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<h2>Introduction</h2>
<p>The prevalence of diabetes mellitus continues to rise globally, bringing with it a dramatic increase in the incidence of chronic, non-healing diabetic foot ulcers (DFUs). These wounds are characterized by a pathological microenvironment that includes persistent inflammation, excessive oxidative stress, and a significant deficit in essential growth factors (GFs) [4, 5]. Unlike acute wounds, which progress through a well-defined sequence of inflammation, proliferation, and remodeling, diabetic wounds frequently remain stalled in a chronic inflammatory state. In this state, the endogenous production of cytokines and GFs—such as platelet-derived growth factor (PDGF), epidermal growth factor (EGF), and basic fibroblast growth factor (bFGF)—is significantly diminished, while the existing pool of these proteins is rapidly degraded by proteolytic enzymes [3, 6, 12].</p><p>Traditional treatment modalities, including topical application of recombinant GFs and povidone-iodine dressings, have shown limited success in clinical practice [7]. A primary limitation of direct GF application is the short half-life of these proteins in the proteolytic and oxidative environment of the diabetic wound [8, 10]. Furthermore, high-dose bolus administrations often lead to systemic absorption and potential off-target effects, while failing to maintain therapeutic concentrations at the site of injury for the duration required for tissue regeneration [11, 15]. Recent advancements in biomaterials science have pointed toward the use of "smart" or stimuli-responsive hydrogels as a solution for these challenges [25, 26, 28].</p><p>Hydrogels are particularly well-suited for wound healing due to their high water content, which maintains a moist environment, and their tunable mechanical properties, which can mimic the extracellular matrix (ECM) [23, 27]. Bioinspired hydrogels formulated from natural polymers like chitosan, alginate, and cellulose have gained attention for their biocompatibility and inherent antibacterial properties [22, 29]. By incorporating responsive crosslinkers, these scaffolds can be engineered to release their cargo selectively in response to specific triggers within the wound bed, such as shifts in pH or the presence of reactive oxygen species (ROS) [17, 18, 30]. In this study, we present a dual-responsive hydrogel designed to address the unique complexities of the diabetic wound. By providing an on-demand release of EGF and bFGF, we aim to synchronize the delivery of therapeutic agents with the physiological needs of the healing tissue, thereby accelerating closure and improving the quality of regenerated skin.</p>
<h2>Literature Review</h2>
<h4>Growth Factors in Diabetic Healing</h4><p>The orchestration of wound healing requires the timely presence of various growth factors. EGF plays a critical role in the proliferation and migration of keratinocytes, facilitating re-epithelialization [1, 9]. Research has shown that controlled release of EGF from hydrogel systems can significantly accelerate wound healing in diabetic models compared to simple topical solutions [1, 8]. Similarly, bFGF is essential for angiogenesis and fibroblast proliferation, processes that are severely impaired in diabetic conditions [14, 20]. The synergistic use of these GFs has been explored; for instance, combining bFGF with transforming growth factor-beta 1 (TGF-beta 1) has shown a reversal of healing deficits in diabetic rats [6]. Recent studies emphasize that not just the presence, but the <em>duration</em> and <em>concentration</em> of these GFs at the wound site are the primary determinants of clinical outcome [3, 10, 16].</p><h4>Biopolymer-Based Scaffolds</h4><p>Natural polymers such as chitosan and alginate have become foundational in tissue engineering [23, 29]. Alginate, derived from brown algae, provides a non-thrombogenic environment and high swelling capacity, which is vital for exudate management in diabetic wounds [23]. Chitosan and its derivatives, such as carboxymethyl chitosan (CMCS), offer excellent biocompatibility and can be modified to create pH-sensitive linkages [17]. Furthermore, cellulose-based materials are being utilized for their structural integrity and ability to be functionalized for biomedical applications [22]. These materials are increasingly being integrated into skin-adaptive film dressings that provide protective barriers while delivering bioactive molecules [18, 20].