Tissue engineered skin substitutes: A comprehensive review of basic design, fabrication using 3D printing, recent advances and challenges

Advanced alginate/58S bioactive glass inks with enhanced printability, mechanical strength, and cytocompatibility for soft tissue engineering

Alginate-based hydrogels are promising biomaterials for extrusion-based bioprinting; however, their poor mechanical properties, printability, and shape integrity limit their utility in mimicking complex tissues and organs. In this study, a novel sodium alginate (Alg)/58S bioactive glass (BG)-based ink was developed for soft tissue engineering applications. The inks were characterised for shear-thinning, flowability, and shape integrity by printing various structures, including single filaments (0° and 90° nozzle movement), scaffolds, and rings. The ABG10 ink (10 wt% 58S BG in Alg) exhibited superior printability, achieving a printing accuracy of over 90 %, compared to a printing accuracy of 30-40 % for pure Alg. Fourier transform infrared spectroscopy revealed interactions between 58S BG and the Alg matrix, while scanning electron microscopy characterised the 58S BG morphology within the matrix. The storage modulus increased from 767 (pure Alg) to 13,604 Pa (ABG10), while compressive strength rose from 23 ± 3 to 43 ± 4 kPa (58 % enhancement). The cytocompatibility of the inks was assessed using an MTT assay (with SH-SY5Y cells), which confirmed that ABG10 ink supports cell viability. Overall, ABG10 hydrogel-based inks exhibited enhanced shear-thinning behaviour, printability, mechanical strength, and cytocompatibility, which could help to develop patient-specific soft tissues.

3D printed bioengineered scaffold containing chitosan, alginate, and Barijeh-loaded niosomes enabled efficient antibiofilm activity and wound healing

In this study, we developed a novel biocompatible wound scaffold by encapsulating Barijeh (Bar), a plant-derived antibacterial compound, with niosome (Nio). The Nio-Bar formulation was incorporated into a chitosan (CS) and alginate (AL) hydrogel mixture, followed by 3D printing to create a three-dimensional scaffold, namely Nio-Bar@CS-AL. The obtained scaffold showed notable degradation, reaching 68 % (w/w) within 14 days. Nio-Bar@CS-AL displayed strong antibacterial activity and led to a >5-log reduction of both Pseudomonas aeruginosa and Staphylococcus aureus, far surpassing the performance of CS-AL scaffolds. Further, it effectively reduced biofilm formation by 74 %-80 % for both pathogens, and showed no cytotoxicity toward human fibroblast (HFF) cells, ensuring safety for wound application. In an in vivo murine wound model, Nio-Bar@CS-AL facilitated over 90 % wound healing after 10-day. Tissue integration was signaled by a twofold increase of TGF-β expression and a reduction of IL-6 expression to near-baseline levels, thereby mitigating inflammation. Histopathological analysis revealed a much higher collagen deposition, a key indicator of effective healing, in scaffold-treated wounds compared to the control. These results suggest that Nio-Bar@CS-AL holds promising clinical potential for treating wound infections and defects, offering a multifaceted strategy to improve wound healing outcomes.

CD133-positive dermal papilla cells are a major driver in promoting hair follicle formation

A major contributing factor to the failure of cell-based human hair follicle (HF) engineering is our inability to cultivate highly specialized, inductive mesenchymal fibroblasts, which reside in a unique niche at the HF base, called the dermal papilla (DP). We and other groups have discovered a unique DP fibroblast subpopulation that can be identified by the cell surface marker CD133. However, the biological difference between CD133-positive (CD133+) and CD133-negative (CD133-) DP cells remains unknown. Using a newly developed double fluorescent transgenic mouse strain, we isolated CD133 + and CD133- DP cells from mouse anagen HFs. In monolayer culture, both DP populations gradually lost expression of the anagen DP signature gene, versican. When maintained in three-dimensional spheroid culture, versican expression was restored in both CD133 + and CD133- DP cells. Importantly, CD133 + DP spheroids appeared more compact, showed stronger alkaline phosphatase staining (AP), and expressed higher levels of DP signature genes. In in vivo skin reconstitution assays, mice grafted with CD133 + DP spheroids grew more hairs in healed wounds than those grafted with CD133- DP spheroids. The data underscore the importance of CD133 + DP cells as a driver of HF formation, which may present a unique opportunity to improve the use of human DP cells in tissue-engineered skin substitutes (TESS).

