3D bioprinting of functional human skin: production and in vivo analysis

A Concise Review on Tissue Engineered Artificial Skin Grafts for Chronic Wound Treatment: Can We Reconstruct Functional Skin Tissue In Vitro?

Chronic wounds occur as a consequence of a prolonged inflammatory phase during the healing process, which precludes skin regeneration. Typical treatment for chronic wounds includes application of autografts, allografts collected from cadaver, and topical delivery of antioxidant, anti-inflammatory, and antibacterial agents. Nevertheless, the mentioned therapies are not sufficient for extensive or deep wounds. Moreover, application of allogeneic skin grafts carries high risk of rejection and treatment failure. Advanced therapies for chronic wounds involve application of bioengineered artificial skin substitutes to overcome graft rejection as well as topical delivery of mesenchymal stem cells to reduce inflammation and accelerate the healing process. This review focuses on the concept of skin tissue engineering, which is a modern approach to chronic wound treatment. The aim of the article is to summarize common therapies for chronic wounds and recent achievements in the development of bioengineered artificial skin constructs, including analysis of biomaterials and cells widely used for skin graft production. This review also presents attempts to reconstruct nerves, pigmentation, and skin appendages (hair follicles, sweat glands) using artificial skin grafts as well as recent trends in the engineering of biomaterials, aiming to produce nanocomposite skin substitutes (nanofilled polymer composites) with controlled antibacterial activity. Finally, the article describes the composition, advantages, and limitations of both newly developed and commercially available bioengineered skin substitutes.

Artificial Biosystems by Printing Biology

The continuous progress of printing technologies over the past 20 years has fueled the development of a plethora of applications in materials sciences, flexible electronics, and biotechnologies. More recently, printing methodologies have started up to explore the world of Artificial Biology, offering new paradigms in the direct assembly of Artificial Biosystems (small condensates, compartments, networks, tissues, and organs) by mimicking the result of the evolution of living systems and also by redesigning natural biological systems, taking inspiration from them. This recent progress is reported in terms of a new field here defined as Printing Biology, resulting from the intersection between the field of printing and the bottom up Synthetic Biology. Printing Biology explores new approaches for the reconfigurable assembly of designed life-like or life-inspired structures. This work presents this emerging field, highlighting its main features, i.e., printing methodologies (from 2D to 3D), molecular ink properties, deposition mechanisms, and finally the applications and future challenges. Printing Biology is expected to show a growing impact on the development of biotechnology and life-inspired fabrication.

Contraction of fibrin-derived matrices and its implications for in vitro human skin bioengineering

It is well-known that fibroblasts play a fundamental role in the contraction of collagen and fibrin hydrogels when used in the production of in vitro bilayered skin substitutes. However, little is known about the contribution of other factors, such as the hydrogel matrix itself, on this contraction. In this work, we studied the contraction of plasma-derived fibrin hydrogels at different temperatures (4, 23, and 37°C) in an isotonic buffer (phosphate-buffered saline). These types of hydrogels presented a contraction of approximately 30% during the first 24 hr, following a similar kinetics irrespectively of the temperature. This kinetics continued in a slowed down manner to reach a plateau value of 40% contraction after 10-15 days. Contraction of commercial fibrinogen hydrogels was studied under similar conditions and the kinetics was completed after 8 hr, reaching values between 20 and 70% depending on the temperature. We attribute these substantial differences to a modulatory effect on the contraction due to plasma proteins which are initially embedded in, and progressively released from, the plasma-based hydrogels. The elastic modulus of hydrogels measured at a constant frequency decreased with increasing temperature in 7-day gels. Rheological measurements showed the absence of a strain-hardening behavior in the plasma-derived fibrin hydrogels. Finally, plasma-derived fibrin hydrogels with and without human primary fibroblast and keratinocytes were prepared in transwell inserts and their height measured over time. Both cellular and acellular gels showed a height reduction of 30% during the first 24 hr likely due to the above-mentioned intrinsic fibrin matrix contraction.

The emerging role of microfluidics in multi-material 3D bioprinting

To assist the transition of 3D bioprinting technology from simple lab-based tissue fabrication, to fully functional and implantable organs, the technology must not only provide shape control, but also functional control. This can be accomplished by replicating the cellular composition of the native tissue at the microscale, such that cell types interact to provide the desired function. There is therefore a need for precise, controllable, multi-material printing that could allow for high, possibly even single cell, resolution. This paper aims to draw attention to technological advancements made in 3D bioprinting that target the lack of multi-material, and/or multi cell-type, printing capabilities of most current devices. Unlike other reviews in the field, which largely focus on variations in single-material 3D bioprinting involving the standard methods of extrusion-based, droplet-based, laser-based, or stereolithographic methods; this review concentrates on sophisticated multi-material 3D bioprinting using multi-cartridge printheads, co-axial nozzles and microfluidic-enhanced printing nozzles.

3D Bioprinting and Its Application to Military Medicine

Introduction

Traditionally, tissue engineering techniques have largely focused on 2D cell culture models—monolayers of immortalized or primary cells growing on tissue culture plastic. Although these techniques have proven useful in research, they often lack physiological validity, because of the absence of fundamental tissue properties, such as multicellular organization, specialized extracellular matrix structures, and molecular or force gradients essential to proper physiological function. More recent advances in 3D cell culture methods have facilitated the development of more complex physiological models and tissue constructs; however, these often rely on self-organization of cells (bottom-up design), and the range of tissue construct size and complexity generated by these methods remains relatively limited. By borrowing from advances in the additive manufacturing field, 3D bioprinting techniques are enabling top-down design and fabrication of cellular constructs with controlled sizing, spacing, and chemical functionality. The high degree of control over engineered tissue architecture, previously unavailable to researchers, enables the generation of more complex, physiologically relevant 3D tissue constructs. Three main 3D bioprinting techniques are reviewed—extrusion, droplet-based, and laser-assisted bioprinting techniques are among the more robust 3D bioprinting techniques, each with its own strengths and weaknesses. High complexity tissue constructs created through 3D bioprinting are opening up new avenues in tissue engineering, regenerative medicine, and physiological model systems for researchers in the military medicine community.

