Beyond 2D: 3D bioprinting for skin regeneration

Bridging the gap between testing and clinics exploring alternative pre-clinical models in melanoma research

Melanoma, the deadliest form of skin cancer, poses a significant clinical challenge for the development of effective treatments. Conventional in vivo animal studies have shown limited translational relevance to humans, raising strength to pre-clinical models for melanoma research. This review provides an in-depth analysis of alternative pre-clinical models including in vitro and ex vivo platforms such as reconstructed skin, spheroids, organoids, organotypic models, skin-on-a-chip, and bioprinting. Through a comprehensive analysis, the specific attributes, advantages, and limitations of each model are elucidated. It discusses the points related to the uniqueness advantages, from capturing complex interactions between melanoma cells and their microenvironment to enabling high-throughput drug screening and personalized medicine approaches. This review is structured covering firstly the roadmap to identify the co-occurrence of discovering new melanoma treatments and the development of its models, secondly it covers a comparative between the most used models followed by a section discussing each of them: the in vitro and ex vivo models. It intends to serve as an asset for researchers of melanoma field and clinicians involved in melanoma therapy, offering insights into the diverse preclinical models available for optimizing their integration into the translational pipeline.

Bioactive materials for in vivo sweat gland regeneration

Loss of sweat glands (SwGs) commonly associated with extensive skin defects is a leading cause of hyperthermia and heat stroke. In vivo tissue engineering possesses the potential to take use of the body natural ability to regenerate SwGs, making it more conducive to clinical translation. Despite recent advances in regenerative medicine, reconstructing SwG tissue with the same structure and function as native tissue remains challenging. Elucidating the SwG generation mechanism and developing biomaterials for in vivo tissue engineering is essential for understanding and developing in vivo SwG regenerative strategies. Here, we outline the cell biology associated with functional wound healing and the characteristics of bioactive materials. We critically summarize the recent progress in bioactive material-based cell modulation approaches for in vivo SwG regeneration, including the recruitment of endogenous cells to the skin lesion for SwG regeneration and in vivo cellular reprogramming for SwG regeneration. We discussed the re-establishment of microenvironment via bioactive material-mediated regulators. Besides, we offer promising perspectives for directing in situ SwG regeneration via bioactive material-based cell-free strategy, which is a simple and effective approach to regenerate SwG tissue with both fidelity of structure and function. Finally, we discuss the opportunities and challenges of in vivo SwG regeneration in detail. The molecular mechanisms and cell fate modulation of in vivo SwG regeneration will provide further insights into the regeneration of patient-specific SwGs and the development of potential intervention strategies for gland-derived diseases.

3D bioprinting of heterogeneous tissue-engineered skin containing human dermal fibroblasts and keratinocytes

Three-dimensional (3D) printed skin substitutes have great potential for wound healing. However, current 3D printed skin models are limited in simulating heterogeneity and complexity of skin tissue due to the lack of customized bioinks optimized for different skin layers. Herein, different gelatin methacrylate (GelMA)/nano-cellulose (BNC) bioink formulations were used to develop heterogeneous tissue-engineered skin (HTS) containing layers of fibroblast networks with larger pores, basal layers with smaller pores, and multilayered keratinocytes. The results revealed that the 10%GelMA/0.3%BNC bioink was better to model bioprinted dermis due to its high printability and cell-friendly sparse microenvironment. Additionally, the 10%GelMA/1.5%BNC bioink as the basal layer presented a dense network and sufficient material stiffness to support the establishment of keratinocyte confluent monolayers. The HTS not only had the ability to remodel the extracellular matrix but also supported epidermis reconstruction and stratification in vitro, with the epidermal thickness growing to 80 μm after 14 days. Furthermore, the full-thickness wound healing experiments demonstrated that the HTS promoted granulation tissue regeneration and improved wound healing quality. The generated skin of the HTS group had hair follicles and early-stage rete ridge structures, which were similar to normal skin in vivo. The HTS may deliver effective skin grafts for future clinical treatments.

