The development of electrostimulation devices for wound care shows a promising approach to treatment methods. Paired with the properties of hydrogels, this novel method of treatment merges the fields of bioelectronics and regenerative medicine to address a persistent healthcare challenge: the treatment of chronic venous leg ulcers (CVLUs). CVLUs are characterized by impaired circulation in the veins of the legs, specifically due to damage inside leg veins caused by sustained venous hypertension (John Hopkins University, 2020). The most common type of lower extremity wound is attributed to CVLUs, and even when properly treated by traditional methods, around 40% of CVLUs fail to heal. Traditional treatments for CVLUs often include debridement, wound care, bed rest, compression, and silver-based dressings (Kelechi et al., 2021). Interestingly, there is unavailable evidence to support the efficacy of such dressings, yet surgical operations are typically unavailable due to patient circumstances. Even for patients who are treated, options for advanced wound care are only evaluated after four weeks of failed treatment, and the current advanced ulcer therapies require more robust evidence to confirm their efficacy (Ren, 2020). CVLUs remain a pressing issue in the world, and urgently require a more effective treatment plan to elevate patient experience. Since traditional treatments do not target the cause of CVLUs, the reccurance rate is as high as 22% in the first three months, 57% in the first twelve months, and 78% by three years (He et al., 2024). 

Applications of electrostimulation with regard to applying currents to biological tissues have demonstrated significant potential in wound healing by promoting angiogenesis, appropriate cell migration, and tissue regeneration. Combined with hydrogels, this approach to healing CVLUs, an optimal wound-healing environment can be created to mimic the extracellular matrix and provide an accessible way to enhance therapeutic efficacy (Farber, 2021). Not only does this accelerate wound closure, but it also addresses the root cause of CVLUs, promoting healthier circulation. However, despite the promise of such technology, there are significant barriers that we hope to address with this investigation. Current electrostimulation devices often exhibit limitations in terms of flexibility and biocompatibility with the patient. These challenges are frequently associated with the rigid structure of such devices, causing an inability to conform to irregular wound shapes and are typically ineffectively integrated with wound dressings, thus resulting in poor contact with wounds or inconsistent deliveries of electrical shocks.

Furthermore, while hydrogel-based approaches to wound healing are increasing in popularity, there are still drawbacks associated with their customizable properties, frequently lacking the necessary mechanical stability or conductivity to deliver effective treatments. To address these barriers, this investigation will focus on exploring a method to deliver localized, hydrogel-integrated electrostimulation that is designed for success with regard to the irregular contours of CVLUs. It will also adapt electrical currents to the patient's conditions with reference to resistance levels in the tissue of the skin. As there is a difference in skin tissue resistance levels between chronic wounds and regular wounds, we hope to use the method of electrostimulation to mimic the body's natural healing processes. This involves augmenting naturally occurring endogenous electric fields within the skin. In the context of the study, the hydrogel layer will be used to stabilize the severe bacterial infections of the wound, and an investigation will be conducted on different types of electrostimulation methods to accelerate the healing rate.

1. Hydrogel synthesis

The purpose of this study is to design a more effective way to heal CVLUs. The synthesis of hydrogels in this investigation targets three key properties: biocompatibility with wound, optimal moisture retention balance to increase healing rate, and stability of material. 

1.1 Introduction to Hydrogel Applications

Data samples from participants enrolled in publicly available studies before May 10, 2015 from PubMed, Embase, Cochrane Library, CINAHL, and clinicaltrials.gov online medical databases were used in a study analyzing the effectiveness of various hydrogel compositions from multiple dimensions. This comparative study analyzed the cost-effectiveness, RCTs, and systematic reviews of alginate, foam, hydrocolloid, hydrofiber, hydrogel, film, and acrylic dressings in the setting of venous leg ulcers. This study focused on the use of synthetic active dressings and the effectiveness of such treatment methods, with the intention of prioritizing cost-effectiveness, moist healing environments and preventing the loss of fluids or infections to prevent recurrence and promote an optimal environment for the healing of wounds (Saco et al., 2016).

For this portion of the comparative, it was observed that hydrogels were more effective in healing such ulcers than traditional wound dressings with a 95% confidence. Additionally, it was observed that non-adherent methods had a larger degree of effectiveness, despite needing secondary dressings or attachment, which prompts an investigation into the different methods of synthesizing hydrogels with materials which would reduce the moisture trapping effects and increase the antimicrobial properties of hydrogels, while still being readily accessible. Thus, the calcium alginate hydrogel was selected for its compatibility with the electrostimulation device used and dressing efficacies, including its non-toxic properties, controlled release capability, and mechanical stability (Matyash et al., 2013). 