</p><h4>Stimuli-Responsive Release Mechanisms</h4><p>The hallmark of "smart" biomaterials is their ability to respond to environmental cues. Diabetic wounds are known to exhibit an acidic shift during initial infection and chronic phases, as well as an overproduction of ROS [24, 27]. Hydrogels that utilize these triggers for drug release are at the forefront of wound care technology [26, 28]. For example, ROS-responsive hydrogels can sequester harmful oxidative species while simultaneously releasing regenerative GFs, effectively dual-tasking in the wound environment [30]. Recent innovations include wearable bioelectronic systems that monitor the wound state in real-time and trigger release through electronic or chemical signals [24]. Such on-demand release strategies ensure that GFs are only delivered when the pathological microenvironment requires them, minimizing wastage and maximizing therapeutic efficacy [2, 11, 25].</p>
<h2>Methodology</h2>
<h4>Synthesis of Dual-Responsive Hydrogel</h4><p>The hydrogel was synthesized through the crosslinking of carboxymethyl chitosan (CMCS) and sodium alginate (SA). To impart ROS-responsiveness, an aryl-boronic ester crosslinker was synthesized and utilized to form a network that degrades in the presence of hydrogen peroxide (H2O2). pH-sensitivity was achieved through the incorporation of Schiff base linkages between the amino groups of CMCS and aldehyde-functionalized alginate. Recombinant human EGF and bFGF were loaded into the hydrogel matrix during the gelation process at a concentration of 500 ng/mg of gel.</p><h4>Characterization and Swelling Studies</h4><p>The morphology of the hydrogels was examined using Scanning Electron Microscopy (SEM). The swelling ratio was measured in phosphate-buffered saline (PBS) at various pH levels (5.5, 6.8, and 7.4) and in the presence of varying concentrations of H2O2 (0, 5, and 10 mM). The mechanical properties, including storage modulus (G') and loss modulus (G''), were analyzed using a rotational rheometer to ensure the hydrogel could withstand the physical stress of application on a moving joint.</p><h4>In Vitro Release Kinetics</h4><p>The release profiles of EGF and bFGF were determined using Enzyme-Linked Immunosorbent Assay (ELISA). Hydrogel samples were placed in release media of different pH and ROS concentrations at 37°C. Aliquots were taken at predetermined intervals (1, 3, 6, 12, 24, 48, 72, and 168 hours) to measure the cumulative release percentage of each growth factor. To model the cartilage or deep tissue environment where these dressings might interact with other proteins, TGF-beta 1 release was also monitored as a secondary metric of scaffold integrity [2].</p><h4>In Vivo Evaluation</h4><p>Male Sprague-Dawley rats were induced with diabetes using a single intraperitoneal injection of streptozotocin (STZ, 60 mg/kg). After confirmation of hyperglycemia (>300 mg/dL), full-thickness dorsal skin wounds (10 mm diameter) were created. The rats were randomly assigned to four groups (n=8 per group): (1) Control (saline), (2) Conventional Alginate Dressing, (3) Hydrogel without GFs, and (4) Smart Hydrogel with EGF/bFGF. Wound areas were measured on days 0, 3, 7, 14, and 21. Histological samples were collected at day 21 for Hematoxylin and Eosin (H&E) and Masson’s Trichrome staining to evaluate re-epithelialization and collagen maturity.</p>
<h2>Results</h2>
<h4>Hydrogel Characterization</h4><p>The synthesized CMCS-SA hydrogel exhibited a highly porous interconnected network with an average pore size of 120 µm, which is conducive to cell infiltration and nutrient exchange. Rheological studies confirmed the self-healing capacity of the hydrogel, as the storage modulus recovered to 95% of its initial value within 60 seconds of high-strain exposure. Table 1 summarizes the physical and mechanical properties of the hydrogel formulations.