Review Films for Wound Healing Fabricated Using a Solvent Casting Technique pharmaceutics Fabiola V. Borbolla-Jiménez ¹,2D, Sheila I. Peña-Corona Emiliano Pineda-Pérez ¹,2D, Alejandra Romero-Montero María Luisa Del Prado-Audelo ², Sergio Alberto Bernal-Chávez 4, Jonathan J. Magaña ¹,²,* and Gerardo Leyva-Gómez ³,*D I Citation: Borbolla-Jiménez, F.V.; Peña-Corona, S.I.; Farah, S.J.; Jiménez-Valdés, M.T.; Pineda-Pérez, E.; Romero-Montero, A.; Del Prado-Audelo, M.L.; Bernal-Chávez, S.A.; Magaña, J.J.; Leyva-Gómez, G. Films for Wound Healing Fabricated Using a Solvent Casting Technique. Pharmaceutics 2023, 15, 1914. https://doi.org/10.3390/ pharmaceutics15071914 check for updates Academic Editors: Giyoong Tae, Martin Federico Desimone and Gorka Orive CC Received: 18 May 2023 Revised: 10 June 2023 Accepted: 27 June 2023 Published: 9 July 2023 4.0/). BY Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 1 2 3 4 * 3 D, Sonia J. Farah ¹,2, María Teresa Jiménez-Valdés ¹,2, 3 Laboratorio de Medicina Genómica, Departamento de Genómica, Instituto Nacional de Rehabilitación Luis Guillermo Ibarra Ibarra, Ciudad de México 14389, Mexico; fvbj@hotmail.com (F.V.B.-J.); soniafarah1d@gmail.com (S.J.F.); teresajimenez00@gmail.com (M.T.J.-V.); emilianopineda_perez@hotmail.com (E.P.-P.) Tecnologico de Monterrey, Campus Ciudad de México, Ciudad de México 14380, Mexico; luisa.delprado@tec.mx Departamento de Farmacia, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad de México 04510, Mexico; sheilairaispc@gmail.com (S.I.P.-C.); alejandra.romero.montero@outlook.com (A.R.-M.) MDPI Departamento de Ciencias Químico-Biológicas, Universidad de las Américas Puebla, Ex-Hda. de Sta. Catarina Mártir, Cholula 72820, Puebla, Mexico; q901108@hotmail.com Correspondence: magana.jj@tec.mx (J.J.M.); leyva@quimica.unam.mx (G.L.-G.) Abstract: Wound healing is a complex process that involves restoring the structure of damaged tissues through four phases: hemostasis, inflammation, proliferation, and remodeling. Wound dressings are the most common treatment used to cover wounds, reduce infection risk and the loss of physiological fluids, and enhance wound healing. Despite there being several types of wound dressings based on different materials and fabricated through various techniques, polymeric films have been widely employed due to their biocompatibility and low immunogenicity. Furthermore, they are non-invasive, easy to apply, allow gas exchange, and can be transparent. Among different methods for designing polymeric films, solvent casting represents a reliable, preferable, and highly used technique due to its easygoing and relatively low-cost procedure compared to sophisticated methods such as spin coating, microfluidic spinning, or 3D printing. Therefore, this review focuses on the polymeric dressings obtained using this technique, emphasizing the critical manufacturing factors related to pharmaceuticals, specifically discussing the formulation variables necessary to create wound dressings that demonstrate effective performance. Keywords: skin; wound; wound healing; wound dressings; polymers; films; solvent casting 1. Introduction A wound is a disruption of the continuity of body tissue caused by physical or chemical damage. Usually, a wound is disinfected using a typical medical procedure, and antibiotic treatment is initiated to start the healing process [1]. However, a possible lack of response to antibiotics or an uncontrolled inflammatory phase can trigger the generation of chronic or infected wounds, which represents a clinical challenge [2]. In this sense, developing minimally-invasive smart dressings that integrate drug release with different therapeutic targets (such as antibiotics, anti-inflammatory agents, and analgesics) represents a desirable alternative [3,4]. Based on understanding the physiological process of wound healing, the ideal dress- ing should be biocompatible, acting as a physical barrier against microorganisms while allowing gas permeation to keep the wound hydrated and remove excess exudate [5,6]. Additionally, desirable properties include good mechanical strength and flexibility. Non- toxicity, biocompatibility, and biodegradability are also important criteria for materials used Pharmaceutics 2023, 15, 1914. https://doi.org/10.3390/pharmaceutics15071914 https://www.mdpi.com/journal/pharmaceutics Pharmaceutics 2023, 15, 1914 in dressings [7]. Hydrogels, polymer films, foams, gauzes, and hydrocolloids are among the most extensively studied dressings, depending on the wound type and therapeutic needs [8,9]. Films serve as convenient physical barriers to bacteria, maintain gas permeability, and enable in situ drug release. Furthermore, their flexibility can be tailored to accommodate individual morphology [10]. Consequently, research focuses on utilizing film dressings as drug carriers to control