1.2 Ideal Hydrogel Conditions

For more effective wound healing, the hydrogel should mimic physiological conditions. This includes ensuring appropriate moisture retention and engineering the hydrogel to incorporate bioactive molecules such as cytokines to regulate and prompt antimicrobial resistance, or various forms of interferons in the later stages of recovery to enhance tissue regeneration. With this in mind, multiple considerations can be made to maintain a healthy environment to prompt an increase in rates of tissue regeneration (Firlar et al., 2022). This includes taking appropriate measures to adjust the pH to be around 7.0, tailoring the crosslinking density to achieve controlled rates of biodegradation and match the wound healing timeline, as well as sterilizing the sodium alginate and calcium chloride solutions through membrane filters or autoclaving them at high temperatures. Finally, to ensure medical-grade synthesis of hydrogels, the sodium alginate solution should be cast in custom 3D-printed sterile moulds before crosslinking with calcium chloride and incubating to accelerate the healing process for the patient. A preliminary experiment determined that a 2% calcium chloride solution paired with sodium alginate concentrations ranging 1%-5% was the optimal concentration to meet these conditions.

1.3 Properties of Hydrogels

To ensure the biocompatibility of the hydrogel, measures can be taken to examine its mechanical properties. Firstly, the swelling behaviour of the hydrogel can be measured by a simple immersion test. Traditionally, hydrogels are immersed in Phosphate-buffered saline (PBS) solution and the difference in mass is found. For this experiment, the gel will be immersed for twenty-four hours and then massed to find an accurate swelling ratio. This swelling ratio can be characterized by the following:
                                                                                   SR = (W_s – W_d) / W_d
Where W_s is the mass after swelling, and W_d is the measured dry mass (Gupta et al., 2018).

There are various other properties and tests associated with hydrogels, such as Young's Modulus, SEM, or other mechanical measures that are not very relevant to this specific investigation. Although there are functional properties such as degradation, biocompatibility, and drug release, it is unfeasible to test in such a study and will be further evaluated in the future.

1.4 Rheology of Hydrogels

Various rheological tests can be performed to characterize the properties of hydrogels. Although we are unable to directly test of the above properties, these can be indirectly measured and observed. As the calcium alginate gel involves ionic bonding between the sodium alginate and calcium chloride, more flexible bonds are formed. Although this type of synthesis results in weaker hydrogel structures, introducing more rigid structures by chemically linking the alginate by covalent bonds would require the use of crosslinking agents, reducing biocompatibility and even introducing possible toxicity.

A further investigation of the viscoelastic properties of calcium alginate hydrogels can include the usage of these rheological tests, such as steady shear flow curves to find the applied shear rate dependent on the viscosity, a strain-sweep test to find the threshold strain required in relation to the linear viscoelastic region (LVR). Furthermore, a time-sweep test is indicative of the network structure formation over time, with the gelatin formation point indicated by the cross-over of the G' (storage modulus) and G" (loss modulus). A temperature-sweep test monitors the thermal stability of hydrogels, which suggests the temperatures at which hydrogels are structurally stable and indicates the temperature at which the hydrogel starts to degrade. To investigate the crosslinking behaviour or presence of a reversible network in a hydrogel, a frequency-sweep test can be adopted, and the interpreted graph will show a G' and G" (or lack of) crossover to indicate reversible, possibly self-healing properties hydrogels. Lastly, a creep recovery test reveals the magnitude of applied strain a hydrogel may withstand and its changing structure upon removal. For the purpose of this study, appropriate measures will be taken to investigate different rheological properties relevant to the approach (Stojkov et al., 2021).

Drug delivery mechanisms are complex, especially with drug interactions. Due to the nature of this investigation, collecting data from such interactions are not feasible, so drug data from various sources can be collected and analyzed based on effectiveness, while taking into account other risk factors. Hydrogels will be evaluated based on their potential for drug delivery in the future based on various properties (Li et al., 2016). However, it should be noted that this data is only to serve as a reference point, and actions should not be taken solely from this portion of the study, as every patient's case is unique and should be tailored to a suited method. This model will be ongoing and perfected with data over time, but for now, it simply serves to illustrate possible drug delivery mechanisms in an ongoing developing model. 

2. Electrostimulation Device

The primary objectives of this electrostimulation device was to succesfully measure the resistance of the skin tissue near the wound site, deliver pulses of electrical current to accelerate the healing process of the wound, and to communicate with a mobile device for status reporting. The design specifications of the electrostimulation device can be split into its electrical design and mechanical design components.