Immune-privileged cord blood-derived endothelial colony-forming cells: advancing immunomodulation and vascular regeneration

Cord blood-derived endothelial colony-forming cells (CB-ECFCs) hold significant promise for regenerative medicine due to their unique vasculogenic and immunomodulatory properties. These cells exhibit a superior proliferative capacity, robust ability to form vascular networks, and lower immunogenicity compared to adult and embryonic stem cell-derived counterparts. The immune-privileged characteristics of CB-ECFCs, including reduced expression of pro-inflammatory mediators and tolerance-inducing molecules such as HLA-G, further enhance their therapeutic potential. Their low immunogenicity minimizes the risk of immune rejection, making them suitable for allogenic cell therapies. Their application extends to complex tissue engineering and organ revascularization, where their ability to integrate into three-dimensional scaffolds and support vascular tree formation represents a significant advancement. Moreover, CB-ECFCs' capability to adapt to inflammatory stimuli and retain immunological memory highlights their functional versatility in dynamic microenvironments. This review highlights the remarkable ontogeny of ECFCs while unveiling the unparalleled potential of CB-ECFCs in revolutionizing regenerative medicine. From pre-vascularizing engineered tissues and organoids to pioneering cell-based therapies for cardiovascular, dermatological, and degenerative diseases, CB-ECFCs stand at the forefront of cutting-edge biomedical advancements, offering unprecedented opportunities for therapeutic innovation. By leveraging their vasculogenic, immune-regulatory, and regenerative capacities, CB-ECFCs offer a robust alternative for addressing the challenges of vascular repair and organ engineering. Future research should focus on unraveling their transcriptomic and functional profiles to optimize clinical applications and advance the field of regenerative medicine.

Biomimetic Hydrogels - Tools for Regenerative Medicine, Oncology, and Understanding Medical Gas Plasma Therapy

Biomimetic hydrogels enable biochemical, cell biology, and tissue-like studies in the third dimension. Smart hydrogels are also frequently used in tissue engineering and as drug carriers for intra- or extracutaneous regenerative medicine. They have also been studied in bio-sensor development, 3D cell culture, and organoid growth optimization. Yet, many hydrogel types, adjuvant components, and cross-linking methods have emerged over decades, diversifying and complexifying such studies. Here, an evaluative overview is provided, mapping potential applications to the corresponding hydrogel tuning. Strikingly, hydrogels are ideal for studying locoregional therapy modalities, such as cold medical gas plasma technology. These partially ionized gases produce various reactive oxygen species (ROS) types along with other physico-chemical components such as ions and electric fields, and the spatio-temporal effects of these components delivered to diseased tissues remain largely elusive to date. Hence, this work outlines the promising applications of hydrogels in biomedical research in general and cold plasma science in particular and underlines the great potential of these smart scaffolds for current and future research and therapy.

Polysaccharide-Based Self-Healing Hydrogel for pH-Induced Smart Release of Lauric Acid to Accelerate Wound Healing

It is highly desirable yet significantly challenging to fabricate an injectable, self-healing, controlled-release wound dressing that is responsive to the alkaline pH of the wounds. Herein, we propose a facile approach to prepare pH-responsive chitosan-oxidized carboxymethyl cellulose (CS-o-CMC) hydrogel constructs in which gelation was achieved via electrostatic and Schiff base formation. Importantly, the Schiff base was formed in acidic medium and the final pH of pregel solution was intrinsically raised to 7.0-7.4 due to the cross-linking by β-glycerol phosphate. The self-healing behavior of the hydrogel was an enthalpy-driven process and efficient in alkaline compared to acidic pH. The pH responsiveness offered a controlled release of lauric acid (LA) from CS-o-CMC/LA hydrogel and regulated the M2 polarization. Overall, reduction in inflammation led to rapid vascularization, reepithelialization, and significantly accelerated wound healing in rats. Our findings demonstrate a promising strategy for developing injectable, immunomodulatory wound dressings tailored to the challenging environment of wounds.

A tranexamic acid-functionalized acellular dermal matrix sponge co-loaded with magnesium ions: Enhancing hemostasis, vascular regeneration, and re-epithelialization for comprehensive diabetic wound healing

Excessive inflammation, accumulation of wound exudate, and blood seepage are common in diabetic wounds, hindering cell proliferation and disrupting tissue remodeling, leading to delayed healing. This study presents a multifunctional sponge scaffold (P5T3@Mg) created by combining an acellular dermal matrix with tranexamic acid and MgO nanoparticles, designed for hemostatic and anti-inflammatory effects. The P5T3@Mg scaffold effectively absorbs wound fluid while promoting healing. In vivo and in vitro hemostasis experiments demonstrate that the P5T3@Mg sponge exhibits excellent hydrophilicity, enhancing blood absorption at the wound site, inhibiting fibrinolysis, and expediting hemostasis. Additionally, the sustained release of Mg2+ from the P5T3@Mg sponge promotes collagen deposition and angiogenesis in diabetic rat wounds, suppressing chronic inflammation and accelerating tissue remodeling and repair.

