero, H. J. Grande and I. Odriozola, Angew. Chem., Int. Ed., 2015, 2, 218; (g) H. Yang, I. Yoon and Y. Lee, Chem. Eng. J., 2020, 402, 126299; (h) S. S. Mahrous and M. S. Mansy, Appl. Radiat. Isot., 2024, 213, 111493.
60. N. Yang, P. Qi, J. Ren, H. Yu, S. Liu, J. Li, W. Chen, D. L. Kaplan and S. Ling, ACS Appl. Mater. Interfaces, 2019, 11, 23632.
61. A. Renjith and H. C. Barshilia, A non-Newtonian electrolyte enabled portable electrochemical probe assembly for in situ corrosion measurements, Indian patent, Application Number 202411100690, 2024.
62. J. Liu, Q. He, M. Pan, K. Du, C. Gong and Q. Tang, J. Mater. Chem. A, 2022, 10, 25118–25128.
63. J. Li, Z. Zhang, X. Cao, Y. Liu and Q. Chen, Soft Matter, 2018, 14, 6767.
64. E. Pezron, L. Leibler, A. Ricard, F. Lafuma and R. Audebert, Macromolecules, 1989, 22, 1169.
65. K. Arjunan, C. Weng, Y. Sheng and H. Tsao, Langmuir, 2024, 40(32), 17081.
66. S. Wang, Q. Duan, J. Lei and D. Y. W. Yu, J. Power Sources, 2020, 468, 228365.
67. Y. Feng, H. Sun, M. Chen, C. Wu, J. Li, Z. Li, K. Makasheva, N. Liu and G. Zhang, IEEE Open J. Nanotechnol., 2024, 5, 156.

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