Materials and Methods

Recent primary literature and reviews were selected to provide a broad overview of the field of 3D bioprinting and illustrate techniques and examples of 3D bioprinting relevant to military medicine. References were selected to illustrate specific examples of advances and potential military medicine applications in the 3D bioprinting field, rather than to serve as a comprehensive review.

Results

Three classes of 3D bioprinting techniques were reviewed: extrusion, droplet-based, and laser-assisted bioprinting. Advantages, disadvantages, important considerations, and constraints of each technique were discussed. Examples from the primary literature were given to illustrate the techniques. Relevant applications of 3D bioprinting to military medicine, namely tissue engineering/regenerative medicine and new models of physiological systems, are discussed in the context of advancing military medicine.

Conclusions

3D bioprinting is a rapidly evolving field that provides researchers the ability to build tissue constructs that are more complex and physiologically relevant than traditional 2D culture methods. Advances in bioprinting techniques, bioink formulation, and cell culture methods are being translated into new paradigms in tissue engineering and physiological system modeling, advancing the state of the art, and increasing construct availability to the military medicine research community.

Advances in the Research of Bioinks Based on Natural Collagen, Polysaccharide and Their Derivatives for Skin 3D Bioprinting

The skin plays an important role in protecting the human body, and wound healing must be set in motion immediately following injury or trauma to restore the normal structure and function of skin. The extracellular matrix component of the skin mainly consists of collagen, glycosaminoglycan (GAG), elastin and hyaluronic acid (HA). Recently, natural collagen, polysaccharide and their derivatives such as collagen, gelatin, alginate, chitosan and pectin have been selected as the matrix materials of bioink to construct a functional artificial skin due to their biocompatible and biodegradable properties by 3D bioprinting, which is a revolutionary technology with the potential to transform both research and medical therapeutics. In this review, we outline the current skin bioprinting technologies and the bioink components for skin bioprinting. We also summarize the bioink products practiced in research recently and current challenges to guide future research to develop in a promising direction. While there are challenges regarding currently available skin bioprinting, addressing these issues will facilitate the rapid advancement of 3D skin bioprinting and its ability to mimic the native anatomy and physiology of skin and surrounding tissues in the future.

Fibrin-based Bioinks: New Tricks from an Old Dog

For the past 10 years, the main efforts of most bioprinting research teams have focused on creating new bioink formulations, rather than inventing new printing set-up concepts. New tissue-specific bioinks with good printability, shape fidelity, and biocompatibility are based on "old" (well-known) biomaterials, particularly fibrin. While the interest in fibrin-based bioinks is constantly growing, it is essential to provide a framework of material's properties and trends. This review aims to describe the fibrin properties and application in three-dimensional bioprinting and provide a view on further development of fibrin-based bioinks.

Materials and technical innovations in 3D printing in biomedical applications

3D printing is a rapidly growing research area, which significantly contributes to major innovations in various fields of engineering, science, and medicine. Although the scientific advancement of 3D printing technologies has enabled the development of complex geometries, there is still an increasing demand for innovative 3D printing techniques and materials to address the challenges in building speed and accuracy, surface finish, stability, and functionality. In this review, we introduce and review the recent developments in novel materials and 3D printing techniques to address the needs of the conventional 3D printing methodologies, especially in biomedical applications, such as printing speed, cell growth feasibility, and complex shape achievement. A comparative study of these materials and technologies with respect to the 3D printing parameters will be provided for selecting a suitable application-based 3D printing methodology. Discussion of the prospects of 3D printing materials and technologies will be finally covered.

The Construction and Application of Three-Dimensional Biomaterials

Biomaterials have been widely explored and applied in many areas, especially in the field of tissue engineering. The interface of biomaterials and cells has been deeply investigated. However, it has been demonstrated that conventional 2D biomaterials fail to maintain the 3D structures and phenotypes of cells, which is the result of their limited ability to mimic the latter's complex extracellular matrix. To overcome this challenge, cell cultivation dependent on 3D biomaterials has emerged as an alternative strategy to make the recovery of 3D structures and functions of cells possible. Thus, with the thriving development of 3D cell culture in tissue engineering, a holistic review of the construction and application of 3D biomaterials is desired. Here, recent developments in 3D biomaterials for tissue engineering are reviewed. An overview of various approaches to construct 3D biomaterials, such as electro-jetting/-spinning, micro-molding, microfluidics, and 3D bio-printing, is first presented. Their typical applications in constructing cell sheets, vascular structures, cell spheroids, and macroscopic cellular constructs are described as well. Following these two sections, the current status and challenges are analyzed, as well as the future outlook of 3D biomaterials for tissue engineering.

