Editorial on Special Issue “Biomaterials, Polymers and Tissue Engineering”

1. Introduction

The development of new materials, new manufacturing methods, and new techniques has attracted interest from many researchers in recent decades. Polymers used in biomedical applications should be nontoxic, biodegradable in most cases, biocompatible, easy to process and sterilize, and meet regulatory requirements specific to their given application. As compared with metals or ceramics, polymers can be easily processed into various shapes and their overall properties can be tailored by changing the molecular architecture (e.g., crosslinking, branching, and increasing the molecular weight of the polymer chains) and adding different additives. In recent years, significant advances have been made to develop new polymer-based biomaterials in the form of biodegradable and biocompatible scaffolds, diagnostic systems, and pharmaceutical formulations [1]. These polymers can be either of natural or synthetic origin and have been engineered to interact with biological systems to regenerate, repair, and restore the function of damaged tissues or organs. Polymeric biomaterials have gained intensive interest in both research and clinical settings and have contributed to the development of personalized biomedical devices and customized implants.

In light of the above, the present Special Issue was introduced to collect articles that addresses the very important topic of biomaterials, with a special focus on application of polymers in the biomedical engineering field. This collection comprises four research articles, one communication, and five review articles. In the present editorial, the most interesting findings of each research paper are presented, along with a brief description of each review article.

2. Highlights of the Studies Published in the Present Special Issue

In recent years, numerous studies have been undertaken to identify the optimal materials and technologies to create affordable and beneficial wound treatments. Considering this interest, three review articles were published in this Special Issue, providing in-depth discussions surrounding the use of cellulose-based materials, lipid nanoparticles, and plant-derived compounds for applications in wound healing. Cellulose is shown to be effective in wound treatments, as it is biocompatible, has the ability to provide a moist environment, and can match in vivo physicochemical properties. Cellulose is an available and low-cost polysaccharide, with tunable physical, chemical, mechanical, and biological performances [2]. Cellulose-based biomaterials have been used to deliver different bioactive molecules, such as antibiotics, vitamins, growth factors, and peptides. In their review, Abazari et al. provide comprehensive information about the role of wound dressings in the healing process, the challenges identified in the clinical environment, and the desired properties of ideal wound dressings. Furthermore, the most relevant findings in recent literature about cellulose-based biomaterials were clearly presented. Cellulose-acetate-based nanofibers loaded with tetracycline hydrochloride or Manuka honey or impregnated with propolis have been used to fabricate various mats for wound dressings [2]. A summary of hydrogels based on cellulose acetate, carboxymethyl cellulose, and bacterial cellulose, designed as wound dressings, is also presented in this review. Besides cellulose-based biomaterials, lipid nanoparticles are shown also to be successful in promoting the wound-healing process, being very effective in application to burn wounds and chronic wounds [3]. In their review, Matei et al. focus on nanosized, lipid-based drug-delivery systems, describing their numerous applications in managing skin wounds. Lipid nanoparticles can maintain skin hydration due to an occlusive biofilm formed at the surface of the stratum corneum. Interesting research approaches refer to the incorporation of nucleic acid into lipid nanoparticles developed for chronic ischemic wounds or the encapsulation of capsaicin and anti-TNF α to treat skin inflammation, or delivery of fusidic acid using lipid nanoparticles, to treat burn infections. The authors concluded that nanosized, lipid-based drug-delivery systems can enhance the efficacy of wound-healing therapies [3]. The paper by Vivcharenko and Przekora also focuses on wound dressings, specifically the biomaterials using curcumin, vitamins, and essential oils [4]. The article describes the synthesis and modifications of biomaterials with bioactive compounds (including curcumin and essential oils) and summarizes the biological effects of the novel wound dressings on the enhancement of skin regeneration.

Tissue engineering is an interdisciplinary field that seeks to repair, replace, or regenerate tissues or organs. Biomaterials for tissue engineering should have controlled surface chemistry and porosity, tunable biodegradability, and must be able to promote cell adhesion, migration, and differentiation. An overview on the biomaterials used to design scaffolds for retinal tissue engineering is presented in the paper by Nair et al. [5]. These biomaterials include natural polymers, synthetic polymers, hybrid polymers, decellularized tissues, and thermoresponsive hydrogel polymers. The authors discuss the advances in the fabrication of scaffolds for retinal repair using stem-cell-derived grafts, detailing the transplantation studies in animal models and their application in current clinical trials. In this Special Issue, in vivo experiments were reported by Hasanein et al. using adult rats as an animal model [6]. The authors developed a new formulation of oil-in-water-type microemulsions to enhance the bioavailability of spironolactone (SP), a mineralocorticoid used to prevent ischemia- and reperfusion-induced renal injuries. The results showed that the kidney oxidative injury was only partially restored after SP administration, but nano-encapsulated SP has beneficial effects in preventing kidney damage and renal oxidative stress in a rat model.