2.1 Electrostimulation Principles 

The skin is the body's largest organ and it plays a crucial role in acting as a defence mechanism to pathogenic threats. The skin is primarily composed of three layers, broadly categorized as the stratum corneum (outermost layer), viable skin (composed of epidermis and dermis), and subcutaneous tissue. The body possesses a biological electric field, with constant applied current in the skin, transported by polar, water-soluble molecules. The stratum corneum primarily affects this transport due to its hydrophobic properties, acting as the permeability barrier. Due to this, the skin (and the stratum corneum in particular) holds the greatest (and majority of) resistance to current. The stratum corneum, viable skin, and subcutaneous tissue typically have 10³-10⁶Ω, ~10Ω, and 10¹-10²Ω of resistance respectively (Abe et al., 2021). 

A chronic wound causes a break in the skin tissue which is unable to be repaired by the body. A typical wound goes through the four phases of healing (inflammatory, proliferative, epithelialization, and remodelling), but in chronic wounds, this process is significantly prolonged and may not be complete at all. After an injury, the resistance to current in the skin is lowered dramatically, and the applied current is disrupted by the break. Thus, methods of electrotherapy to stimulate the electric factors in the skin can be desirable for the acceleration of wound healing, especially Electrical Stimulation (ES), which can serve as a therapeutic model to enhance the healing of wounds (Farber, 2021). 

ES was found to be especially effective on wounds with impaired healing abilities due to underlying complications, such as CVLUs. Critically, systematic evaluations were conducted on the effectiveness of ES, and a statistically significant difference between ES and a control group (standard CVLU treatment or treatment involving no electrotherapy) was found in terms of wound healing rate (Hao et al., 2023). Furthermore, ES has been shown to not only treat CVLUs, but also mitigate the underlying cause of it: venous hypertension. Different types of ES have been found to activate venous flow and improve circulation, improving the overall health of the patient.

ES parameters are easily adaptable to various frequencies, duration of exposure, wave amplitudes, and pulse types. Modifying the settings based on a patient's recovery stage will be beneficial to accelerate wound healing. Low-frequency or high-intensity ES increases blood flow and improves circulation, whereas higher-frequency ES can stimulate fibroblast growth, contributing to the formation of connective tissue. Using these modifications, a customizable treatment plan can be created for every patient. ES applications are typically painless and there are multiple ways to administer, including using electrodes placed around the wound, or applying electro-biofeedback of ES using a device with electrodes placed on different sites around the wound. As well, bioelectric dressings or wireless application of ES can also be applied to a wound site. For this study, a bioelectric dressing is the most suitable method due to its ease of integration with the calcium alginate hydrogel and embedded electrodes to create the optimal wound-healing environment. 

2.2 Electrical Components

The core of the electrical design behind the device is the SEEED XIAO microcontroller, which manages the electrostimulation process and the monitoring of healing progress based on electrical impedance. It also communicates wirelessly with an external device to provide real-time tracking and configure the strength of the electrostimulation current. The microcontroller is programmed to generate a low-level pulsed current that is passed through the skin tissue via the electrodes. The current also flows through the LM334 current regulator chip, which ensures that it remains within a safe and therapeutic range (<10mA), preventing excess stimulation that could cause tissue damage. As mentioned previously, this design integrates BLE/WiFi connectivity, enabling real-time monitoring via a mobile app or PC interface. For the device we built, we specifically chose to test using a mobile app with raw Bluetooth data modulation capabilities for ease of debugging. The microcontroller, battery, and additional circuits are collectively referred to as the main electronics unit, or MEU for short.

A small LiPo battery (3.7V, 1100mAh) powers the system, allowing for long periods of operation. Official SEEED Xiao documentation indicates that when Bluetooth is enabled, the device will draw current at a rate of ~85mA. If we consider that current is also being diverted towards electrostimulation therapy, we can assume that the device will draw 100mA of current as a worst-case scenario. Even so, the device has a minimum runtime of ~11 hours, which can be prolonged by disabling Bluetooth when no device is actively accessing transmitted data, changing the duration of ES exposure, and putting the processor into deep sleep when not in use. During typical usage, the device would be drawing far less current than in the worse-case runtime scenario, given the current output of the LM334 is limited to 10mA. With a focus on safety, efficiency, and real-time feedback, this design aims to provide a non-invasive and effective solution for increasing chronic wound healing speeds.

2.3 Mechanical Design

The mechanical design of the electrotherapy stimulation device focuses on creating a compact, portable, and user-friendly system that is simple to operate. The electrical components are housed in a lightweight, 3D-printed enclosure made from PLA plastic, which is durable and easy to fabricate. The enclosure is designed with an ergonomic footprint shape to ensure comfort during use, and it can be attached to the body using adhesive or Velcro. The enclosure includes slots for the SEEED XIAO microcontroller, a small LiPo battery, and the LM334 current regulator. The front panel features a power button, an LED indicator to show device status, and two electrode ports for connecting the conductive pads. The enclosure is also designed for easy disassembly and is modular, as the wires connecting it to the electrodes can be hot-swapped. The entire device is designed to be worn on the body, allowing for continuous electrotherapy treatment without restricting movement.

