INTRODUCTION
General introduction of hydrogels
Hydrogels are three-dimensional networks of hydrophilic polymers known for their ability to absorb significant amounts of water or biological fluids Their high water content allows them to mimic the properties of living tissues, making them ideal for various applications These include biosensors, drug delivery systems, and as carriers or matrices in tissue engineering.
Recent advancements in hydrogels have revealed exceptional properties, including electrical conductivity, self-healing capabilities, and biocompatibility These characteristics make hydrogels highly applicable in various engineering fields.
Alginate, a natural polysaccharide polymer extracted from brown algae, is widely utilized for hydrogel preparation due to its environmental sustainability, high biocompatibility, and biodegradability Composed of multiblock copolymers, alginate consists of poly-D-mannuronic acid (M unit) and poly-L-guluronic acid (G unit) The G block effectively binds to multivalent cations like Ca2+, facilitating the formation of Ca-alginate hydrogels.
Conductive hydrogels are polymeric blends formed by integrating crosslinked hydrogel networks with conductive materials such as conductive polymers, carbon-based substances, metal nanoparticles, and monovalent ionic salts In May 2022, Donghwan Ji and colleagues developed a highly robust conductive hydrogel by incorporating the electrically conductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) into an alginate matrix Additionally, in 2014, Devaki et al created a conductive nano hydrogel using silver nanoparticles, achieved through in situ polymerization of acrylic acid and reduction of silver ions, with methylol urea serving as both a gelling and reducing agent at room temperature, resulting in an impressive electrical conductivity of 5.72 × 10^1 S/m.
The development of environmentally friendly materials is enhanced by the application of alginate hydrogels This study focuses on evaluating the conductivity, mechanical properties, and self-welding capabilities of Conductive Ca-alginate hydrogels, emphasizing their potential for innovative design.
This capstone project explores the fabrication of conductive alginate-based hydrogels, emphasizing their enhanced mechanical properties and conductivity The study specifically focuses on the hydrogels' self-welding capabilities, showcasing their potential applications in various fields.
Topic importance
Recent advancements in conductive hydrogels are paving the way for their use in biomedical and engineering applications, particularly in bioelectrodes like neuroprosthetic electrodes These hydrogels mimic the mechanical properties of human skin, offering a promising alternative to metallic electrodes by enhancing their mechanical qualities and ensuring long-term stability within living organisms The rapid development of this field is driven by global collaboration among scientists and engineers, leading to the creation of high-quality alginate-based hydrogels with excellent electrical conductivity and mechanical characteristics Additionally, the development of self-welding capabilities further expands the potential applications of these innovative materials.
Aims of this capstone project
This capstone project centers on the fabrication of conductive Ca-alginate hydrogels, utilizing advanced techniques to enhance their electrical conductivity through increased active carbon content Additionally, we assess the mechanical properties and self-welding capabilities of these hydrogels via tensile testing Our investigation aims to explore the improvements in the mechanical properties, electrical conductivity, and self-welding abilities of the hydrogels.
Research limitations
This study focuses on the fabrication of initial conductive hydrogels using varying concentrations of Na-alginate (3wt% to 5wt%) and activated carbon (1wt% to 12wt%), with a constant Ca 2+ concentration of 0.5M The dried gels were prepared through two methods: free drying at room temperature and drying under clamping conditions at temperatures of room temperature, 50ºC, and 75ºC.
Approaching methods
A survey on fabrications and applications of conductive hydrogels;
Fabricating various conductive alginate-based hydrogels and determining their mechanical properties and conductivity;
Preparing self-welding hydrogels and determining their lap shear stress;
Applications of conductive hydrogels in electrical devices;
Molecular structure of sodium alginate;
Fabrication process of conductive alginate-based hydrogels via a diffusion method;
Evaluation of mechanical properties of hydrogels via tensile test;
FABRICATING ALGINATE HYDROGELS
Hydrogels are commonly produced through two primary methods: the utilization of natural polymers like cellulose, alginate, pectin, chitosan, and collagen, or the use of synthetic polymers such as polyacrylamide, polyethylene glycol, acrylic acid, and polyvinyl alcohol.