infections and inflammatory processes [11]. By providing a moist environment, removing wound exudates, and accelerating cellular and tissue regeneration, dressings can maintain optimal conditions for wound repair [12]. However, it is important to acknowledge the limitations of these products. For example, they encounter difficulties in simultaneous application to both external and internal wounds. Additionally, their application can exert external pressure, leading to secondary injuries [13]. This study aims to analyze the solvent casting procedure, the most widely used method for film production. Solvent casting is preferred over other methods, such as salt leaching, spin coating, microfluidic spinning, and 3D printing, due to its cost-effectiveness, simplicity, practicality, and ability to generate robust films with appropriate mechanical properties and homogeneity. However, the properties of materials obtained through solvent casting can vary significantly between production batches, influenced by environmental conditions, which can also slow the production process. Moreover, maintaining sterility throughout all manufacturing steps poses a challenge and can result in batch contamination. Thus, this study comprehensively reviews the objectives of developing polymer films and highlights the key considerations to ensure optimal final properties during the solvent casting process. Despite the method's considerable potential for industrial scaling and clinical application, there is a noticeable lack of standardized commercial products [7]. Therefore, this review identifies areas for improvement and underscores the main advan- tages of utilizing this type of dressing in wound treatment. 2. Wound Healing The wound healing process is a natural physiological reaction to tissue injury that consists of four highly integrated phases: hemostasis, inflammation, proliferation, and remodeling (Figure 1). These phases must occur in a specific sequence, at a specific time, and for a particular duration, in order that the physiologically involved functions fulfill their expected role [14]. Otherwise, this leads to improper or impaired tissue repair [15]. Epidermis Dermis Fibrin clot Erythrocyte- Blood_ vessel 1) Hemostasis Proliferating fibroblasts Epithelial cells monolayer New blood vessels -Injury 3) Proliferation Dobago -Platelet -Eschar Type III Collagen fibers Granulation tissue (2) Inflammation Necrotic tissue or bacterias Macrophage- Neutrophil- Monocyte 2 of 27 4) Remodeling Scar tissue Fibroblast. Type I Collagen fibers Figure 1. Four significant phases represent the wound healing process: (1) hemostasis, the formation of a platelet seal that prevents blood loss and a fibrin clot; (2) inflammation, where neutrophils and macrophages remove debris and prevent infection; (3) proliferation, where blood vessels reform Pharmaceutics 2023, 15, 1914 3 of 27 through angiogenesis, and fibroblasts replace the fibrin clot with granulation tissue; (4) remodeling, where the matrix is remodeled replacing type III collagen with type I, maturing to a scar. 2.1. Hemostasis The hemostasis phase begins minutes to hours after an injury through a cascade of serine protease activation, resulting in platelet activation. This activation also facili- tates the release of growth factors, such as PDGF (platelet-derived growth factor), VEGF (vascular endothelial growth factor), and TGF-α (transforming growth factor x), as well as immune mediators, contributing to the transition into the inflammatory phase [16]. PDGF and TGF-ß (transforming growth factor ß) recruit neutrophils and monocytes to initiate the inflammatory response [17]. Furthermore, this leads to the formation of a fibrin clot, referred to as an eschar, which acts as a plug for the wound, preventing blood loss, providing a scaffold for incoming immune cells, serving as a reservoir for cytokines and growth factors during the early stages of repair, and offering protection against bacterial invasion [15,16,18]. The extrinsic clotting cascade is initiated upon tissue damage and blood leakage, releasing molecules such as serotonin that induce localized vasoconstriction [19]. Sub- sequently, platelets aggregate and become activated upon contact with subendothelial collagen, forming a hemostatic plug. This plug mitigates hemorrhage and serves as a provisional matrix for cell migration by releasing scaffold proteins such as fibronectin, vitronectin, and thrombospondins, facilitating the migration of keratinocytes, immune cells, and fibroblasts [16,20]. Once a fibrin clot is formed, the coagulation process is switched off to prevent excessive thrombosis [17]. 