A Novel Plasma-Based Bioink Stimulates Cell Proliferation and Differentiation in Bioprinted, Mineralized Constructs

Extrusion-based bioprinting, also known as 3D bioplotting, is a powerful tool for the fabrication of tissue equivalents with spatially defined cell distribution. Even though considerable progress has been made in recent years, there is still a lack of bioinks which enable a tissue-like cell response and are plottable at the same time with good shape fidelity. Herein, we report on the development of a bioink which includes fresh frozen plasma from full human blood and thus a donor/patient-specific protein mixture. By blending of the plasma with 3 w/v% alginate and 9 w/v% methylcellulose, a pasty bioink (plasma-alg-mc) was achieved, which could be plotted with high accuracy and furthermore allowed bioplotted mesenchymal stromal cells (MSC) and primary osteoprogenitor cells to spread within the bioink. In a second step, the novel plasma-based bioink was combined with a plottable self-setting calcium phosphate cement (CPC) to fabricate bone-like tissue constructs. The CPC/plasma-alg-mc biphasic constructs revealed open porosity over the entire time of cell culture (35 d), which is crucial for bone tissue engineered grafts. The biphasic structures could be plotted in volumetric and clinically relevant dimensions and complex shapes could be also generated, as demonstrated for a scaphoid bone model. The plasma bioink potentiated that bioplotted MSC were not harmed by the setting process of the CPC. Latest after 7 days, MSC migrated from the hydrogel to the CPC surface, where they proliferated to 20-fold of the initial cell number covering the entire plotted constructs with a dense cell layer. For bioplotted and osteogenically stimulated osteoprogenitor cells, a significantly increased alkaline phosphatase activity was observed in CPC/plasma-alg-mc constructs in comparison to plasma-free controls. In conclusion, the novel plasma-alg-mc bioink is a promising new ink for several forms of bioprinted tissue equivalents and especially gainful for the combination with CPC for enhanced, biofabricated bone-like constructs.

From Shape to Function: The Next Step in Bioprinting

In 2013, the "biofabrication window" was introduced to reflect the processing challenge for the fields of biofabrication and bioprinting. At that time, the lack of printable materials that could serve as cell-laden bioinks, as well as the limitations of printing and assembly methods, presented a major constraint. However, recent developments have now resulted in the availability of a plethora of bioinks, new printing approaches, and the technological advancement of established techniques. Nevertheless, it remains largely unknown which materials and technical parameters are essential for the fabrication of intrinsically hierarchical cell-material constructs that truly mimic biologically functional tissue. In order to achieve this, it is urged that the field now shift its focus from materials and technologies toward the biological development of the resulting constructs. Therefore, herein, the recent material and technological advances since the introduction of the biofabrication window are briefly summarized, i.e., approaches how to generate shape, to then focus the discussion on how to acquire the biological function within this context. In particular, a vision of how biological function can evolve from the possibility to determine shape is outlined.

Vat polymerization-based bioprinting-process, materials, applications and regulatory challenges

Over the years, the field of bioprinting has attracted attention for its highly automated fabrication system that enables the precise patterning of living cells and biomaterials at pre-defined positions for enhanced cell-matrix and cell-cell interactions. Notably, vat polymerization (VP)-based bioprinting is an emerging bioprinting technique for various tissue engineering applications due to its high fabrication accuracy. Particularly, different photo-initiators (PIs) are utilized during the bioprinting process to facilitate the crosslinking mechanism for fabrication of high-resolution complex tissue constructs. The advancements in VP-based printing have led to a paradigm shift in fabrication of tissue constructs from cell-seeding of tissue scaffolds (non-biocompatible fabrication process) to direct bioprinting of cell-laden tissue constructs (biocompatible fabrication process). This paper, presenting a first-time comprehensive review of the VP-based bioprinting process, provides an in-depth analysis and comparison of the various biocompatible PIs and highlights the important considerations and bioprinting requirements. This review paper reports a detailed analysis of its printing process and the influence of light-based curing modality and PIs on living cells. Lastly, this review also highlights the significance of VP-based bioprinting, the regulatory challenges and presents future directions to transform the VP-based printing technology into imperative tools in the field of tissue engineering and regenerative medicine. The readers will be informed on the current limitations and achievements of the VP-based bioprinting techniques. Notably, the readers will realize the importance and value of highly-automated platforms for tissue engineering applications and be able to develop objective viewpoints towards this field.

Three Dimensional Bioprinting of a Vascularized and Perfusable Skin Graft Using Human Keratinocytes, Fibroblasts, Pericytes, and Endothelial Cells

Multilayered skin substitutes comprising allogeneic cells have been tested for the treatment of nonhealing cutaneous ulcers. However, such nonnative skin grafts fail to permanently engraft because they lack dermal vascular networks important for integration with the host tissue. In this study, we describe the fabrication of an implantable multilayered vascularized bioengineered skin graft using 3D bioprinting. The graft is formed using one bioink containing human foreskin dermal fibroblasts (FBs), human endothelial cells (ECs) derived from cord blood human endothelial colony-forming cells (HECFCs), and human placental pericytes (PCs) suspended in rat tail type I collagen to form a dermis followed by printing with a second bioink containing human foreskin keratinocytes (KCs) to form an epidermis. In vitro, KCs replicate and mature to form a multilayered barrier, while the ECs and PCs self-assemble into interconnected microvascular networks. The PCs in the dermal bioink associate with EC-lined vascular structures and appear to improve KC maturation. When these 3D printed grafts are implanted on the dorsum of immunodeficient mice, the human EC-lined structures inosculate with mouse microvessels arising from the wound bed and become perfused within 4 weeks after implantation. The presence of PCs in the printed dermis enhances the invasion of the graft by host microvessels and the formation of an epidermal rete. Impact Statement Three Dimensional printing can be used to generate multilayered vascularized human skin grafts that can potentially overcome the limitations of graft survival observed in current avascular skin substitutes. Inclusion of human pericytes in the dermal bioink appears to improve both dermal and epidermal maturation.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