Hydrogels are synthetic polymers formed by polymerizing hydrophilic monomers, with or without crosslinkers, creating a three-dimensional network capable of absorbing significant amounts of water They can be classified based on the nature of their crosslinked junctions, which can be chemically stable networks formed by covalent bonds or physically crosslinked bonds influenced by environmental factors such as temperature and pH Common hydrophilic monomers include acrylamide and methacrylic acid, while N,N’-methylenebis(acrylamide) and ethylene glycol dimethacrylate are frequently used as crosslinkers Free-radical polymerization is the predominant method for hydrogel preparation, utilizing thermal or photo initiators This process involves three stages: initiation, where an initiator creates a free radical; propagation, where the radical reacts with monomers to grow polymer chains; and termination, where polymer chains become saturated or combine to form longer chains.
Chemically crosslinked hydrogels are synthesized by dissolving monomers, crosslinkers, and initiators in water, resulting in covalently crosslinked networks However, these hydrogels often have poor mechanical properties due to fixed crosslinking points and limited energy dissipation mechanisms In contrast, physical hydrogels are created by copolymerizing two monomers in a water medium, leading to noncovalent bonds that form three-dimensional networks The reversible nature of these noncovalent interactions allows for effective energy dissipation during deformation, resulting in enhanced mechanical properties Additionally, various strategies for hydrogel fabrication include ionic and covalent crosslinking, phase transition, cell crosslinking, and free radical polymerization.
Figure 1 Chemical structure of some commonly used monomers and crosslinkers in fabrication of hydrogels
The mechanism of free radical polymerization involves the use of ammonium persulfate (APS) as an initiator and N,N,N',N'-tetramethylethylenediamine (TEMED) as an accelerator to generate free radicals These free radicals react with acrylamide (AAm) monomers, creating the initial radical necessary for the polymerization process As the reaction progresses, additional AAm monomers are added to the growing chain The polymerization concludes when two growing chains couple, resulting in the formation of a single, long polymer chain.
Figure 3 Schematic representation of the formation of (a) chemical and (b) physical crosslinked hydrogels
Alginate hydrogels are typically formed in aqueous solutions using divalent ions like Ca²⁺ and Mg²⁺ as crosslinking agents, which create an egg-box structure by interacting with G-block branches However, the high solubility of CaCl₂, the most common crosslinking agent, leads to an uncontrollable gelation rate that negatively impacts the uniformity and mechanical properties of the hydrogels To manage this, alternative crosslinking agents such as CaSO₄ or CaCO₃ can be utilized due to their slower solubility, which helps delay gelation Additionally, temperature plays a crucial role in the gelation process; lowering the temperature reduces the activity of divalent cations, resulting in a more stable cross-linking network and improved mechanical properties of the hydrogels.
23 cation activity, resulting in a more stably arranged crosslinking network, leading to higher quality mechanical properties of hydrogel
Covalent cross-linking agents are utilized to bond two polymer chains, facilitating the creation of hydrogels This process enables the formation of a three-dimensional network by reacting the functional groups of both synthetic and natural polymers.
Alginate hydrogels are formed through covalent crosslinking, involving the reaction of carboxylic groups in alginate branches with crosslinking molecules that contain primary diamines The choice of crosslinking molecules and their density significantly influences the mechanical properties, such as porosity and elasticity, as well as the absorption and water holding capacity of the hydrogels While the composition of crosslinkers plays a crucial role in determining absorbance and water retention, the density of these crosslinkers primarily affects the mechanical characteristics of alginate hydrogels This approach holds promise for biomedical applications, provided that the mechanical properties can be effectively controlled through varying the interactions of crosslinking density with different types of crosslinking molecules.
Figure 4 Schematic showing of covalent crosslinking of alginate using adipic acid dihydrazide as cross-linker
Thermoresponsive phase transition is a widely utilized method for preparing hydrogels, occurring when the solution's low critical temperature is exceeded This process involves a thermosensitive polymer that transitions from a coil to a globule state at temperatures below approximately 32°C Upon increasing the temperature, the thermosensitive polymer precipitates into a solid gel, making it a key technique in hydrogel formation.