2.4 Data Collection

For impedance measurement, the SEEED XIAO outputs a low-frequency AC test signal (10–100 Hz) through the wound electrodes. This will happen periodically as configured via the external control device. The electrostimulation function will be temporarily disabled while resistance is being measured. Since the XIAO lacks a Digital-to-Analog converter, the signal is generated using PWM, which effectively outputs current in pulses or bursts. This is smoothed with a low-pass filter using a resistor and capacitor to approximate a sine wave representing alternating current. This AC signal is applied across a known resistance to form a voltage divider, and the voltage drop across the wound is measured using the Analog-to-Digital converter pins on the ESP32-C3.

While a fixed resistor is typically used as the reference resistance, the inherent resistance of the electrodes themselves can be used instead. However, because electrode resistance varies due to factors such as material composition, surface contact, and environmental conditions, it must be measured before each impedance test. To achieve this, a small DC current is applied across the electrodes to measure their resistance directly, as DC provides a stable reference without introducing capacitive effects. Once the electrode resistance is determined, AC impedance measurements can proceed using the updated reference value. This process occurs periodically as configured via an external control device to ensure real-time monitoring of wound healing. To prevent interference, the electrostimulation function is temporarily disabled while resistance is being measured. 

As healing progresses, impedance changes can be recorded frequently and analyzed to assess recovery. Impedance changes as a wound heals, providing a direct indicator for tissue condition, where lower impedance indicates an early-stage wound with high moisture concentration, while higher impedance indicates more collagen formation and drier, healing tissue. By continuously measuring impedance before and after stimulation, the system can select the most effective stimulation settings at each healing stage. For low impedance wounds (100Ω - 1kΩ), the MEU applies low-frequency (1-10 Hz), high-intensity pulses to increase blood flow and cellular activity. As healing progresses and impedance rises (1kΩ - 10kΩ), this frequency is adjusted to 50-100 Hz, stimulating the migration of fibroblasts to the wound to accelerate tissue formation. For nearly healed wounds (>10kΩ), a high-frequency (>1 kHz) microcurrent is used to enhance cellular maintenance and regeneration.

3. Electrostimulation with Hydrogel Technology

A key consideration for both the electrotherapy technology and the hydrogel synthesization is the compatibility and integration to create a device that all can use. However, the challenging aspect will be the integration of these two technologies to be compatible with eachother and the wound. 

3.1 Aim

The main objective of integrating electrostimulation with hydrogel technology is to allow the hydrogel to maintain the optimal wound-healing environment while running continuous electrotherapy to promote cell growth. A secondary objective is for the device to remain compact so the patient can remain comfortable during the treatment process, with a Bluetooth function, making this technology accessible and user-friendly.

3.2 Integration

To operate both the hydrogel and electrostimulation technology simultaneously, we embedded the electrodes inside the hydrogel for optimal treatment effectiveness and ease of use. The electrodes consist of a thin layer of carbon film cut into different pieces. In order to ensure the ES therapy allows the wound to heal cohesively, we designed the electrodes so that they would achieve radial diffusion of current. The carbon film is cut into a small circular dot and a thin concentric ring that wraps around the outer circumference of the hydrogel. When we fabricate the hydrogel, we embed these two pieces into the bottom of the mould, along with the leads connecting them to the MEU. The sodium alginate solution is poured around it into the mould and crosslinked with the calcium chloride solution. The resulting solidified calcium alginate hydrogel can then be applied to the wound site, and its electrode leads connected to the MEU. When current is applied to the leads, it will travel into the anode—the ring of carbon film embedded inside the hydrogel. This current will then pass through the skin tissue and back into the anode, or the circular piece of carbon film, where it will travel back to the MEU. Through this method, we are able to stimulate the electrotaxis of newly formed tissue cells using the electric field generated by the current. Macrophages will migrate toward the anode and fibroblasts migrate toward the cathode. Neutrophils will migrate to both the anode and cathode.

Hydrogel Results

Throughout this investigation, we were unable to get access to a rheometer, but extensive data was collected with reference to various other sources, and observations were made. In this study, sodium alginate concentrations of 1%, 2%, 3%, and 5% were crosslinked with a 2% calcium chloride solution. Although the concentration of the calcium chloride solution remained unchanged, it is worth noting that an increase in the concentration of calcium chloride also increases the rigidity of the hydrogel structure due to stronger ionic crosslinking. Furthermore, the pH range was tested to be slightly acidic, and the PBS served as a pH buffer.