Recombinant fibrous protein biomaterials meet skin tissue engineering

Natural biomaterials, particularly fibrous proteins, are extensively utilized in skin tissue engineering. However, their application is impeded by batch-to-batch variance, limited chemical or physical versatility, and environmental concerns. Recent advancements in gene editing and fermentation technology have catalyzed the emergence of recombinant fibrous protein biomaterials, which are gaining traction in skin tissue engineering. The modular and highly customizable nature of recombinant synthesis enables precise control over biomaterial design, facilitating the incorporation of multiple functional motifs. Additionally, recombinant synthesis allows for a transition from animal-derived sources to microbial sources, thereby reducing endotoxin content and rendering recombinant fibrous protein biomaterials more amenable to scalable production and clinical use. In this review, we provide an overview of prevalent recombinant fibrous protein biomaterials (collagens, elastin, silk proteins and their chimeric derivatives) used in skin tissue engineering (STE) and compare them with their animal-derived counterparts. Furthermore, we discuss their applications in STE, along with the associated challenges and future prospects.

3D skin bioprinting as promising therapeutic strategy for radiation-associated skin injuries

Both cutaneous radiation injury and radiation combined injury (RCI) could have serious skin traumas, which are collectively referred to as radiation-associated skin injuries in this paper. These two types of skin injuries require special managements of wounds, and the therapeutic effects still need to be further improved. Cutaneous radiation injuries are common in both radiotherapy patients and victims of radioactive source accidents, which could lead to skin necrosis and ulcers in serious conditions. At present, there are still many challenges in management of cutaneous radiation injuries including early diagnosis, lesion assessment, and treatment prognosis. Radiation combined injuries are special and important issues in severe nuclear accidents, which often accompanied by serious skin traumas. Mass victims of RCI would be the focus of public health concern. Three-dimensional (3D) bioprinting, as a versatile and favourable technique, offers effective approaches to fabricate biomimetic architectures with bioactivity, which provides potentials for resolve the challenges in treating radiation-associated skin injuries. Combining with the cutting-edge advances in 3D skin bioprinting, the authors analyse the damage characteristics of skin wounds in both cutaneous radiation injury and RCI and look forward to the potential value of 3D skin bioprinting for the treatments of radiation-associated skin injuries.

Gelatine-collagen photo-crosslinkable 3D matrixes for skin regeneration

Immediate care of skin wounds and burns is essential to repair this mechanical and chemical barrier to infections. Hydrogels have become one of the standard methods for wound care. Here, gelatine-collagen photo-crosslinkable matrixes or hydrogels were manufactured by two-photon polymerization (TPP) or one-photon UV exposure using a Digital Light Processing (DLP) setup. Both techniques are able to construct matrixes from computer-aided design models, which is important for future clinical applications in which wound dressings should be customized. Although TPP can mimic the 3D dermo-epidermal junction with a high spatial resolution (i.e., ∼6 μm3), the manufacturing time was too slow to produce large wound dressings. Therefore, a DLP setup was explored in this study to fabricate large 2D matrixes of several cm2 using the same photo-resist as for TPP, except for the photoinitiator. The fibroblast viability, adherence, and proliferation were analysed in time on both 3D and 2D matrixes in vitro using two-photon microscopy. For both types of matrixes, the adherence and proliferation of fibroblasts (3T3-NIH) were optimal for stiff structures with a Young's modulus of 191 ± 35 kPa compared to softer matrixes of 37 ± 12 kPa. Fibroblast showed complete confluence on Day 14 after seeding on these matrixes, which may create the granulation tissue composed of fibronectin, collagen, and various proteoglycans in the future dermis before repair of the epidermis and disintegrating of their host matrix. For the monitoring of this repair, gelatine-collagen matrixes can easily incorporate bio-optical sensors for the simultaneous monitoring of inflammation processes and wound healing in time.

Recent Developments in 3D-(Bio)printed Hydrogels as Wound Dressings

Wound healing is a physiological process occurring after the onset of a skin lesion aiming to reconstruct the dermal barrier between the external environment and the body. Depending on the nature and duration of the healing process, wounds are classified as acute (e.g., trauma, surgical wounds) and chronic (e.g., diabetic ulcers) wounds. The latter take several months to heal or do not heal (non-healing chronic wounds), are usually prone to microbial infection and represent an important source of morbidity since they affect millions of people worldwide. Typical wound treatments comprise surgical (e.g., debridement, skin grafts/flaps) and non-surgical (e.g., topical formulations, wound dressings) methods. Modern experimental approaches include among others three dimensional (3D)-(bio)printed wound dressings. The present paper reviews recently developed 3D (bio)printed hydrogels for wound healing applications, especially focusing on the results of their in vitro and in vivo assessment. The advanced hydrogel constructs were printed using different types of bioinks (e.g., natural and/or synthetic polymers and their mixtures with biological materials) and printing methods (e.g., extrusion, digital light processing, coaxial microfluidic bioprinting, etc.) and incorporated various bioactive agents (e.g., growth factors, antibiotics, antibacterial agents, nanoparticles, etc.) and/or cells (e.g., dermal fibroblasts, keratinocytes, mesenchymal stem cells, endothelial cells, etc.).