Thermosensitive polymers undergo a reversible phase transition in response to temperature changes When the temperature exceeds the critical solution temperature, weak interactions between hydrophilic groups and water molecules diminish, leading to increased hydrophobic interactions among isopropyl polymer groups This process transforms the polymer into a solid gel, or globule phase Conversely, when the temperature falls below the critical solution temperature, the polymer branches revert to random coils due to the reorganization of hydrogen bonds between water molecules and hydrophilic groups, converting the solid hydrogel back into a liquid solution, known as the coil phase.
Alginate gels can be produced not only through various physical and chemical methods but also by utilizing cells that crosslink with polymer chains When alginate chains are modified with cell adhesion peptides, such as the RGD sequence (arginine-glycine-aspartic acid), they gain the ability to interact with cell surface receptors, facilitating cell adhesion This modification enables the formation of long-distance, reversible polymer networks without the need for a chemical crosslinking agent As cells are introduced to the RGD-modified alginate solution, they become evenly distributed, leading to the development of a polymer network driven by specific receptor-ligand interactions.
Alginate is a natural substance derived from the cell walls of brown algae species such as Ascophyllum nodosum, Laminaria hyperborea, and Macrocystis pyrifera, as well as certain bacteria like Azotobacter vinelandii and Pseudomonas spp The extraction process involves using diluted HCl to obtain alginic acid from the algae, which then forms a fibrous precipitate of sodium alginate or calcium alginate when sodium chloride or calcium chloride is added to the solution The final alginate powder is produced through a series of steps including precipitation removal, purification, and lyophilization.
Alginate powder is primarily used to create alginate hydrogels, which are formed when sodium ions are replaced by divalent cations like Ca²⁺ and Mg²⁺ This process results in the development of a three-dimensional network, making alginate hydrogels a valuable material in various applications.
Figure 5 A typical process for the extraction of sodium alginate from brown algae [10] 2.2 Alginate chemical structure
Alginate is a water-soluble linear polysaccharide composed of M and G blocks, which can be organized in homogeneous (poly-G, poly-M) or heterogeneous (MG) geometries The unique configurations of the monomers result in diverse geometries for the G-blocks, which appear warped, and the M-blocks, which resemble ribbons By increasing the G-block content and the polymer's molecular weight, stronger and more porous alginate hydrogels can be produced, while a higher proportion of M-blocks yields softer, more elastic hydrogels Unlike monovalent alginate salts that are soluble in water and form stable solutions, alginic acid is insoluble in both water and organic solvents Additionally, a pH lower than the pKa of 3.38 – 3.65 leads to alginate precipitation, with ion density also playing a crucial role in the water solubility of alginate salts.
Figure 6 The conformation of monomers and blocks distribution of alginate salt [10] 2.3 General properties of alginate
The physical properties of alginate, including viscosity, mechanical characteristics of alginate hydrogel, and water absorption rates, are significantly influenced by the diversity in composition, molecular weight, and distribution patterns of the M and G blocks Additionally, lowering the pH can enhance alginate viscosity through the formation of hydrogen bonds, while commercial alginate typically has a molecular weight ranging from 33,000.
Alginate, with a molecular weight of 400,000 g/mol, significantly influences the physical properties of alginate hydrogels, particularly as its molecular weight increases Unlike monovalent alginate salts or esters, alginic acid is insoluble in both water and organic solvents Due to its remarkable gel-forming ability, alginate is widely utilized in the pharmaceutical and biomedical industries as a suspension and viscosity enhancer, as well as in artificial organ implants and various other applications, owing to its non-toxic, eco-friendly, biodegradable, and biocompatible characteristics.