Advances in bioprinting using additive manufacturing

Since its conception in the 1980's, several advances in the field of additive manufacturing have led to exploration of alternate as well as combination biomaterials. These progresses have directed the use of 3D printing in wider applications such as printing of dermal layers, cartilage, bone defects, and surgical implants. Furthermore, the incorporation of live and functional cells with or atop biomaterials has laid the foundation for its use in tissue engineering. The purpose of this review is to summarize the advances in 3D printing and bioprinting of several types of tissues such as skin, cartilage, bones, and cardiac valves. This review will address the current 3D technologies used in tissue construction and study the biomaterials being investigated. There are several requirements that need to be addressed, in order to reconstruct functional tissue such as mechanical strength, porosity of the replicate and cellular incorporation. Researchers have focused their studies to answer questions regarding these requirements.

Current Strategies and Future Perspectives of Skin-on-a-Chip Platforms: Innovations, Technical Challenges and Commercial Outlook

The skin is the largest and most exposed organ in the human body. Not only it is involved in numerous biological processes essential for life but also it represents a significant endpoint for the application of pharmaceuticals. The area of in vitro skin tissue engineering has been progressing extensively in recent years. Advanced in vitro human skin models strongly impact the discovery of new drugs thanks to the enhanced screening efficiency and reliability. Nowadays, animal models are largely employed at the preclinical stage of new pharmaceutical compounds development for both risk assessment evaluation and pharmacokinetic studies. On the other hand, animal models often insufficiently foresee the human reaction due to the variations in skin immunity and physiology. Skin-on-chips devices offer innovative and state-of-the-art platforms essential to overcome these limitations. In the present review, we focus on the contribution of skin-on-chip platforms in fundamental research and applied medical research. In addition, we also highlighted the technical and practical difficulties that must be overcome to enhance skin-on-chip platforms, e.g. embedding electrical measurements, for improved modeling of human diseases as well as of new drug discovery and development.

Opportunities and challenges of translational 3D bioprinting

3D-printed orthopaedic devices and surgical tools, printed maxillofacial implants and other printed acellular devices have been used in patients. By contrast, bioprinted living cellular constructs face considerable translational challenges. In this Perspective, we first summarize the most recent developments in 3D bioprinting for clinical applications, with a focus on how 3D-printed cartilage, bone and skin can be designed for individual patients and fabricated using the patient's own cells. We then discuss key translational considerations, such as the need to ensure close integration of the living device with the patient's vascular network, the development of biocompatible bioinks and the challenges in deriving a physiologically relevant number of cells. Lastly, we outline untested regulatory pathways, as well as logistical challenges in material sourcing, manufacturing, standardization and transportation.

Three-dimensional bioprinting for organ bioengineering: promise and pitfalls

PURPOSE OF REVIEW

Loss of organ function is a critical issue that threatens a patient's life. Currently, the only available treatment is organ transplantation; however, shortage of donor organs, histocompatibility, and life-long immunosuppression present major challenges. Three-dimensional bioprinting technology holds a promising solution for treating organ failure by fabricating autologous tissues and organs for transplantation. To biofabricate a functional tissue, target-cell types are combined with an appropriate biomaterial for structural support and a bioink that supports cell function and maturation. Bioprinted structures can mimic the native tissue shape and functionality.

RECENT FINDINGS

The main goal of three-dimensional bioprinting is to produce functional tissues/organs; however, whole organ printing has not been achieved. There have been recent advances in the successful three-dimensional bioprinting of numerous tissues. This review will discuss the types of bioprinters, biomaterials, bioinks, and the fabrication of various constructs for repair of vascular, cartilage, skin, cardiac, and liver tissues. These bioprinted tissue constructs have the potential to be used to treat tissues and organs that have been damaged by injury or disease.

SUMMARY

Three-dimensional bioprinting technology offers the ability to fabricate three-dimensional tissue structures with high precision, fidelity, and stability at human clinical scale. The creation of complex tissue architectures with heterogeneous compositions has the potential to revolutionize transplantation of tissues and organs.

Redirecting differentiation of mammary progenitor cells by 3D bioprinted sweat gland microenvironment

Background

Mammary progenitor cells (MPCs) maintain their reproductive potency through life, and their specific microenvironments exert a deterministic control over these cells. MPCs provides one kind of ideal tools for studying engineered microenvironmental influence because of its accessibility and continually undergoes postnatal developmental changes. The aim of our study is to explore the critical role of the engineered sweat gland (SG) microenvironment in reprogramming MPCs into functional SG cells.

Methods

We have utilized a three-dimensional (3D) SG microenvironment composed of gelatin-alginate hydrogels and components from mouse SG extracellular matrix (SG-ECM) proteins to reroute the differentiation of MPCs to study the functions of this microenvironment. MPCs were encapsulated into the artificial SG microenvironment and were printed into a 3D cell-laden construct. The expression of specific markers at the protein and gene levels was detected after cultured 14 days.

Results

Compared with the control group, immunofluorescence and gene expression assay demonstrated that MPCs encapsulated in the bioprinted 3D-SG microenvironment could significantly express the functional marker of mouse SG, sodium/potassium channel protein ATP1a1, and tend to express the specific marker of luminal epithelial cells, keratin-8. When the Shh pathway is inhibited, the expression of SG-associated proteins in MPCs under the same induction environment is significantly reduced.

Conclusions

Our evidence proved the ability of differentiated mouse MPCs to regenerate SG cells by engineered SG microenvironment in vitro and Shh pathway was found to be correlated with the changes in the differentiation. These results provide insights into regeneration of damaged SG by MPCs and the role of the engineered microenvironment in reprogramming cell fate.