Advances and Innovations of 3D Bioprinting Skin

Three-dimensional (3D) bioprinted skin equivalents are highlighted as the new gold standard for alternative models to animal testing, as well as full-thickness wound healing. In this review, we focus on the advances and innovations of 3D bioprinting skin for skin regeneration, within the last five years. After a brief introduction to skin anatomy, 3D bioprinting methods and the remarkable features of recent studies are classified as advances in materials, structures, and functions. We will discuss several ways to improve the clinical potential of 3D bioprinted skin, with state-of-the-art printing technology and novel biomaterials. After the breakthrough in the bottleneck of the current studies, highly developed skin can be fabricated, comprising stratified epidermis, dermis, and hypodermis with blood vessels, nerves, muscles, and skin appendages. We hope that this review will be priming water for future research and clinical applications, that will guide us to break new ground for the next generation of skin regeneration.

Three-Dimensional Skin Tissue Printing with Human Skin Cell Lines and Mouse Skin-Derived Epidermal and Dermal Cells

Since the skin covers most surfaces of the body, it is susceptible to damage, which can be fatal depending on the degree of injury to the skin because it defends against external attack and protects internal structures. Various types of artificial skin are being studied for transplantation to repair damaged skin, and recently, the production of replaceable skin using three-dimensional (3D) bioprinting technology has also been investigated. In this study, skin tissue was produced using a 3D bioprinter with human skin cell lines and cells extracted from mouse skin, and the printing conditions were optimized. Gelatin was used as a bioink, and fibrinogen and alginate were used for tissue hardening after printing. Printed skin tissue maintained a survival rate of 90% or more when cultured for 14 days. Culture conditions were established using 8 mM calcium chloride treatment and the skin tissue was exposed to air to optimize epidermal cell differentiation. The skin tissue was cultured for 14 days after differentiation induction by this optimized culture method, and immunofluorescent staining was performed using epidermal cell differentiation markers to investigate whether the epidermal cells had differentiated. After differentiation, loricrin, which is normally found in terminally differentiated epidermal cells, was observed in the cells at the tip of the epidermal layer, and cytokeratin 14 was expressed in the lower cells of the epidermis layer. Collectively, this study may provide optimized conditions for bioprinting and keratinization for three-dimensional skin production.

Applications of the amniotic membrane in tissue engineering and regeneration: the hundred-year challenge

The amniotic membrane (Amnio-M) has various applications in regenerative medicine. It acts as a highly biocompatible natural scaffold and as a source of several types of stem cells and potent growth factors. It also serves as an effective nano-reservoir for drug delivery, thanks to its high entrapment properties. Over the past century, the use of the Amnio-M in the clinic has evolved from a simple sheet for topical applications for skin and corneal repair into more advanced forms, such as micronized dehydrated membrane, amniotic cytokine extract, and solubilized powder injections to regenerate muscles, cartilage, and tendons. This review highlights the development of the Amnio-M over the years and the implication of new and emerging nanotechnology to support expanding its use for tissue engineering and clinical applications.

3D bioprinting for fabricating artificial skin tissue

As an organ in direct contact with the external environment, the skin is the first line of defense against external stimuli, so it is the most vulnerable to damage. In addition, there is an increasing demand for artificial skin in the fields of drug testing, disease research and cosmetic testing. Traditional skin tissue engineering has made encouraging progress after years of development. However, due to the complexity of the skin structures, there is still a big gap between existing artificial skin and natural skin in terms of function. Three-dimensional (3D) bioprinting is an advanced biological manufacturing method. It accurately deposits bioinks into pre-designed three-dimensional shapes to create complex biological tissues. This technology aims to print artificial tissues and organs with biological activities and complete physiological functions, thereby alleviating the problem of tissues and organs in short supply. Here, based on the introduction to skin structure and function, we systematically elaborate and analyze skin manufacturing methods, 3D bioprinting biomaterials and strategies, etc. Finally, the challenges and perspectives in 3D bioprinting skin field are summarized.

Applications of 3D Bio-Printing in Tissue Engineering and Biomedicine

In recent years, 3D bio-printing technology has developed rapidly and become an advanced bio-manufacturing technology. At present, 3D bio-printing technology has been explored in the fields of tissue engineering, drug testing and screening, regenerative medicine and clinical disease research and has achieved many research results. Among them, the application of 3D bio-printing technology in tissue engineering has been widely concerned by researchers, and it contributing many breakthroughs in the preparation of tissue engineering scaffolds. In the future, it is possible to print fully functional tissues or organs by using 3D bio-printing technology which exhibiting great potential development prospects in th applications of organ transplantation and human body implants. It is expected to solve thebiomedical problems of organ shortage and repair of damaged tissues and organs. Besides,3Dbio-printing technology will benefit human beings in more fields. Therefore, this paper reviews the current applications, research progresses and limitations of 3D bio-printing technology in biomedical and life sciences, and discusses the main printing strategies of 3D bio-printing technology. And, the research emphases, possible development trends and suggestions of the application of 3D bio-printing are summarized to provide references for the application research of 3D bio-printing.