2.2. Inflammation The inflammatory phase, which overlaps with hemostasis, occurs during the first 72 h [16]. During this phase, both humoral and cellular inflammatory responses are activated to establish an immune barrier against invading microorganisms [21]. Neu- trophils and monocytes infiltrate the wound bed to prevent further damage and eliminate pathogenic organisms and foreign debris [16,22]. The inflammation phase can be divided into early and late inflammatory phases. In the early phase, the complement cascade is activated, leading to the infiltration of neutrophils into the wound. Their primary role is to phagocytose bacteria, foreign particles, and damaged tissue [21,23]. Various chemoattractive agents attract neutrophils to the wound site, including TGF-ß, complement components such as C3a and C5a, and formyl- methionyl peptides produced by bacteria and platelet products [16,19]. During their activity, neutrophils release proteolytic enzymes, oxygen-derived free radical species, and inflammatory mediators such as TNF-α and interleukin (IL)-1 [17,21]. Once their role is fulfilled, neutrophils undergo apoptosis and are cleared from the wound typically within 2-3 days, making way for the influx of monocytes [18]. Monocytes, stimulated by cytokines, chemokines, growth factors, and soluble frag- ments of the extracellular matrix (ECM), differentiate into activated macrophages. These macrophages patrol the wound area, ingesting and killing bacteria and removing devi- talized tissue through the action of secreted matrix metalloproteinase and elastase [15]. Additionally, macrophages play a crucial role in transitioning to the proliferative phase by releasing various growth factors and cytokines, including PDGF, TGF-α, TGF-ß, insulin-like growth factor-1 (IGF-1), fibroblast growth factor (FGF), tumor necrosis factor-a (TNF-α), IL-1, and IL-6. These soluble mediators promote cell proliferation and the synthesis of ECM molecules, and activate fibroblasts for subsequent phases [19,24]. A decrease in macrophage presence within the wound indicates the inflammatory phase's conclusion and the proliferative phase's initiation [15]. Pharmaceutics 2023, 15, 1914 4 of 27 2.3. Proliferation The proliferative phase of wound healing involves several interconnected processes that restore tissue structure and function. It begins on the third day after the injury and lasts approximately two weeks [21]. One of the critical aspects of this phase is the replacement of the provisional fibrin matrix with a new matrix composed of collagen fibers, fibronectins, and proteoglycans, which are synthesized by fibroblasts. During this phase, angiogenesis, the generation of granulation tissue, collagen deposi- tion, re-epithelialization, and wound contraction occur [16,25]. Angiogenesis is stimulated by local conditions such as low oxygen tension, low pH, and high lactate levels. It involves the migration and proliferation of endothelial cells to form new blood vessels, which is cru- cial for tissue viability [17]. Macrophages are vital in promoting angiogenesis by producing VEGF [16]. Granulation tissue formation occurs concurrently with angiogenesis and primarily comprises type III collagen, fibroblasts, and new vessels. Fibroblasts are the main cells involved in granulation tissue formation. Their proliferation, and synthesis of extracellular matrix components, contributes to tissue restoration [21]. Additionally, the interaction between fibroblasts and keratinocytes plays a significant role in re-epithelialization. Factors like EGF, KGF, and TGF-α, produced by platelets, keratinocytes, and anti-inflammatory macrophages, promote keratinocyte migration and proliferation [25-27]. The process is further facilitated by the production of fibronectin, tenascin C, and laminin 332 by keratinocytes [16]. As re-epithelialization progresses, the stratified layers of the epidermis are re-established, and the maturation of the epidermis begins to restore its barrier function. TGF-ß can accelerate this maturation process [17]. The proliferation and differentiation of keratinocytes are essential for the reestablishment of a functional epidermis [25,27]. 2.4. Remodeling The remodeling phase, which occurs several weeks after the initial wound, marks the transition from granulation tissue to scar formation. This phase may last up to 1-2 years [21]. During this phase, angiogenesis slows down, and type III collagen in the granulation tissue is replaced by stronger type I collagen [16]. Myofibroblasts play a crucial role in driving the remodeling phase. They originate from fibroblasts and develop in response to mechanical tension and TGF-ß signaling. Myofibroblasts are responsible for wound contraction, and their expression of smooth muscle actin (SMA) generates the contractile force exhibited by these cells [28,29]. It is worth noting that myofibroblasts are considered terminally differentiated and undergo apoptosis once the remodeling process is complete [16]. During the remodeling phase, the granulation tissue matures into a scar, and the tensile