The field of induced pluripotent stem cells (iPSCs) has gained importance in recent years, contributing to the development of therapeutic research and regenerative medicine. Their potential to differentiate into various tissues or cells attracted the interest of researchers. For instance, Yeom et al. developed an alternative method for reprogramming, using small molecules and external stimuli, without the use of transgenes [7]. The authors showed that vitamin C improved iPSC generation from mouse and human somatic cells with transgenes.

Current economic and environmental issues drew the attention of researchers and motivated them to develop new strategies to valorize the biomass into valuable products. An example is the work of Botelho et al., published in this Special Issue. The authors used chicken feathers as source of keratin through microbial degradation. The keratin peptides were fractioned, according to their molecular weight, and their effect on keratinocyte migration and metabolic activity as well as on macrophage release of TNF-α was evaluated [8]. The resulting peptides induced significant changes in the viability and migration rate of keratinocytes, depending on the size distribution (fraction). Thus, the cell’s response can be modulated to either increase or decrease in their migration rate and metabolic activity.

Additive manufacturing has attracted tremendous attention due to its high versatility and functionality. Different 3D-printing techniques hold an important place in numerous biological and biomedical applications. Specifically, stereolithography (SLA) is an emerging technology for the fabrication of complex structures, such as scaffolds for tissue engineering, microfluidic devices, implantable drug-delivery systems, and items for dental reconstruction [9,10,11,12]. With the advancement of the SLA technique, the fabrication of 3D-printed composites with enhanced functionality and efficacity, with improved performance and widened applications, can be achieved. In this context, continuous innovation is needed to produce new materials suitable for SLA, which are easily processable, cost-effective, and characterized by good stability during storage. New discoveries surrounding the composition of the resins used in 3D printing include the addition of nanofillers, which make them feasible for manufacturing composites with superior mechanical strength. Graphene is a nanofiller of choice and several studies developing successful graphene/photopolymer resins for the SLA technique have been reviewed [13].

Another 3D-printing process is fused deposition modelling (FDM), which is widely used in the automobile industry, aerospace industry, and in the medical sector, due to its simplicity and cost-effectiveness. Polylactic acid (PLA) is one of the most popular materials for 3D printing with FDM, due to its ease of use. PLA is a thermoplastic, semi-crystalline polyester, with good strength and stiffness; however, it possesses brittleness and slow crystallization. Adam and Weltsch have shown that a styrene–ethylene–butylene–styrene (SEBS) thermoplastic elastomer acts as a nucleating agent in blends with PLA, and increases the initial temperature of crystallization [14]. Moreover, the impact strength increased with the addition of SEBS (up to 3%); meanwhile, in the bending tests, the specimens containing more than 3% were no longer broken. Furthermore, the addition of CNT (0.06% and 0.1%) modified the crystal structure of the blend, as revealed by the presence of another peak which appeared on the DSC curve; moreover, the addition of CNT prevented the total fracture of the specimens. The authors concluded that PLA blends containing 1% SEBS and 0.06 or 0.1% CNTs are suitable raw materials for the 3D printing of shape-memory products or prototypes.

Graphene oxide (GO) has been applied to fabricate biomedical devices due to its mechanical performance, biocompatibility, biodegradation, and antibacterial properties. The article authored by Rojas et al. evaluated the influence of the sonication time, GO percentage in the liquid phase, and the percentage of benzoyl peroxide in the solid phase, on the mechanical and setting properties established for cements [15]. The incorporation of graphene oxide into acrylic bone cements has a reinforcing effect even at low concentrations of 0.3 wt%, leading to an increase in the mechanical properties in compression and bending. Additionally, GO reduced the maximum temperature reached during polymerization and increased the setting time, an important parameter in orthopedics, related to the time the surgeon has to mold and apply the cement during the procedure.

3. Conclusions

This Special Issue focuses on biomaterials for medical applications and contains both research and review articles, gathering research results from multiple disciplines. Several approaches have been evaluated for wound healing, and modern techniques such as 3D printing have been assessed for the development of biomaterials for use in personalized medicine. Validation of research concepts through in vivo studies has been also investigated and presented.