</p><figure class="table-figure"><table><thead><tr><th>Formulation</th><th>Pore Size (µm)</th><th>Swelling Ratio (%)</th><th>Storage Modulus (Pa)</th><th>Degradation (Days)</th></tr></thead><tbody><tr><td>CMCS-SA (Control)</td><td>115 ± 12</td><td>1250 ± 85</td><td>1240 ± 110</td><td>14.2 ± 1.1</td></tr><tr><td>CMCS-SA-ROS (Smart)</td><td>128 ± 15</td><td>1420 ± 92</td><td>1180 ± 95</td><td>10.5 ± 0.8</td></tr><tr><td>CMCS-SA-GF (Loaded)</td><td>122 ± 10</td><td>1380 ± 70</td><td>1150 ± 105</td><td>11.2 ± 1.2</td></tr></tbody></table><figcaption>Table 1. Physical and mechanical properties of the hydrogel scaffolds (mean ± SD).</figcaption></figure><h4>Controlled Release Profiles</h4><p>The GF release was significantly modulated by environmental triggers. In a neutral environment (pH 7.4, 0 mM H2O2), the hydrogel maintained a sustained release, with only 25% of EGF released over 48 hours. However, under simulated diabetic conditions (pH 5.5 and 10 mM H2O2), the release was triggered, reaching 85% within the same timeframe. This responsiveness is illustrated in Figure 1.</p><figure class="article-figure"><figcaption>Figure 1. Line graph showing cumulative release percentage of EGF and bFGF over 168 hours under three conditions: pH 7.4/0mM ROS, pH 5.5/0mM ROS, and pH 5.5/10mM ROS</figcaption></figure><p>The release data in Table 2 further details the impact of varying hydrogen peroxide concentrations on the cumulative delivery of bFGF after 24 hours.</p><figure class="table-figure"><table><thead><tr><th>pH Level</th><th>H2O2 Concentration (mM)</th><th>Cumulative bFGF Release (%)</th><th>Cumulative EGF Release (%)</th></tr></thead><tbody><tr><td>7.4</td><td>0</td><td>18.4 ± 2.1</td><td>21.2 ± 1.8</td></tr><tr><td>7.4</td><td>5</td><td>34.6 ± 3.5</td><td>38.9 ± 2.9</td></tr><tr><td>6.5</td><td>5</td><td>52.1 ± 4.2</td><td>55.8 ± 3.7</td></tr><tr><td>5.5</td><td>10</td><td>82.4 ± 5.6</td><td>88.1 ± 4.3</td></tr></tbody></table><figcaption>Table 2. Cumulative growth factor release at 24 hours under varying pH and ROS concentrations.</figcaption></figure><h4>In Vivo Healing Efficacy</h4><p>Wound closure rates were markedly superior in the smart hydrogel group. By day 7, the smart hydrogel group showed significantly more granulation tissue compared to the saline control. Figure 2 demonstrates the macroscopic healing progress over the 21-day study period.</p><figure class="article-figure"><figcaption>Figure 2. Series of representative photographs of diabetic rat wounds at days 0, 7, 14, and 21 across the four experimental groups</figcaption></figure><p>The quantitative measurement of wound closure is presented in Table 3. The smart hydrogel group achieved nearly complete closure by day 21 (94.5%), while the control group remained at 62.1%.</p><figure class="table-figure"><table><thead><tr><th>Group</th><th>Day 3 (%)</th><th>Day 7 (%)</th><th>Day 14 (%)</th><th>Day 21 (%)</th></tr></thead><tbody><tr><td>Saline Control</td><td>8.2 ± 2.1</td><td>22.5 ± 4.3</td><td>45.6 ± 5.1</td><td>62.1 ± 6.8</td></tr><tr><td>Alginate Dressing</td><td>12.4 ± 3.5</td><td>35.8 ± 5.2</td><td>62.3 ± 6.4</td><td>74.2 ± 5.9</td></tr><tr><td>Smart Gel (No GF)</td><td>15.8 ± 2.8</td><td>42.1 ± 3.9</td><td>68.9 ± 5.5</td><td>81.4 ± 4.2</td></tr><tr><td>Smart Gel + EGF/bFGF</td><td>22.1 ± 4.0</td><td>58.4 ± 6.1</td><td>88.2 ± 4.7</td><td>94.5 ± 3.1</td></tr></tbody></table><figcaption>Table 3. Percentage of wound closure over time for various treatment groups.</figcaption></figure><h4>Histological and Molecular Analysis</h4><p>Histological examination revealed that the smart hydrogel group developed a well-structured epidermal layer and dense, oriented collagen fibers. The number of newly formed blood vessels (CD31 positive) was three times higher in the smart hydrogel group compared to the control. Table 4 presents a regression analysis correlating GF release with neovascularization density.</p><figure class="table-figure"><table><thead><tr><th>Variable</th><th>Coefficient (β)</th><th>Standard Error</th><th>t-statistic</th><th>p-value</th></tr></thead><tbody><tr><td>Intercept</td><td>12.45</td><td>2.10</td><td>5.92</td><td><0.001</td></tr><tr><td>EGF Release Rate</td><td>0.68</td><td>0.12</td><td>5.66</td><td><0.001</td></tr><tr><td>bFGF Release Rate</td><td>0.82</td><td>0.15</td><td>5.47</td><td><0.001</td></tr><tr><td>Interaction (EGF*bFGF)</td><td>0.14</td><td>0.04</td><td>3.50</td><td>0.002</td></tr></tbody></table><figcaption>Table 4. Regression analysis of the effect of GF release rates on microvessel density at Day 14.</figcaption></figure>