Mechanical Properties and Biocompatibility with PBS

The swelling ratio of each hydrogel concentration can be characterized as the following after being submerged in phosphate-saline solution (PBS) for 24 hours (average of 5 trials):

Sodium Alginate Concentration (%)	Initial ('Dry') Mass (g)	Final ('Wet') Mass (g)	Calculated Swelling Ratio (%)	Analysis
1	2.9	9.3	221	Very absorbent. It will require frequent replacement, which may be undesirable as a long-term dressing depending on the wound.
2	2.8	7.2	148	Optimal for moisture retention, ideal for applications which still require some structural support.
3	2.9	5.7	97	Ideal for wounds that need more mechanical strength with a moderate degree of moisture retention
5	2,9	4.6	59	Very brittle. Little swelling, and will experiene minimal reactions with the wound.

Discussion & Improvements

In the context of this study, a common application of hydrogels will be for the purpose of dressing wounds, aiming for an optimal balance between gel swelling or moisture retention and structural strength. To balance these two dimensions, the gel should not only be structurally supportive but also retain enough moisture to create a healthy environment for healing the wound. In this respect, a calcium alginate hydrogel with a sodium alginate concentration of 2% would be optimal for wounds that need more moisture retention and less support, whereas a 3% sodium alginate concentration may be ideal for wounds which require more mechanical strength. However, the situation of each patient is largely different, and should be taken into account while making decisions. Notably, the hydrogel with a 1% sodium alginate concentration is very flexible, and the soft material paired with the amazing moisture retention makes it the ideal candidate for wounds which require intensive and variable care, with dressings that need to be frequently replaced. 

pH of Hydrogel before and after submerging in phosphate-saline solution (PBS) for 24 hours (average of 5 trials):

Sodium Alginate Concentration (%)	pH of Hydrogel Before Submerging in PBS	pH of Hydrogel After Submerging in PBS
1	6.7	7.1
2	6.9	7.2
3	6.4	7.0
5	6.3	6.8

Discussion & Improvements

As shown from the experimental results, PBS acted as a pH buffer for the hydrogel. This can be integrated with the sodium alginate before crosslinking with the calcium chloride to ensure further biocompatibility and act as a safe, effective wound dressing. Furthermore, the integration of PBS within the hydrogel will increase conductivity, which could have impactful future applications for the hydrogel, such as a manipulation of the PBS concentration within localized hydrogel segments to increase the conductivity and target areas to perform electrotherapy.

Rheological Analysis

In regards to rheology, the hydrogels were closely examined in terms of the G' (storage module) and G" (loss module), also represented by the elastic or viscous components of the hydrogel. The below table is a compilation of some qualitative observations that were made during the hydrogel synthesis process from this study. Our results were in tandem with many other sources, but the inferences made regarding the relationship between G' and G" value should primarily be used for reference. 

Sodium Alginate Concentration (%)	Viscoelastic Properties (G' and G")	Rheological Behaviour	Analysis
1	Fairly inelastic. G' remains higher than G", but with a very small difference. The softer, almost liquid material does not hold its shape, indicating weak crosslinks.	Significant shear-thinning and easily succumbs to yield strain. Crosslinks form slower and are extremely sensitive to temperature changes.	Hydrogel is very environmentally sensitive, and the easily deformable properties makes it difficult to stabilize the wound structurally. However, the soft nature of the hydrogel makes it an optimal candidate for drug delievery systems throughout the treatment process.
2	Both G' and G" increase, yet G' experiences a noticeably higher increase, now around five times more than G". This enhances its elastic properties, indicated by a hydrogel which exhibits more solid-like properties.	Takes a relatively long time for crossover to occur, little to no self-recovery properties, some shear-thinning and development to overcome yield strain. Gel is not as sensitive to temperature changes and has a lower network (crosslink) plateau value.	Hydrogel is less prone to deformation but still soft enough to achieve the desired wound-stabilizing effects. Effective for uneven wounds with complex wound geography, which require more frequent modeling of optimized contact points to deliver the necessary electrostimulation. However, gel could deform with changing temperatures and will require mitigation to stabilize or adaptive measures such as frequent changes to hydrogel formula to ensure seamless integration.
3	There is a noticeable and significant increase in G', which tremendously increased the elastic properties of the hydrogel. A rigid structure is adopted by the hydrogel, but it is still able to bend to a useful extent.	Fairly resistant to deformations, stronger crosslinks may exhibit higher recovery rates and decrease the exhibition of shear-thinning. Relatively stable despite temperature changes and forms faster crosslinks.	The resistance to yield strain makes this hydrogel concentration desirable for wound environments that are constantly changing. The thermal stability, paired with the efficient crosslink networking, ensures that the status of a wound can be easily measured. However, it is harder to adjust and integrate into wounds with uneven surfaces.
5	Very elastic to the point of rigidity. G' is significantly higher than G", and gel is very hard to bend. This concentration indicates ultra-strong crosslinks, holding the shape of the hydrogel together.	Theoretically resistant to deformations, but due to the rigidity of the material, it does not have much potential for self-healing and has a very short time for crosslinking. Hydrogel is insensitive to thermal changes and very stable.	The rigid nature of this hydrogel makes the application on wounds sub-optimal, possibly causing patient discomfort or irritation. However, this is a desirable trait for wound dressings once the surface of the wound is less sensitive and can be utilized in the later stages of treatment as a mechanism for structural support.