Electronic supplementary material

The online version of this article (10.1186/s41038-019-0167-y) contains supplementary material, which is available to authorized users.

The future of skin toxicology testing - Three-dimensional bioprinting meets microfluidics

Over the years, the field of toxicology testing has evolved tremendously from the use of animal models to the adaptation of in vitro testing models. In this perspective article, we aim to bridge the gap between the regulatory authorities who performed the testing and approval of new chemicals and the scientists who designed and fabricated these in vitro testing models. An in-depth discussion of existing toxicology testing guidelines for skin tissue models (definition, testing models, principle, and limitations) is first presented to have a good understanding of the stringent requirements that are necessary during the testing process. Next, the ideal requirements of toxicology testing platform (in terms of fabrication, testing, and screening process) are then discussed. We envisioned that the integration of three-dimensional bioprinting within miniaturized microfluidics platform would bring about a paradigm shift in the field of toxicology testing; providing standardization in the fabrication process, accurate, and rapid deposition of test chemicals, real-time monitoring, and high throughput screening for more efficient skin toxicology testing.

Healing of Chronic Wounds: An Update of Recent Developments and Future Possibilities

Chronic wounds are the result of disruptions in the body's usual process of healing. They are not only a source of significant pain and discomfort but also, more importantly, an unguarded port of entry for pathogens into the body. While our current understanding of this phenomenon is far from complete, findings in physiological patterns and advancements in wound healing technologies have helped develop wound management and healing solutions to this long-standing medical challenge. This review presents an overview of known wound healing mechanics, abnormalities that lead to chronic wounds, and a summary of established and new wound healing technologies. Various approaches to heal wounds are discussed, from dermal replacements to advanced biomaterial-based treatments, from cell-, synthetic-, and composite-based approaches to preclinical approaches, which make developing such products possible. While tested breakthrough products are described, the authors focused more on recently developed innovations, which are at varying stages of maturity. The review concludes with a note on future perspectives and opinions on where the field and industry are headed and where they should be. Impact Statement Wound healing is an important area of research and clinical practice, and has captured the attention of tissue engineers since the nascent beginnings of the discipline. Tissue-engineered skin was the first FDA-approved product, achieved in 1996. Despite this success, and the passage of time, healing wounds, particularly chronic wounds, remains a vexing challenge. This comprehensive review article will provide readers with a synopsis of current issues, research approaches, animal models, technologies, and products that span the continuum from early development to clinical studies, in the hope of fueling new interests and ideas to overcome this long-standing medical challenge.

Bioprinting functional tissues

Despite the numerous lives that have been saved since the first successful procedure in 1954, organ transplant has several shortcomings which prevent it from becoming a more comprehensive solution for medical care than it is today. There is a considerable shortage of organ donors, leading to patient death in many cases. In addition, patients require lifelong immunosuppression to prevent graft rejection postoperatively. With such issues in mind, recent research has focused on possible solutions for the lack of access to donor organs and rejections, with the possibility of using the patient's own cells and tissues for treatment showing enormous potential. Three-dimensional (3D) bioprinting is a rapidly emerging technology, which holds great promise for fabrication of functional tissues and organs. Bioprinting offers the means of utilizing a patient's cells to design and fabricate constructs for replacement of diseased tissues and organs. It enables the precise positioning of cells and biologics in an automated and high throughput manner. Several studies have shown the promise of 3D bioprinting. However, many problems must be overcome before the generation of functional tissues with biologically-relevant scale is possible. Specific focus on the functionality of bioprinted tissues is required prior to clinical translation. In this perspective, this paper discusses the challenges of functionalization of bioprinted tissue under eight dimensions: biomimicry, cell density, vascularization, innervation, heterogeneity, engraftment, mechanics, and tissue-specific function, and strives to inform the reader with directions in bioprinting complex and volumetric tissues. STATEMENT OF SIGNIFICANCE: With thousands of patients dying each year waiting for an organ transplant, bioprinted tissues and organs show the potential to eliminate this ever-increasing organ shortage crisis. However, this potential can only be realized by better understanding the functionality of the organ and developing the ability to translate this to the bioprinting methodologies. Considering the rate at which the field is currently expanding, it is reasonable to expect bioprinting to become an integral component of regenerative medicine. For this purpose, this paper discusses several factors that are critical for printing functional tissues including cell density, vascularization, innervation, heterogeneity, engraftment, mechanics, and tissue-specific function, and inform the reader with future directions in bioprinting complex and volumetric tissues.

3D and 4D Printing of Polymers for Tissue Engineering Applications

Three-dimensional (3D) and Four-dimensional (4D) printing emerged as the next generation of fabrication techniques, spanning across various research areas, such as engineering, chemistry, biology, computer science, and materials science. Three-dimensional printing enables the fabrication of complex forms with high precision, through a layer-by-layer addition of different materials. Use of intelligent materials which change shape or color, produce an electrical current, become bioactive, or perform an intended function in response to an external stimulus, paves the way for the production of dynamic 3D structures, which is now called 4D printing. 3D and 4D printing techniques have great potential in the production of scaffolds to be applied in tissue engineering, especially in constructing patient specific scaffolds. Furthermore, physical and chemical guidance cues can be printed with these methods to improve the extent and rate of targeted tissue regeneration. This review presents a comprehensive survey of 3D and 4D printing methods, and the advantage of their use in tissue regeneration over other scaffold production approaches.