MATERIALS AND METHODS
Sodium Alginate with a viscosity of 80 to 120 Cp was sourced from FUJIFILM Wako Pure Chemical Corporation in Japan, while anhydrous Calcium Chloride was obtained from Xilong Scientific Co., Ltd in the Republic of China Steam-activated Charcoal was purchased from Duchefa Biochemie in the Netherlands, Sodium Chloride was acquired from Supleco, and 99% Glycerol was obtained from Fisher All compounds were utilized in their original state without further purification, and distilled water was used for hydrogel preparation An electronic scale was employed to accurately measure the chemicals.
Figure 10 Sodium Alginate 80~120 (Na-alginate, viscosity 80~120Cp)
Figure 11 Calcium chloride (CaCl 2 ) Figure 12 Charcoal (steam activated)
Figure 13 Sodium chloride Figure 14 Glycerol 99%
Figure 15 Khanh Hoi distilled water Figure 16 Electronic scales
2 Methods of fabrications conductive Ca-alginate hydrogels and self-welding hydrogels
The conductive calcium alginate [Ca-alginate@C/0.5/x/y] hydrogels, where x (3,
4, and 5 wt%), y (1, 4, 8, and 12wt%) stands for the initial concentration of weight percent of Na-alginate (wt%) and weight percent of activated carbon, were prepared by the following process (Figrue 17)
The activated carbon alginate solution (Figure 18) was created by combining x wt% Na-alginate and y wt% activated carbon in water and letting it dissolve naturally for
To create conductive Ca-alginate hydrogels, a 0.5M CaCl2 solution was prepared by dissolving CaCl2 powder in water A reaction mold was formed by joining two glass pieces with a 3mm spacer, leaving the upper end open for pouring The mold was filled halfway with a Na-alginate solution, and the remaining space was filled with the 0.5M CaCl2 solution, allowing Ca2+ ions to disperse within the alginate Upon contact, gelation occurred, leading to the formation of Ca-alginate hydrogels Once fully gelled, the hydrogels were carefully removed and immersed in a 0.5M CaCl2 solution to complete the crosslinking process Finally, the conductive hydrogels were washed with water for two days to eliminate any non-crosslinked salts and polymers, resulting in the desired physical properties.
Figure 17 Schematic illustration of fabrication of isotropic conductive Ca-alginate hydrogels
Figure 18 The alginate@C solution containing 8wt% activated carbon
Figure 19 Fabrication of 0.5M CaCl 2 aq solution
Free dried (FD) hydrogels were prepared by the following process (Figure 20) [Ca-alginate@C/0.5/x/y] hydrogels were cut into a samples with dimensions of ~ 50 × ~
The samples of calcium alginate hydrogels, designated as [Ca-alginate@C/0.5/x/y], were prepared in a size of 50 mm (width × length) They were initially placed on glass and air-dried for approximately 2 days Following this, the dried hydrogels were re-swelled in distilled water for around 2 days to achieve the desired FD calcium alginate hydrogels.
Figure 20 Schematic illustration of fabrication of conductive Ca-alginate hydrogels free dried at room temperature
2.3 Clamp dried conductive hydrogels a Clamp dried at room temperatue
The conductive calcium alginate [Ca-alginate@C/0.5/x/y] hydrogels, where x (3,
The initial concentrations of Na-alginate were set at 4 and 5 weight percent (wt%), while activated carbon concentrations were prepared at 1, 4, 8, and 12 wt% These concentrations were achieved using the specified method illustrated in Figure 21.
To create clamp-dried conductive [Ca-alginate@C/0.5/x/y] hydrogels, the samples are cut into 50 × 50 mm pieces The clamp consists of two square mica plates, featuring a central square cutout.
40 mm Hydrogels samples were wrapped in 4-sided paper and clamped in place, keeping them at room temperature (26°C, humidity ~ 65%) for 2 days After that time, they were
The hydrogels were soaked in distilled water for one day, after which the clamps were gently removed The resuscitated samples were then washed in distilled water for approximately two days Finally, the dried hydrogels were preserved in distilled water for future use.