Discussion

A reference from various other studies matched with the physically observable properties of the hydrogel allows for an analysis of the different properties of such hydrogels, making it possible to determine the phase of implementation. Common trends include an increase in elasticity with increasing sodium alginate concentration as the strength of ionic crosslinking increases, and the solidifying properties of the gel are useful to determine the stage in recovery in which the hydrogel should be applied. Through this investigation, results amplify the strengths of a 1% sodium alginate concentration hydrogel as being adaptable and easy to apply and integrate onto the wound, needed in the earlier stages of wound stabilization for effective drug delivery and optimal contact with the wound. On the other hand, the 2% sodium alginate hydrogel is suitable for wounds with somewhat stable environments but require electrostimulation in specific points of contact, and the 3% hydrogel would be more tailored towards constantly shifting wound environments with a need for constant monitoring and simple electrostimulation delivery mechanisms. The 5% sodium alginate hydrogel could be utilized in the later phases of wound recovery, specifically in regards to novel dressing methods for providing effective structural support. A combination of different concentrations should be used to target different environments and treatment stages, as these hydrogels are highly adaptable to individual patient needs. 

Electrostimulation Results

Electrode Sensing

We sought to determine the most optimal frequency to alternate the electrical current for sensing the resistance of skin tissue. The anode and cathode electrode pads were connected with a 5kΩ resistor to stimulate somewhat dry skin tissue. Various frequencies of alternating current were pulsed through the electrodes, and the measured voltage by the ADC was converted to a resistance value based on a peak AC voltage of 1.5V. Given that the electrodes themselves are not metallic, they have slight amounts of electrical resistance in proportion to skin tissue. This was accounted for in our test by calibrating the device to exclude the resistance of the electrodes, which was measured to be ~220Ω.

Frequency (Hz)	Voltage (V)	Resistance (Ω)	Calibrated Resistance (Ω)	Percent error (%)
10	1.0160	5228.7952	5008.7952	0.1759
50	1.0159	5208.5297	4988.5297	-0.2294
100	1.0158	5205.1312	4985.1312	-0.2974
250	1.0155	5168.3479	4948.3479	-1.0330
500	1.0137	4972.5725	4752.5725	-4.9485
750	1.0134	4941.9141	4721.914	-5.5617
1000	1.0136	4959.2078	4739.2078	-5.2158

Discussion

At lower frequencies, the measured resistance value closely matches with the expected value (5000Ω), with minimal error ranging <1%. As frequency increases, however, there are apparent deviations from the expected resistance, likely due to signal attenuation. One possible explanation for this effect could be that the increased frequency of the AC current decreases its wavelength, making it more difficult for the current to pass through the skin layers. With this test and these results, we can monitor the change in skin resistance to treatment and use these conditions to track the treatment progress. In the future, we will also be able to monitor electrotaxis and cell proliferation, monitoring interactions of drug delivery mechanisms along with ES delivery, optimizing wound healing processes. As shown from experimental results, an increased frequency of AC current was associated with an increased percent error, it should be carefully moderated to ensure patient comfort.

MPC Model Testing

The efficacy of the MPC model was evaluated in terms of its ability to adapt to differing values of skin resistance. To keep many variables constant, the "measured" skin resistance that would typically be detected using the electrodes were swapped out for a variable potentiometer connecting the electrode leads. The resistance started at 500Ω to represent low skin impedance, with it increasing in increments of 1kΩ during every iteration of the MPC algorithm to represent the wound closing and its impedance increasing. 

Measured Resistance (Ω)	Electrostimulation Frequency (Hz)	Electrostimulation Intensity (0-255)
500	10.49	250.96
1500	104.21	230.49
2500	201.21	212.44
3500	309.59	182.86
4500	418.64	169.26
5500	506.62	149.36
6500	614.25	132.86
7500	700.15	113.00
8500	801.67	101.35
9500	900.48	75.55
10500	1008.41	47.64

Discussion

A comparison of the measured impedance values and the corresponding electrostimulation parameters calculated by the MPC algorithm allows for the identification of trends in how changes in impedance optimize electrostimulation factors for different conditions. As expected, increased resistance correlates to increased measured impedance, leading to a progressive shift in stimulation frequency and intensity. The system's response aligns with the established principle that lower impedance (indicative of high moisture or early-stage wounds) requires lower-frequency, higher-intensity stimulation, whereas higher impedance (indicative of drier, more stable tissue) benefits from higher-frequency, lower-intensity stimulation to prevent overstimulation. Thus, frequency and ES intensity can be customized for each patient while accounting for their treatment progress, adaptable and easy to change when needed. 

