Biomaterial Fabrication Techniques E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This reference is a guide to biomaterial fabrication techniques. The book comprises ten chapters introducing the reader to a range of biomaterial synthesis while highlighting biomedical applications. Each chapter presents a review of the topic followed by updated information about relevant core and applied concepts in an easy to understand format. The first two chapters present vital information about biomaterial components, such as polymer nanocomposites and scaffolds, and the strategies used for their fabrication. The proceeding chapters explain the principles of the most widely used fabrication techniques, and their application in detail. These include freeze drying, electrospinning, 3D printing, multiphoton lithography, particulate leaching, supramolecular self assembly, solvent casting and melt molding. The book is an essential primer on biomaterial synthesis for students and early career researchers in the field of biomedical engineering, applied chemistry and tissue engineering.
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Seitenzahl: 461
Veröffentlichungsjahr: 2003
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FOREWORD
This book provides an up-to-date, comprehensive, and authoritative overview of advancements in scaffold manufacture and uses in tissue engineering, combining the foundations for a wide understanding of scaffolds for tissue growth and development. The chapters cover a wide range of issues, including innovative materials and methodologies for scaffold preparation, difficulties, and future prospects. The chapters include topics such as novel materials and techniques for scaffold preparation, challenges, future prospects, and much more. The authors have carefully analyzed and summarized recent research findings in the aforementioned areas, providing an in-depth understanding of scaffold that maintains a balance among a variety of topics related to tissue engineering, including biology, chemistry, material science, and engineering, among others, while prioritizing study topics that are likely to be useful in the future.
PREFACE
This book is a collection of research and review articles from various parts of the world, highlighting the pivotal importance of biomaterials and their potential biomedical application. The articles link new findings and critically review the fundamental concepts and principles that are making the base of innovation. The book comprises ten chapters; the first two chapters deal with vital information about biomaterials and the strategies used for their fabrication. The rest of the chapters highlight the most widely used technique, their principle and their application in detail. The book contains up-to-date knowledge of biomaterials, their fabrication technique and their potential application, which is beneficial both for the experience as well as new researchers.
List of Contributors
Introduction to Biomaterials and Scaffolds for Tissue Engineering
Khalil K. Hussain1,*,Muhammad Naeem2Abstract
Biomaterials are essential elements in various fields, especially medicine. They can help restore biological functions and speed up the healing process after injury or disease. Natural or synthetic biomaterials are used in clinical applications to provide support, replace damaged tissue, or restore biological function. The study of such types of biomaterials is an active area of research, particularly in the field of tissue engineering (TE). In general, the term TE describes the regeneration, growth, and repair of damaged tissue due to disease or injury. TE is a modern science that combines biology, biochemistry, clinical medicine and biomaterials, which led to the research and development of various applications. For example, in the field of regenerative medicine, biomaterials can serve as a support (scaffold) to promote cell growth and differentiation, which ultimately facilitates the healing process of tissues. This chapter describes the various properties of biomaterials, a detailed discussion of scaffolds in terms of design, properties and production techniques, and future directions for TE.
INTRODUCTION
The U.S. National Institute of Health defines biomaterials as “any substance or combination of substances, other than drugs, of synthetic or natural origin, that can be used for any period of time, partially or completely augments or replaces a tissue, organ, or function of the body to maintain or improve the quality of life of the individual” [1]. Interestingly, the use of biomaterials dates back to ancient times, when the Romans and Egyptians used plant fibres to suture skin wounds and made prosthetic limbs from wood [2]. Since then, the use of biomaterials has gone through different phases. In the industrial era, biomaterials have changed dramatically, leading to the synthesis of novel biomaterials for various applic-
ations, especially in regenerative medicine and tissue engineering strategies. In general, biomaterials can be divided into three groups: Ceramics, synthetic polymers and natural polymers. However, each group has advantages and disadvantages [3]. In humans, the extracellular matrix (ECM) is considered a natural template biomaterial that provides support, spatial organisation, and maintenance of a biologically active microenvironment. The matrix is composed of different proteins that serve different functions, e.g., structural support proteins such as collagen and elastin, adhesion proteins such as fibronectin and laminin, and swellable proteins that contain polysaccharides such as glycosaminoglycans (GAGs) and proteoglycans [4]. The restructuring and remodelling of the ECM support tissue regeneration, cell survival, proliferation, and other functions [5]. Based on the functions of ECM, researchers are working to synthesise biomaterials that can mimic the role of ECM, which is currently not possible. Therefore, the most typical approach in the field of biomaterials is to understand the ECM mechanisms at the cellular level [6]. The approach has led to the emergence of a new field called tissue engineering (TE), which enables the formation of functional tissues. However, the equation is not simple, as the host response to biomaterials is complex and can trigger a proinflammatory response [7, 8]. TE is a multifaceted field that connects many disciplines, as shown in Fig. (1). Interestingly, in recent studies, macrophages play a crucial positive role in remodelling by secreting cytokines and/or scaffold degradation products [9-12].