Figure 21 Schematic illustration of fabrication of conductive Ca-alginate hydrogels clamp dried at room temperature b Clamp dried at 𝟓𝟎℃ and 𝟕𝟓℃
Fabrication clamp dried conductive hydrogels at 50ºC and 75ºC requires preparation of a heating furnace Hydrogels samples will be placed in the furnace (Figure
22) and allowed to dry at different temperatures (50ºC and 75ºC) After 1 day, take them out and soak the hydrogels in distilled water for ~ 1 day Gently remove the clamp and leave the hydrogels in distilled water for ~ 1 day Finally, clamp dried conductive hydrogels were preserved in distilled water
Figure 22 Heating furnace for fabricating clamp dried hydrogels
To utilize the self-welding method for hydrogels, begin by cutting freeze-dried hydrogel samples into dimensions of approximately 30 x 8 mm Soak these samples in a sodium chloride solution for the designated time, followed by a 45-minute wash with distilled water to eliminate sodium Prepare a filler solution by mixing glycerol (99%) with Na-alginate at a 1:1 ratio, and apply this filler to the welding surface After 15 minutes, scrape off the old layer and reapply new layers every 15 minutes for up to 2 hours Finally, immerse the self-welding samples in a calcium solution for about one day to achieve the desired self-welded hydrogels, which should be stored in distilled water.
Figure 23 Schematic illustration of fabrication of self-welding hydrogels
Following the weights of the hydrogels that were water-equilibrated (W wet ) and dried by a heating apparatus (Figure 24) at 120°C for 24 hours (W dry ), the water content of each hydrogel was determined:
Water content (wt%) = [ W wet −W dry
Figure 24 Heating equipment for drying hydrogels
Electrical conductivity, also referred to as specific conductance, measures a material's ability to carry electrical current This intrinsic property is essential for understanding how different materials conduct electricity.
Electrical conductivity (σ) is the reciprocal of the electrical resistivity (ρ): σ = 1
𝜌 (2) where resistivity for a material with a uniform cross section is: ρ = 𝑅×𝐴
𝑙 (3) where R is the electrical resistance, A is the cross-sectional area, and l is the length of the material
Figure 25 Dedicated resistance tester CD800A
A tensile test was conducted on Ca-alginate conductive hydrogels using a tensile strength tester (PT-1699vdo, PRO TEST, Taiwan) and a 50-kg load cell at ambient conditions (26 °C, 65% humidity) to assess their mechanical properties The dimensions of the hydrogels, cut into rectangular samples measuring 50 x 5 mm, were measured with an electronic caliper.
The samples measured 43 mm in length and width, with a 10 mm distance between the clamps During tensile testing, each sample was secured at both ends, and the upper clamp was pulled upward at a constant deformation speed of 500% per minute until fracture occurred Each gel was tested three times to ensure accuracy To determine Young's modulus values, tensile stress-strain curves were linearly fitted within a strain range of 1-3%.
Figure 26 Tensile strength tester (PT-1699vdo, PRO TEST, Taiwan) and 50-kg load cell
FABRICATION OF CONDUCTIVE CA-ALGINATE HYDROGELS, SELF-WELDING HYDROGEL, EVALUATION OF THEIR
HYDROGELS, SELF-WELDING HYDROGEL, EVALUATION OF THEIR
The conductive calcium alginate hydrogels, designated as [Ca-alginate@C/0.5/x/y], were synthesized using a straightforward diffusion method In this formulation, 'x' represents the initial alginate concentration at 4 wt%, while 'y' indicates the varying weight percentages of activated carbon, specifically at 1, 4, 8, and 12 wt%.
The fabrication process of [Ca-alginate@C/0.5/x/y] hydrogels involves several key steps First, activated carbon is mixed with a sodium alginate solution and poured into a reaction mold Next, a 0.5M CaCl2 aqueous solution is introduced, allowing Ca2+ ions to disperse throughout the alginate solution Gelation occurs when the alginate solution interacts with the Ca2+ ions, leading to the formation of Ca-alginate hydrogels The resulting conductive hydrogels are then thoroughly washed with water for two days to remove any non-crosslinked salts and polymers Finally, the desired 0.5M Ca2+ physical conductive hydrogels are obtained.