Functional Skin Grafts: Where Biomaterials Meet Stem Cells

Skin tissue engineering has attained several clinical milestones making remarkable progress over the past decades. Skin is inhabited by a plethora of cells spatiotemporally arranged in a 3-dimensional (3D) matrix, creating a complex microenvironment of cell-matrix interactions. This complexity makes it difficult to mimic the native skin structure using conventional tissue engineering approaches. With the advent of newer fabrication strategies, the field is evolving rapidly. However, there is still a long way before an artificial skin substitute can fully mimic the functions and anatomical hierarchy of native human skin. The current focus of skin tissue engineers is primarily to develop a 3D construct that maintains the functionality of cultured cells in a guided manner over a period of time. While several natural and synthetic biopolymers have been translated, only partial clinical success is attained so far. Key challenges include the hierarchical complexity of skin anatomy; compositional mismatch in terms of material properties (stiffness, roughness, wettability) and degradation rate; biological complications like varied cell numbers, cell types, matrix gradients in each layer, varied immune responses, and varied methods of fabrication. In addition, with newer biomaterials being adopted for fabricating patient-specific skin substitutes, issues related to escalating processing costs, scalability, and stability of the constructs under in vivo conditions have raised some concerns. This review provides an overview of the field of skin regenerative medicine, existing clinical therapies, and limitations of the current techniques. We have further elaborated on the upcoming tissue engineering strategies that may serve as promising alternatives for generating functional skin substitutes, the pros and cons associated with each technique, and scope of their translational potential in the treatment of chronic skin ailments.

Engineered Skin Tissue Equivalents for Product Evaluation and Therapeutic Applications

The current status of skin tissue equivalents that have emerged as relevant tools in commercial and therapeutic product development applications is reviewed. Due to the rise of animal welfare concerns, numerous companies have designed skin model alternatives to assess the efficacy of pharmaceutical, skincare, and cosmetic products in an in vitro setting, decreasing the dependency on such methods. Skin models have also made an impact in determining the root causes of skin diseases. When designing a skin model, there are various chemical and physical considerations that need to be considered to produce a biomimetic design. This includes designing a structure that mimics the structural characteristics and mechanical strength needed for tribological property measurement and toxicological testing. Recently, various commercial products have made significant progress towards achieving a native skin alternative. Further research involve the development of a functional bilayered model that mimics the constituent properties of the native epidermis and dermis. In this article, the skin models are divided into three categories: in vitro epidermal skin equivalents, in vitro full-thickness skin equivalents, and clinical skin equivalents. A description of skin model characteristics, testing methods, applications, and potential improvements is presented.

Fabrication of Bioengineered Skin by Injection Molding: A Feasibility Study on Automation

The use of bioengineered skin has facilitated fundamental and applied research because it enables the investigation of complex interactions between various cell types as well as the extracellular matrix. The predominantly manual fabrication of these living tissues means, however, that their quality, standardization, and production volume are extremely dependent on the technician's experience. Simple laboratory automation could facilitate the use of living tissues by a greater number of research groups. We developed and present here an injection molding technique for the fabrication of bilayered skin equivalents. The tissue was formed automatically by two separate injections into a customized mold to produce the dermal and epidermal skin layers. We demonstrated the biocompatibility of this fabrication process and confirmed the resulting bilayered morphology of the bioengineered skin using histology and immunohistochemistry. Our findings highlight the possibility of fabricating multilayered living tissue by injection molding, suggesting that further investigation into this automation method could result in the rapid and low-cost fabrication of standardized bioengineered skin.

Lung tissue bioengineering for chronic obstructive pulmonary disease: overcoming the need for lung transplantation from human donors

Introduction: Chronic obstructive pulmonary disease (COPD) affects more than 380 million people, causing more than 3 million deaths annually worldwide. Despite this enormous burden, currently available therapies are largely limited to symptom control. Lung transplant is considered for end-stage disease but is severely limited by the availability of human organs. Furthermore, the pre-transplant course is a complex orchestration of locating and harvesting suitable lungs, and the post-transplant course is complicated by rejection and infection. Lung tissue bioengineering has the potential to relieve the organ shortage and improve the post-transplant course by generating patient-specific lungs for transplant. Additionally, emerging progenitor cell therapies may facilitate in vivo regeneration of pulmonary tissue, obviating the need for transplant. Areas Covered: We review several lung tissue bioengineering approaches including the recellularization of decellularized scaffolds, 3D bioprinting, genetically-engineered xenotransplantation, blastocyst complementation, and direct therapy with progenitor cells. Articles were identified by searching relevant terms (see Key Words) in the PubMed database and selected for inclusion based on novelty and uniqueness of their approach. Expert Opinion: Lung tissue bioengineering research is in the early stages. Of the methods reviewed, only direct cell therapy has been investigated in humans. We anticipate a minimum of 5-10 years before human therapy will be feasible.

Biomaterials to Mimic and Heal Connective Tissues

Connective tissue is one of the four major types of animal tissue and plays essential roles throughout the human body. Genetic factors, aging, and trauma all contribute to connective tissue dysfunction and motivate the need for strategies to promote healing and regeneration. The goal here is to link a fundamental understanding of connective tissues and their multiscale properties to better inform the design and translation of novel biomaterials to promote their regeneration. Major clinical problems in adipose tissue, cartilage, dermis, and tendon are discussed that inspire the need to replace native connective tissue with biomaterials. Then, multiscale structure-function relationships in native soft connective tissues that may be used to guide material design are detailed. Several biomaterials strategies to improve healing of these tissues that incorporate biologics and are biologic-free are reviewed. Finally, important guidance documents and standards (ASTM, FDA, and EMA) that are important to consider for translating new biomaterials into clinical practice are highligted.