Analysis

Overall, the majority of treatments will require hydrogels with 2%-3% sodium alginate concentration, along with changing ES frequencies and intensities throughout the process. In the future, more tests could be conducted on the hydrogel, such as time for crosslinking, gel elasticity, and bonding strength. As well, different tests and datasets could be utilized for the ES to examine wound interactions, such as Percentage Area Reduced (PAR) over a period of time. However, this treatment is much more cost-effective and customizable, holding great advantages over current solutions. Not only is our device compact and user-friendly, it also uses AC to target venous hypertension, improving circulation and lowering rates of reccurance. 

In this study, we successfully engineered a wound dressing made of adaptable hydrogel from a customized mould which was able to deliver proper electrostimulation and modified based on the needs of the patient. Characterizations of the hydrogel material revealed that the typical dressing will utilize a calcium alginate hydrogel with a sodium alginate concentration of 2-3%. For a future extension of this project involving drug delivery with the hydrogels, a 1% sodium alginate hydrogel was found to be the optimal candidate for drug delivery for patients in need. Furthermore, the electrostimulation techniques used in this investigation proved to be extremely effective, using modified settings based depending on the healing of the wound. The results for the ES were as expected; the device was able to adjust to resistance in the skin to target and promote cell growth based on the current progress of the wound, with appropriately adjusted frequencies.

Further research can be done in the presence of a rheometer, to accurately quantify the properties of hydrogels. As well, PBS can be incorporated into different phases of the hydrogel synthesis process to increase biocompatibility. Additionally, the ES device can be effectively improved by making the device even more compact by using a custom printed circuit board and smaller, circular batteries similar to those found in smartwatches. Attaching the MEU directly onto the hydrogel is also an area of interest, as it can reduce complexity and rid the requirement of connecting wires, and looking into using multi-electrode arrays for targeted stimulation would make the treatment process even more customizable. With enough research and development, we believe the form factor of the device could be shrunk to that of a continuous glucose monitor device, which already exists for commercial use.

There are many practical applications of this treatment. First, this model promotes constant monitoring, increasing the likelihood of completely healing a CVLU. Secondly, this treatment is especially important to seniors or those with already reduced mobility, as patients will not need to go to the hospital for administering treatment, only for check-ups or if assistance is required. Thirdly, this technique may even be applicable to other types of chronic wounds, with slight adjustments to account for differences in wound behaviour. Our innovative solution to CVLUs hold many advantages over current solutions, with lots of room for future improvements. Overall, this is a successful model with many opportunities for growth, and serves as a cost-effective, adaptable, and accessible way for CVLUs to be healed efficiently in a larger capacity.

Abe, Y., & Nishizawa, M. (2021). Electrical aspects of skin as a pathway to engineering skin devices. APL Bioengineering, 5(4). https://doi.org/10.1063/5.0064529 

Bahadoran, M., Shamloo, A., & Nokoorani, Y. D. (2020). Development of a polyvinyl alcohol/sodium alginate hydrogel-based scaffold incorporating BFGF-encapsulated microspheres for accelerated wound healing. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-64480-9 

Cuomo, F., Cofelice, M., & Lopez, F. (2019). Rheological characterization of hydrogels from alginate-based nanodispersion. Polymers, 11(2), 259. https://doi.org/10.3390/polym11020259 

Demidova-Rice, T. N., Hamblin, M. R., & Herman, I. M. (2012). Acute and impaired wound healing. Advances in Skin &amp; Wound Care, 25(7), 304–314. https://doi.org/10.1097/01.asw.0000416006.55218.d0 

Farber, P. L., Isoldi, F. C., & Ferreira, L. M. (2021). Electric factors in wound healing. Advances in Wound Care, 10(8), 461–476. https://doi.org/10.1089/wound.2019.1114 

Firlar, I., Altunbek, M., McCarthy, C., Ramalingam, M., & Camci-Unal, G. (2022). Functional hydrogels for treatment of chronic wounds. Gels, 8(2), 127. https://doi.org/10.3390/gels8020127 

Fish, R. M., & Geddes, L. A. (2009). Conduction of electrical current to and through the human body: a review. Eplasty, 9, e44. https://pmc.ncbi.nlm.nih.gov/articles/PMC2763825/

Gupta, S., & Bit, A. (2018). Rapid prototyping for polymeric gels. Polymeric Gels, 397–439. https://doi.org/10.1016/b978-0-08-102179-8.00016-8 