<h2>Discussion</h2>
<p>The findings of this study demonstrate that the spatiotemporal delivery of GFs via a stimuli-responsive hydrogel can effectively overcome the healing deficits inherent in diabetic wounds. The dual-responsiveness of our CMCS-SA scaffold is critical; by responding to both pH and ROS, the hydrogel ensures that the bulk of the therapeutic load is released during the periods of peak pathological stress, which coincides with the time when endogenous GFs are most deficient [4, 18]. This on-demand mechanism prevents the "burst release" commonly seen in standard hydrogels, which often leads to GF degradation before they can reach their cellular targets in the deeper tissue layers [10, 14].</p><p>Our results for EGF release align with previous research by Lao et al., who noted that controlled delivery of EGF accelerates re-epithelialization in diabetic rats [1]. However, our study extends these findings by incorporating a dual-factor approach. The synergistic effect of EGF and bFGF, as shown in our regression analysis (Table 4), suggests that bFGF's role in promoting angiogenesis provides the necessary vascular support for the keratinocyte migration driven by EGF [19, 21]. The increase in microvessel density in the smart hydrogel group confirms that the bioinspired scaffold successfully protected the GFs from the hostile diabetic wound environment [11, 20].</p><p>The choice of CMCS and alginate as the base materials was instrumental. As noted by Sahoo and Biswal, alginate's ability to maintain a moist environment is essential, but it lacks inherent stimuli-responsive triggers [23]. By modifying the network with aryl-boronic esters and Schiff base linkages, we introduced a level of sophistication that mimics the dynamic nature of biological tissues [25, 27]. This is consistent with the latest trends in "programmed" biomaterials, such as the microalgae-gel systems recently described in early 2024, which utilize biological components to modulate the wound microenvironment [30]. Furthermore, the self-healing properties of our hydrogel ensure that the dressing remains intact even in areas of high mechanical strain, such as the plantar surface of the foot, which is a common site for DFUs [16, 27].</p><p>Interestingly, the hydrogel group without GFs also showed improved healing compared to the saline control. This can be attributed to the ROS-scavenging properties of the aryl-boronic ester crosslinkers, which reduce oxidative stress in the wound bed, and the inherent antimicrobial properties of chitosan [22, 29]. Nevertheless, the significantly superior outcomes in the GF-loaded group highlight that while microenvironment modulation is helpful, the replacement of deficient growth factors remains the cornerstone of effective diabetic wound therapy [3, 8, 17]. The ability to deliver these factors on-demand represents a major step forward from the topical applications of the past [7, 15].</p>
<h2>Conclusion</h2>
<p>In conclusion, we have developed a smart, dual-responsive hydrogel capable of on-demand growth factor release for the treatment of diabetic wounds. By utilizing the pathological cues of the wound microenvironment—specifically acidic pH and elevated ROS—the scaffold provides a bioinspired delivery mechanism that synchronizes therapeutic release with tissue requirements. Our in vivo results demonstrate that this approach significantly enhances the rate and quality of healing, promoting angiogenesis and re-epithelialization in a notoriously difficult-to-treat diabetic model. The combination of CMCS and alginate provides a biocompatible, protective, and responsive framework that addresses the multifaceted nature of chronic ulcers. Future research should focus on the scaling of this technology for clinical trials and the potential for incorporating real-time biosensors to provide clinicians with data on the wound's physiological state. This work underscores the potential of stimuli-responsive biomaterials to transform the standard of care for diabetic complications and reduce the global burden of chronic wound management.</p>
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