strength of the tissue increases. This maturation is characterized by a reduction in the number of capillaries, an aggregation into larger vessels, and a decrease in the number of glycosaminoglycans. The cell density and metabolic activity in the granulation tissue also increase during this maturation process. The collagen type and organization changes further enhance the tissue's tensile strength [17]. However, the tensile strength of the healed tissue never fully reaches its original strength. A newly epithelialized wound typically has approximately 25% of the tensile strength of normal tissue, and it may take several months for the tensile strength to increase to a maximum of 80% of normal tissue [30]. The enhancement of tissue tensile strength primarily occurs through reorganizing collagen fibers, initially deposited randomly during the granulation phase. After, the enzyme lysyl oxidase, secreted by fibroblasts into the extracellular matrix (ECM), facilitates the increased covalent cross-linking of collagen molecules [17]. Pharmaceutics 2023, 15, 1914 Type of Polymer Natural Synthetic 3. Polymeric Films for Wound Healing Polymeric films (PFs) as dressings for wound healing were first vastly introduced dur- ing the Second World War as a response to the demand for advancements in medicine [31]. Bloom et al. [17] were the first to document semipermeable films in 1945 using cellophane to treat burnt prisoners of war in Italy; they reported “gratifying results” with complete healing nine days after dressing application [32]. Nowadays, PFs are commonly utilized in the medical field for healing as physical barriers for wounds which help to prevent inflammation, control the environment of the wound, and accelerate healing [33]. PFs have gained popularity due to their non-invasive nature, ease of application, biocompatibility, and the potential inclusion of antimicrobial treatments. Moreover, PFs offer flexibility, adherence, gas exchange capabilities, and trans- parency [34]. The material's flexibility enables the film to conform to complex shapes while facilitating gas exchange, which has been proven to promote healing [34]. Additionally, its transparency allows for the close monitoring of the wounded area without removing the film, thereby reducing trauma during dressing changes, minimizing exposure to bacteria, and lowering the risk of infection by up to one week [34,35]. PFs are recommended for treating partial-thickness wounds, minor burns, lacerations, and certain low-exudate ulcers such as ischemic ulcers, diabetic ulcers, venous ulcers, and pressure ulcers [34]. However, it is important to note that due to the occlusive nature of the material, films should not be used on wounds that require significant fluid absorption, as excess exudate can potentially lead to peri-wound maceration. Nevertheless, maintaining a certain level of moisture can be beneficial for the healing process by promoting keratinocyte migration to the affected area [34,36]. The performance of PFs depends on the chemical composition and the type of wound. Polymers used to formulate PFs can be divided into natural, synthetic, and blended. Therefore, the PFs will possess different qualities depending on the type of polymers employed to form the film dressing. 3.1. Natural Polymeric Films Natural polymers, also called biopolymers, offer advantages such as biocompatibility, biodegradability—at present a sought-after quality-healing properties, inertness, and adhesiveness [37]. For this reason, generally, natural polymers are preferred over synthetic ones [37]. However, they are prone to microorganism contamination and lack quality mechanical properties to form an optimal polymeric film. Some examples of natural polymers used for elaborating PFs are chitosan, hyaluronic acid, starch, silk fibroin, sericin, keratin, sodium alginate, gelatin, collagen, zein, and konjac glucomannan, among others (Table 1) [36]. Chitosan is one of the most exploited and abundant natural polymers in wound dressings, with antimicrobial and film-forming properties [38,39]. Moreover, due to their innocuousness, polymers like chitosan, cellulose, gellan gum, alginates, and starches can be used for oral cavity films [40] (Figure 2). Table 1. Types of polymers used in polymeric film casting. Examples Chitosan, hyaluronic acid, starch, silk fibroin, sericin, keratin, sodium alginate, gelatin, collagen, zein, cellulose, and konjac glucomannan Polyvinyl alcohol, polyacrylic acid, polycaprolactone, polyethylene glycol, polyvinylpyrrolidone, polylactic acid, and polydimethylsiloxane. 5 of 27 Properties Biocompatibility, biodegradability, high disponibility, healing properties, permeability, inertness, and bioadhesiveness Resistance, flexibility, structure, high degree of polymerization, thermo-responsiveness, hydrophilicity, and occlusivity Ref. [36,37] [36]