3D Bioprinting Methods and Techniques: Applications on Artificial Blood Vessel Fabrication

Three-dimensional (3D) printing has rapidly evolved, with major applications in the field of medicine. One of the greatest advances leading to 3D bioprinting was the development of biomaterials, cells and supporting components for the fabrication of functional living tissues. Several different methods and techniques of 3D bioprinting are briefly described in this review article, and applications of 3D printing for the fabrication of artificial blood vessels and grafts are presented. Advances in additive manufacturing techniques, medical imaging modalities, biomaterials and cellular engineering will lead to further developments in the fabrication of patient-specific vascular tissue constructs. Future multidisciplinary research and innovations are expected to further transform the fields of tissue engineering and regenerative medicine.

Current Biomedical Applications of 3D Printing and Additive Manufacturing

Additive manufacturing (AM) has emerged over the past four decades as a cost-effective, on-demand modality for fabrication of geometrically complex objects. The ability to design and print virtually any object shape using a diverse array of materials, such as metals, polymers, ceramics and bioinks, has allowed for the adoption of this technology for biomedical applications in both research and clinical settings. Current advancements in tissue engineering and regeneration, therapeutic delivery, medical device fabrication and operative management planning ensure that AM will continue to play an increasingly important role in the future of healthcare. In this review, we outline current biomedical applications of common AM techniques and materials.

3D Cell Printing of Perfusable Vascularized Human Skin Equivalent Composed of Epidermis, Dermis, and Hypodermis for Better Structural Recapitulation of Native Skin

Although skin cell-printing has exhibited promises for fabrication of functional skin equivalents, existing skin models through 3D cell printing are still composed of dermal and epidermal layers. However, a key hope for printing skin is to improve structural complexity of human skin over conventional construction, enabling the precise localization of multiple cell types and biomaterials. Here, the complexity of skin anatomy is increased using 3D cell printing. A novel printing platform is suggested for engineering a matured perfusable vascularized 3D human skin equivalent composed of epidermis, dermis, and hypodermis. The skin model is evaluated using functional markers representing each region of epidermis, dermis, and hypodermis to confirm tissue maturation. It is hypothesized that the vascularized dermal and hypodermal compartments that provide a more realistic microenvironment can promote cross-talks with the epidermal compartment, producing better recapitulation of epidermal morphogenesis. Skin stemness in epithelial tissue is investigated. These findings reveal that the full-thickness skin has more similarities to the native human skin compared with the dermal and epidermal skin model, indicating that it better reflects the actual complexity of native human skin. It is envisioned that it offers better predictive and reliable in vitro platform for investigation of mechanisms of pathological research and skin disease modeling.

Bioengineered Skin Intended for Skin Disease Modeling

Clinical use of bioengineered skin in reconstructive surgery has been established for more than 30 years. The limitations and ethical considerations regarding the use of animal models have expanded the application of bioengineered skin in the areas of disease modeling and drug screening. These skin models should represent the anatomical and physiological traits of native skin for the efficient replication of normal and pathological skin conditions. In addition, reliability of such models is essential for the conduction of faithful, rapid, and large-scale studies. Therefore, research efforts are focused on automated fabrication methods to replace the traditional manual approaches. This report presents an overview of the skin models applicable to skin disease modeling along with their fabrication methods, and discusses the potential of the currently available options to conform and satisfy the demands for disease modeling and drug screening.

Skin tissue regeneration for burn injury

The skin is the largest organ of the body, which meets the environment most directly. Thus, the skin is vulnerable to various damages, particularly burn injury. Skin wound healing is a serious interaction between cell types, cytokines, mediators, the neurovascular system, and matrix remodeling. Tissue regeneration technology remarkably enhances skin repair via re-epidermalization, epidermal-stromal cell interactions, angiogenesis, and inhabitation of hypertrophic scars and keloids. The success rates of skin healing for burn injuries have significantly increased with the use of various skin substitutes. In this review, we discuss skin replacement with cells, growth factors, scaffolds, or cell-seeded scaffolds for skin tissue reconstruction and also compare the high efficacy and cost-effectiveness of each therapy. We describe the essentials, achievements, and challenges of cell-based therapy in reducing scar formation and improving burn injury treatment.

Current and Future Perspectives on Skin Tissue Engineering: Key Features of Biomedical Research, Translational Assessment, and Clinical Application

The skin is responsible for several important physiological functions and has enormous clinical significance in wound healing. Tissue engineered substitutes may be used in patients suffering from skin injuries to support regeneration of the epidermis, dermis, or both. Skin substitutes are also gaining traction in the cosmetics and pharmaceutical industries as alternatives to animal models for product testing. Recent biomedical advances, ranging from cellular-level therapies such as mesenchymal stem cell or growth factor delivery, to large-scale biofabrication techniques including 3D printing, have enabled the implementation of unique strategies and novel biomaterials to recapitulate the biological, architectural, and functional complexity of native skin. This progress report highlights some of the latest approaches to skin regeneration and biofabrication using tissue engineering techniques. Current challenges in fabricating multilayered skin are addressed, and perspectives on efforts and strategies to meet those limitations are provided. Commercially available skin substitute technologies are also examined, and strategies to recapitulate native physiology, the role of regulatory agencies in supporting translation, as well as current clinical needs, are reviewed. By considering each of these perspectives while moving from bench to bedside, tissue engineering may be leveraged to create improved skin substitutes for both in vitro testing and clinical applications.