The isotropic calcium alginate hydrogels, designated as Ca-alginate@C/0.5/x/y, are produced using free drying and clamp drying methods In this formulation, x represents the initial concentration of sodium alginate at 5wt%, while y indicates the weight percentage of activated carbon at 8wt%.
Conductive hydrogels were created using a 5wt% alginate concentration and an 8wt% activated carbon concentration Square samples measuring approximately 50 x 50 mm were carefully placed on a transparent glass plate and allowed to dry naturally for about two days Following this, the dried hydrogels were re-swelled in distilled water for an additional two days to achieve the desired final product.
Figure 28 Fabricating process of free dried [Ca-alginate@C/0.5/5/8] hydrogels (a) As- prepared [Ca-alginate@C/0.5/5/8] hydrogels, (b) Completely dried [Ca- alginate@C/0.5/5/8] hydrogels, (c) Re-swelled conductive Ca-alginate hydrogels
Samples of hydrogels samples were created for drying at different temperatures conditions Clamp dried at room temperature (Figure 30), clamp dried at 50ºC (Figure
31), and clamp dried at 75ºC (Figure 32) In order for the results to be comparable, the weight percent concentration of activated carbon is likewise maintained at 8wt%
Hydrogel samples were prepared by cutting them into squares measuring approximately 50 x 50 mm To prevent breakage during clamping, the edges of the hydrogels were wrapped in paper, and care was taken to tighten the clamp gently After the drying process, the hydrogels were soaked in distilled water for about one day Once soaked, the clamps were carefully removed, allowing the hydrogels to be preserved in distilled water.
Figure 29 The hydrogels clamp a Clamp dried hydrogels at room temperatue
The fabrication process of Clamp dried [Ca-alginate@C/0.5/5/8] hydrogels involves several key steps Initially, the hydrogels are prepared and then wrapped in paper around their outer edges before clamping Once clamped at room temperature, the hydrogels undergo a drying process Finally, they are re-swelled in distilled water, demonstrating their ability to regain moisture Additionally, clamp dried hydrogels can also be subjected to a drying process at 50ºC for enhanced results.
The fabrication process of [Ca-alginate@C/0.5/5/8] hydrogels involves several key steps: initially, the hydrogels are prepared, followed by wrapping their outer edges in paper before clamping The hydrogels are then clamped at a temperature of 50ºC, resulting in a re-swelled clamp-dried hydrogel once immersed in distilled water.
47 c Clamp dried hydrogels at 75ºC
The fabrication process of [Ca-alginate@C/0.5/5/8] hydrogels involves several key steps: initially, the hydrogels are prepared, followed by wrapping their outer edges in paper before clamping The hydrogels are then subjected to clamping at a temperature of 75ºC Finally, the clamped hydrogels are re-swelled in distilled water, resulting in the final product.
Welding hydrogels is a time-consuming process that involves removing calcium from the network After free-drying the hydrogels at room temperature for two days, they are immersed in a sodium chloride (NaCl) solution As sodium is absorbed and the binding fibers begin to separate, it is crucial to replace the NaCl solution to maintain the integrity of the hydrogels during the process.
The hydrogels were soaked in distilled water for 20 minutes to neutralize sodium, leading to swelling and separation of free fibers in the structure Next, a filler layer composed of a 1:1 solution of 5wt% sodium alginate and 99% glycerol was applied to the welding surface The hydrogels were then immersed in calcium ions for approximately one day, followed by rinsing with distilled water for another day Finally, the self-welding hydrogels were stored in distilled water for preservation.
48 a Self-welding hydrogels using [Ca-alginate@C/0.5/5/8] hydrogels
Figure 33 Fabrication process of self-welding using [Ca-alginate@C/0.5/5/8] hydrogels
The [Ca-alginate@C/0.5/5/8] hydrogels, measuring approximately 30 x 5 mm in length and width, undergo a soaking process in Sodium Chloride solution for about 1.5 hours Following this, they are immersed in distilled water for roughly 45 minutes These hydrogels feature a filler coating that enhances their ability to bond together Additionally, after being soaked in a 0.5M Ca²⁺ solution for approximately one day, they are further treated with distilled water.