Review of Low-Cost 3D Bioprinters: State of the Market and Observed Future Trends

Three-dimensional (3D) bioprinting has become mainstream for precise and repeatable high-throughput fabrication of complex cell cultures and tissue constructs in drug testing and regenerative medicine, food products, dental and medical implants, biosensors, and so forth. Due to this tremendous growth in demand, an overwhelming amount of hardware manufacturers have recently flooded the market with different types of low-cost bioprinter models-a price segment that is most affordable to typical-sized laboratories. These machines range in sophistication, type of the underlying printing technology, and possible add-ons/features, which makes the selection process rather daunting (especially for a nonexpert customer). Yet, the review articles available in the literature mostly focus on the technical aspects of the printer technologies under development, as opposed to explaining the differences in what is already on the market. In contrast, this paper provides a snapshot of the fast-evolving low-cost bioprinter niche, as well as reputation profiles (relevant to delivery time, part quality, adherence to specifications, warranty, maintenance, etc.) of the companies selling these machines. Specifically, models spanning three dominant technologies-microextrusion, droplet-based/inkjet, and light-based/crosslinking-are reviewed. Additionally, representative examples of high-end competitors (including up-and-coming microfluidics-based bioprinters) are discussed to highlight their major differences and advantages relative to the low-cost models. Finally, forecasts are made based on the trends observed during this survey, as to the anticipated trickling down of the high-end technologies to the low-cost printers. Overall, this paper provides insight for guiding buyers on a limited budget toward making informed purchasing decisions in this fast-paced market.

Three-Dimensionally Printed Skin Substitute Using Human Dermal Fibroblasts and Human Epidermal Keratinocytes

INTRODUCTION

Wound healing affects millions of people annually. After injury, keratinocytes from the wound edge proliferate, migrate, and differentiate to recapitulate the 3-dimensional (3D) structure needed to provide a barrier function. If the wound is too large, skin grafting may be required. We are interested in discovering novel strategies to enhance the wound healing process. It may be possible to recreate a viable and histologically accurate skin tissue using 3D printing. We hypothesize that keratinocytes and dermal fibroblasts can be bioprinted into a viable skin substitute.

METHODS

Adult human dermal fibroblasts (HDFa) and adult human epidermal keratinocytes (HEKa) were cultured and subsequently printed with a 3D bioprinter within a hydrogel scaffold. After printing the HDFa and HEKa separately, cell viability and histological appearance were determined by sectioning the printed tissue and performing hematoxylin and eosin staining. The stained histological sections were analyzed for tissue morphology.

RESULTS

The HEKa and HDFa cells suspended in the hydrogel were successfully printed into 3D scaffolds that resembled skin with hematoxylin and eosin staining.

CONCLUSIONS

The HEKa and HDFa cells can be grown on 3D-printed hydrogels successfully. In addition, HEKa and HDFa cells can survive and grow when suspended in a hydrogel and 3D printed. Future potential applications of these results could lead to the creation of viable skin tissue for wound healing and surgical repair.

Recent Advances in the Design of Three-Dimensional and Bioprinted Scaffolds for Full-Thickness Wound Healing