Therapeutic strategies for skin regeneration based on biomedical substitutes

Regenerative medicine and tissue engineering (TE) have experienced significant advances in the development of in vitro engineered skin substitutes, either for replacement of lost tissue in skin injuries or for the generation of in vitro human skin models to research. However, currently available skin substitutes present different limitations such as expensive costs, abnormal skin microstructure and engraftment failure. Given these limitations, new technologies, based on advanced therapies and regenerative medicine, have been applied to develop skin substitutes with several pharmaceutical applications that include injectable cell suspensions, cell-spray devices, sheets or 3Dscaffolds for skin tissue regeneration and others. Clinical practice for skin injuries has evolved to incorporate these innovative applications to facilitate wound healing, improve the barrier function of the skin, prevent infections, manage pain and even to ameliorate long-term aesthetic results. In this article, we review current commercially available skin substitutes for clinical use, as well as the latest advances in biomedical and pharmaceutical applications used to design advanced therapies and medical products for wound healing and skin regeneration. We highlight the current progress in clinical trials for wound healing as well as the new technologies that are being developed and hold the potential to generate skin substitutes such as 3D bioprinting-based strategies.

Skin bioprinting: the future of burn wound reconstruction?

Burns are a significant cause of trauma, and over the years, the focus of patient care has shifted from just survival to facilitation of improved functional outcomes. Typically, burn treatment, especially in the case of extensive burn injuries, involves surgical excision of injured skin and reconstruction of the burn injury with the aid of skin substitutes. Conventional skin substitutes do not contain all skin cell types and do not facilitate recapitulation of native skin physiology. Three-dimensional (3D) bioprinting for reconstruction of burn injuries involves layer-by-layer deposition of cells along with scaffolding materials over the injured areas. Skin bioprinting can be done either in situ or in vitro. Both these approaches are similar except for the site of printing and tissue maturation. There are technological and regulatory challenges that need to be overcome for clinical translation of bioprinted skin for burn reconstruction. However, the use of bioprinting for skin reconstruction following burns is promising; bioprinting will enable accurate placement of cell types and precise and reproducible fabrication of constructs to replace the injured or damaged sites. Overall, 3D bioprinting is a very transformative technology, and its use for wound reconstruction will lead to a paradigm shift in patient outcomes. In this review, we aim to introduce bioprinting, the different stages involved, in vitro and in vivo skin bioprinting, and the various clinical and regulatory challenges in adoption of this technology.

In Situ Bioprinting of Autologous Skin Cells Accelerates Wound Healing of Extensive Excisional Full-Thickness Wounds

The early treatment and rapid closure of acute or chronic wounds is essential for normal healing and prevention of hypertrophic scarring. The use of split thickness autografts is often limited by the availability of a suitable area of healthy donor skin to harvest. Cellular and non-cellular biological skin-equivalents are commonly used as an alternative treatment option for these patients, however these treatments usually involve multiple surgical procedures and associated with high costs of production and repeated wound treatment. Here we describe a novel design and a proof-of-concept validation of a mobile skin bioprinting system that provides rapid on-site management of extensive wounds. Integrated imaging technology facilitated the precise delivery of either autologous or allogeneic dermal fibroblasts and epidermal keratinocytes directly into an injured area, replicating the layered skin structure. Excisional wounds bioprinted with layered autologous dermal fibroblasts and epidermal keratinocytes in a hydrogel carrier showed rapid wound closure, reduced contraction and accelerated re-epithelialization. These regenerated tissues had a dermal structure and composition similar to healthy skin, with extensive collagen deposition arranged in large, organized fibers, extensive mature vascular formation and proliferating keratinocytes.

Beyond 2D: 3D bioprinting for skin regeneration

Essential cellular functions that are present in tissues are missed by two-dimensional (2D) cell monolayer culture. It certainly limits their potential to predict the cellular responses of real organisms. Engineering approaches offer solutions to overcome current limitations. For example, establishing a three-dimensional (3D)-based matrix is motivated by the need to mimic the functions of living tissues, which will have a strong impact on regenerative medicine. However, as a novel approach, it requires the development of new standard protocols to increase the efficiency of clinical translation. In this review, we summarised the various aspects of requirements related to well-suited 3D bioprinting techniques for skin regeneration and discussed how to overcome current bottlenecks and propel these therapies into the clinic.

Cell traction force in a confined microenvironment with double-sided micropost arrays

Three-dimensional (3D) cell migrations are regulated by force interactions between cells and a 3D extracellular matrix (ECM). Mapping the 3D traction force generated by cells on the surrounding ECM with controlled confinement and contact area will be useful in understanding cell migration. In this study, double-sided micropost arrays were fabricated. The cell traction force was mapped by microposts on the top and bottom of opposing surfaces with a controlled separating distance to create different confinements. The density of micropost arrays was modified to investigate the effect of cell contact area on 3D traction force development. Using MC3T3-E1 osteoblastic cells, the leading traction force was found to increase with additional contact surface on the top. Summing force vectors on both surfaces, a large force imbalance was found from the leading to trailing regions for fast migrating cells. With 10 μm separation and densely arranged microposts, the traction force on the top surface was the largest at 28.6 ± 2.5 nN with the highest migration speed of 0.61 ± 0.07 μm min⁻¹. Decreasing the density of the top micropost arrays resulted in a reduced traction force on the top and lower migration speed. With 15 μm separation, the cell traction force on the top and migration speed further decreased simultaneously. These results revealed traction force development on 3D ECM with varied degrees of confinement and contact area, which is important in regulating 3D cell migration.

Double-sided micropost arrays to monitor three-dimensional cell traction force development over time on top and bottom surfaces with controlled confinement and contact area.