~ 1 day, self-welding hydrogels are obtained
Figure 34 Fabrication process of self-welding using [Ca-alginate@C/0.5/5/8] hydrogels
The [Ca-alginate@C/0.5/5/8] hydrogels, measuring approximately 30 x 5 mm (length x width), undergo a soaking process in Sodium Chloride solution for about 2.5 hours Following this, they are immersed in distilled water for approximately 45 minutes These hydrogels feature a filler coating that facilitates their welding together Additionally, they are soaked in a 0.5M Ca²⁺ solution for around one day before being placed in distilled water.
~ 1 day, self-welding hydrogels are obtained
Figure 35 Fabrication process of self-welding using [Ca-alginate@C/0.5/5/8] hydrogels
The [Ca-alginate@C/0.5/5/8] hydrogels, measuring approximately 30 x 5 mm (length x width), undergo a soaking process in Sodium Chloride solution for around 3.5 hours, followed by immersion in distilled water for about 45 minutes These hydrogels feature a filler coating that enhances their structural integrity by welding them together Additionally, they are soaked in a 0.5M Ca²⁺ solution for approximately one day before further immersion in distilled water.
~ 1 day, self-welding hydrogels are obtained b Self-welding hydrogels using [Ca-alginate@C/0.5/5/8] hydrogels and [Ca- alginate/0.5/5] hydrogels
The fabrication process of self-welding hydrogels involves the use of [Ca-alginate@C/0.5/5/8] and [Ca-alginate/0.5/5] hydrogels Initially, the [Ca-alginate/0.5/5] hydrogels, measuring approximately 30 x 5 mm, are soaked in a Sodium Chloride solution for about one hour Following this, they are immersed in distilled water for roughly 45 minutes To facilitate welding with [Ca-alginate@C/0.5/5/8] hydrogels, the [Ca-alginate/0.5/5] hydrogels receive a filler coating after being soaked in NaCl solution for approximately 2.5 hours Finally, after a one-day soak in a 0.5M Ca²⁺ solution and an additional day in distilled water, the self-welding hydrogels are successfully obtained.
The fabrication process of self-welding hydrogels involves the use of [Ca-alginate@C/0.5/5/8] and [Ca-alginate/0.5/5] hydrogels, measuring approximately 30 x 8 mm Initially, the hydrogels are soaked in a Sodium Chloride solution for about 2.5 hours, followed by a 45-minute immersion in distilled water A filler coating is applied to facilitate the welding process Finally, the hydrogels are treated with a 0.5M Ca²⁺ solution for one day, and then soaked in distilled water for another day to achieve the desired self-welding properties.
CONDUCTIVE Ca-ALGINATE HYDROGELS APPLICATION
1 Main reasons for choosing conductive Ca-alginate hydrogels
Hydrogels have a vast range of applications across various fields, including health, agriculture, and daily life Among them, conductive hydrogels are emerging as revolutionary materials for flexible electronics, conductors, and soft robotics Unlike traditional rigid silicon-based electronics, flexible devices offer exceptional mechanical deformation capabilities and enhanced responsiveness for human-machine interactions The rise of flexible electronics presents challenges in meeting the demands for mechanical deformation, such as bending, folding, twisting, and stretching Conductive hydrogels are gaining significant attention as soft conductors for these devices due to their flexibility, high tensile strength, and conductivity, making them biodegradable and environmentally friendly Additionally, their self-welding ability positions them as promising candidates for flexible circuit applications.
1.2 Main reasons for choosing conductive Ca-alginate hydrogels
Global electronic waste is on the rise, as reported by various UN-affiliated agencies and organizations Improper recycling of electronic components, particularly those containing heavy metals, poses significant risks to human health, wildlife, and the environment Countries like China, India, and several African nations, including Nigeria's Lagos and Ghana's Accra, are particularly vulnerable due to lax regulations surrounding e-waste recycling and disposal.