Three-dimensional (3D) printed scaffolds have recently emerged as an innovative treatment option for patients with critical-sized skin wounds. Current approaches to managing life-threatening wounds include skin grafting and application of commercially sourced skin substitutes. However, these approaches are not without several challenges. Limited donor tissue and donor site morbidity remain a concern for tissue grafting, while engineered skin substitutes fail to fully recapitulate the complex native environment required for wound healing. The implementation of 3D printed dermal scaffolds offers a potential solution for these shortcomings. Spatial control over scaffold structure, the ability to incorporate multiple materials and bioactive ingredients, enables the creation of conditions specifically optimized for wound healing. Three-dimensional bioprinting, a subset of 3D printing, allows for the replacement of lost cell populations and secreted active compounds that contribute to tissue repair and recovery. The replacement of damaged and lost cells delivers beneficial effects directly, or synergistically, supporting injured tissue to recover its native state. Despite encouraging results, the promise of 3D printed scaffolds has yet to be realized. Further improvements to current material formulations and scaffold designs are required to achieve the goal of clinical adoption. Herein, we provide an overview of 3D printing techniques and discuss several strategies for healing of full-thickness wounds by using 3D printed acellular scaffolds or bioprinted cellular scaffolds, aimed at translating this technology to the clinical management of skin lesions. We identify the challenges associated with designing and optimizing printed tissue replacements, and discuss the future perspectives of this emerging option for managing patients who present with critical-sized life-threatening cutaneous wounds.

Extrusion-Based Bioprinting of Multilayered Nanocellulose Constructs for Cell Cultivation Using In Situ Freezing and Preprint CaCl2 Cross-Linking

Extrusion-based bioprinting with a preprint cross-linking agent and an in situ cooling stage provides a versatile method for the fabrication of 3D structures for cell culture. We added varying amounts of calcium chloride as a precross-linker into native nanofibrillated cellulose (NFC) hydrogel prior to 3D bioprinting to fabricate structurally stable multilayered constructs without the need for a separate cross-linking bath. To further enhance their stability, we bioprinted the multilayered structures onto an in situ temperature-controlled printing stage at 25, 0, and -10 °C. The extruded and subsequently freeze-dried volumetric constructs maintained their structures after being immersed into a cell culture medium. The ability to maintain the shape after immersion in cell media is an essential feature for the fabrication of stem cell-based artificial organs. We studied the viability and distribution of mouse embryonic fibroblast cells into the hydrogels using luminescence technique and confocal microscopy. Adding CaCl2 increased the stability of the multilayered nanocellulose structures, making them suitable for culturing cells inside the 3D hydrogel environment. Lower stage temperature considerably improved the structural stability of the 3D printed structures, however, had no effect on cell viability.

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.

Challenges of 3D printing technology for manufacturing biomedical products: A case study of Malaysian manufacturing firms

Additive manufacturing has attracted increasing attention worldwide, especially in the healthcare, biomedical, aerospace, and construction industries. In Malaysia, insufficient acceptance of this technology by local industries has resulted in a call for government and local practitioners to promulgate the development of this technology for various industries, particularly for biomedical products. The current study intends to frame the challenges endured by biomedical industries who use 3D printing technology for their manufacturing processes. Qualitative methods, particularly in-depth interviews, were used to identify the challenges faced by manufacturing firms when producing 3D printed biomedical products. This work was able to identify twelve key challenges when deploying additive manufacturing in biomedical products and these include issues related to binder selection, poor mechanical properties, low-dimensional accuracy, high levels of powder agglomeration, nozzle size, distribution size, limited choice of materials, texture and colour, lifespan of materials, customization of fit and design, layer height, and, lastly, build-failure. Furthermore, there also are six challenges in the management of manufacturing biomedical products using 3D printing technology, and these include staff re-education, product pricing, limited guidelines, cyber-security issues, marketing, and patents and copyright. This study discusses the reality faced by 3D printing players when producing biomedical products in Malaysia, and presents a primary reference for practitioners in other developing countries.

Tissue Engineering and Regenerative Medicine: Achievements, Future, and Sustainability in Asia

Exploring innovative solutions to improve the healthcare of the aging and diseased population continues to be a global challenge. Among a number of strategies toward this goal, tissue engineering and regenerative medicine (TERM) has gradually evolved into a promising approach to meet future needs of patients. TERM has recently received increasing attention in Asia, as evidenced by the markedly increased number of researchers, publications, clinical trials, and translational products. This review aims to give a brief overview of TERM development in Asia over the last decade by highlighting some of the important advances in this field and featuring major achievements of representative research groups. The development of novel biomaterials and enabling technologies, identification of new cell sources, and applications of TERM in various tissues are briefly introduced. Finally, the achievement of TERM in Asia, including important publications, representative discoveries, clinical trials, and examples of commercial products will be introduced. Discussion on current limitations and future directions in this hot topic will also be provided.

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.

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.