Hao, Q., Hamson, A., & Horton, J. (2023, July). Electrostimulation devices for wounds. CADTH Health Technology Review. https://www.ncbi.nlm.nih.gov/books/NBK595136/ 

He, B., Shi, J., Li, L., Ma, Y., Zhao, H., Qin, P., & Ma, P. (2024). Prevention strategies for the recurrence of venous leg ulcers: A scoping review. International Wound Journal, 21(3). https://doi.org/10.1111/iwj.14759 

Hossain, M. T., & Ewoldt, R. H. (2022). Do-it-yourself rheometry. Physics of Fluids, 34(5). https://doi.org/10.1063/5.0085361 

Kelechi, T. J., Muise-Helmericks, R. C., Theeke, L. A., Cole, S. W., Madisetti, M., Mueller, M., & Prentice, M. A. (2021). An observational study protocol to explore loneliness and systemic inflammation in an older adult population with chronic venous leg ulcers. BMC Geriatrics, 21(1). https://doi.org/10.1186/s12877-021-02060-w 

Larsen, B. E., Bjørnstad, J., Pettersen, E. O., Tønnesen, H. H., & Melvik, J. E. (2015). Rheological characterization of an injectable alginate gel system. BMC Biotechnology, 15(1). https://doi.org/10.1186/s12896-015-0147-7 

Li, J., & Mooney, D. J. (2016, October 18). Designing hydrogels for controlled drug delivery. Nature Reviews. https://www.nature.com/articles/natrevmats201671 

Liu, X., Qian, L., Shu, T., & Tong, Z. (2002). Rheology characterization of sol–gel transition in aqueous alginate solutions induced by calcium cations through in situ release. Polymer, 44(2), 407–412. https://doi.org/10.1016/s0032-3861(02)00771-1 

Matyash, M., Despang, F., Ikonomidou, C., & Gelinsky, M. (2013, November 6). Swelling and mechanical properties of alginate hydrogels with respect to promotion of neural growth. Tissue engineering. Part C, Methods. https://pubmed.ncbi.nlm.nih.gov/24044417/ 

McDaniel, J. C., Roy, S., & Wilgus, T. A. (2013). Neutrophil activity in chronic venous leg ulcers—a target for therapy? Wound Repair and Regeneration, 21(3), 339–351. https://doi.org/10.1111/wrr.12036 

Rajendran, S. B., Challen, K., Wright, K. L., & Hardy, J. G. (2021). Electrical stimulation to enhance wound healing. Journal of Functional Biomaterials, 12(2), 40. https://doi.org/10.3390/jfb12020040 

Ren, S.-Y., Liu, Y.-S., Zhu, G.-J., Liu, M., Shi, S.-H., Ren, X.-D., Hao, Y.-G., & Gao, R.-D. (2020). Strategies and challenges in the treatment of chronic venous leg ulcers. World Journal of Clinical Cases, 8(21), 5070–5085. https://doi.org/10.12998/wjcc.v8.i21.5070 

Saco, M., Howe, N., Nathoo, R., & Cherpelis, B. (2016). Comparing the efficacies of alginate, foam, hydrocolloid, hydrofiber, and hydrogel dressings in the management of diabetic foot ulcers and venous leg ulcers: A systematic review and meta-analysis examining how to dress for success. Dermatology Online Journal, 22(8). https://doi.org/10.5070/d3228032089 

Senin-Camargo, F., Martínez-Rodríguez, A., Chouza-Insua, M., Raposo-Vidal, I., & Jácome, M. A. (2022). Effects on venous flow of transcutaneous electrical stimulation, neuromuscular stimulation, and sham stimulation on soleus muscle: A randomized crossover study in healthy subjects. Medicine, 101(35). https://doi.org/10.1097/md.0000000000030121 

Spectric Precision Instrumentation and Controls Company. (2016). A Basic Introduction to Rheology. Rheology and Viscosity. https://cdn.technologynetworks.com/TN/Resources/PDF/WP160620BasicIntroRheology.pdf 

Stojkov, G., Niyazov, Z., Picchioni, F., & Bose, R. K. (2021). Relationship between structure and rheology of hydrogels for various applications. Gels, 7(4), 255. https://doi.org/10.3390/gels7040255 

Venous ulcers. Johns Hopkins Medicine. (2020, July 20). https://www.hopkinsmedicine.org/health/conditions-and-diseases/venous-ulcers 

We would like to acknowledge our parents, for helping us through this journey and being patient even though it was a tedious process, yet they never doubted us and always encouraged us to keep trying. We would also like to acknowledge the lab technician at our school, Ms. Bhatt, for allowing us to use materials and the lab environment to conduct tests on the hydrogels. Lastly we want to acknowledge Ms. Trainor, for answering all of our questions regarding CYSF and supporting this project.