The European Environment Agency (EEA) reports that global e-waste generation reaches approximately 40 million tons annually, with this category of waste increasing at a rate three times faster than other waste types This rapid growth is largely attributed to the rising consumption of electronic products.
66 continuously increasing, especially in populous and fast-growing countries such as China and India
Finding eco-friendly alternatives to heavy metals in electronic components is a pressing concern Conductive hydrogels, a cutting-edge material made from alginate extracts derived from algae and non-toxic cations like Ca²⁺, offer a non-toxic and environmentally friendly solution With excellent conductivity and self-welding capabilities, these hydrogels present a promising option for the electronics manufacturing industry The rapid advancement in this field is fueled by the collaborative efforts of scientists and engineers from various disciplines around the globe.
To meet the demands for durable and well-shaped finished products, it's essential to evaluate and enhance the mechanical properties of materials We have selected free dried [Ca-alginate@C/0.5/5/8] hydrogels for their optimal mechanical properties, making them suitable for various applications.
To illustrate the creation of electric conductors, we prepared free-dried [Ca-alginate@C/0.5/5/8] hydrogels, following the method outlined in chapter 3 Two samples measuring 30 x 5 mm were cut from the hydrogels and connected to a power source of 25V - 10A, with a light-emitting diode (LED) attached at the other end As the voltage was gradually increased from 17V to 25V, the brightness of the LED correspondingly intensified, demonstrating the relationship between voltage and light output.
Figure 52 Croocodie clips creating a temporary electrical connection
Figure 53 Pulse source with voltage of 25V
Figure 54 Light-emitting diode (LED) illumination using an electrical circuit comprising the [Ca-alginate@C/0.5/5/8] hydrogels, respectively, under an applied voltage of 25 V
2.2 Application 2: Fabrication flexible circuit board
Free dried [Ca-alginate@C/0.5/5/8] hydrogels were prepared using the method outlined in chapter 3 (Figure 20) These hydrogels were then cut into various dimensions for use as electric conductors The welding process involved attaching [Ca-alginate@C/0.5/5/8] hydrogels to a substrate made from isotropic [Ca-alginate/0.5/5] hydrogels, measuring 80 x 80 mm Leveraging the self-soldering properties of these hydrogels enables the creation of flexible circuit boards suitable for flexible electronic devices.
Figure 55 Diagram of the circuit used for the light-emitting diode (LED) testing The circuit consisted of three resistors 100Ω, one transistor (A1015), and one LED
Figure 56 Fabrication flexible electric circuit board (a) A substrate based on isotropic
[Ca-alginate/0.5/5] hydrogels (with dimensions is 80 x 80 mm), (b) Welding samples of
[Ca-alginate@C/0.5/5/8] hydrogels onto a substrate based on isotropic [Ca-alginate/0.5/5] hydrogels to make a flexible circuit board
The flexible electric circuit board, illustrated in Figure 57, showcases a bonded Ca 2+ structure created by welding hydrogels of [Ca-alginate@C/0.5/5/8] onto isotropic [Ca-alginate/0.5/5] hydrogels This innovative design successfully powers an LED prototype when connected to a 55 V power supply.
CONCLUSIONS
Recent advancements in conductive alginate hydrogels have been significantly informed by previous research, providing a comprehensive understanding of their synthesis, chemical structure, properties, and applications Conductive alginate hydrogels, prepared through the diffusion method, exhibit enhanced mechanical properties following structural modifications, with Young's modulus, tensile strength, and strain showing remarkable improvements compared to initial hydrogels The incorporation of activated carbon has notably increased their electrical conductivity, while the hydrogels' self-welding capability presents new opportunities for flexible electronic devices Practical applications have been successfully implemented, including electric conductors and flexible circuit boards, aligning with current scientific and engineering advancements Future research should focus on enhancing electrical and thermal conductivity, as well as self-healing properties, to unlock further applications in biomedicine, micro-nano robotics, and flexible electronics, capitalizing on the superior load-bearing capabilities of these innovative materials Continued exploration is essential to fully realize the potential of these advanced hydrogels.
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