Fig. (1)) Basic components in TE: Biomaterial scaffold serving as a template for tissue formation. Cells for regeneration, and signal either chemically from growth factors or physically from bioreceptor.BIOMATERIALS FOR SCAFFOLD FABRICATION
As mentioned earlier, biomaterials play an important role in tissue replacement and regeneration. So far, various types of materials have been synthesised and used as scaffolds in TE. In the following section, these biomaterials are described in detail.
Ceramics
Ceramic-based biomaterials are inorganic compounds of natural or synthetic origin that can be doped or un-doped with metals. Ceramics are an ideal choice as biomaterials because they have excellent properties, such as biocompatibility and osteoinductivity. This type of material has a similar chemical composition to natural human bone and hardly triggers any immune response. They also help in cell migration and facilitate osteogenic differentiation. Therefore, these types of biomaterials are popular to rebuild injured body parts, especially in bone regeneration. However, ceramics have some disadvantages that limit their use in scaffold fabrication, such as fragility and slow degradation [13-15]. There are three types of ceramic biomaterials: (I) inert to the biological environment; (II) resorbable: subject to in vivo degradation by phagocytosis; and (III) bioactive by chemically bonding with the cell surface [16]. Commonly used ceramic biomaterials include (a) calcium phosphate (CaP) biomaterials such as hydroxyapatite (HA), beta-tricalcium phosphate (BTP), a mixture of HA and BTP, (b) bioactive glass, (c) alumina, and (d) zirconia.
Natural HA is derived from a certain type of bovine ribbon phosphate and contains minute amounts of magnesium, sodium, carbon trioxide and fluorine. Synthetic HA, on the other hand, is prepared by various methods, including chemical deposition, biomimetic deposition and wet chemical precipitation [17]. Several reports have been published on synthetic HA. For example, Ray and colleagues reported synthetic HA with biocompatible and biomimetic properties. The prepared material was used for bone tissue engineering and iliac wings of dogs [18]. Similarly, Calabrese prepared a bilayer type 1 collagen HA /Mg scaffold and used it for osteochondral regeneration in vitro and in vivo [19-21]. Bioglass is composed of different elements with different weight percentages in the following order: SiO2, CaO, Na2O, and P2O5 with weight percentages of 45, 24.5, 24.5, and 6.0, respectively. It was first described by Hench and named 45S5 Bioglass, which has been used in biomedical applications [22]. Since then, various methods for the synthesis of bioglass have been reported, such as polymer foam replication, thermal bonding, and sol-gel. Bioglass and HA have similar properties, such as higher Ca to P content, making them ideal for bone grafts. The role of Bioglass in bone regeneration is outlined in Fig. (2). Moreover, bioglass has excellent osteoinductivity, controlled degradation rate and high bioactivity. However, they suffer from low strength and toughness [23, 24].
Fig. (2)) Schematic representation of the integration of bioglass with bone. The two stages show the interaction of the bioglass with the physiological environment and later the two stages of the process of new bone formation.Alumina (Al2O3) is a polycrystalline ceramic material with low porosity and small grain size. Zirconia, on the other hand, is a polymorphic structure with low thermal conductivity and a higher coefficient of thermal expansion. They are also an excellent choice for bone grafts and prostheses due to their higher biocompatibility and fracture load. So far, different ceramics have been used for different applications. The clinical application of such materials is limited due to their brittleness, mechanical flexibility and low mechanical strength, as well as the difficulty in controlling the rate of degradation.
Polymers
Polymers are a popular material for scaffold fabrication because they are biocompatible and bioactive. Therefore, different types of polymers have been introduced, either natural or synthetic. Among the naturally derived polymers, collagen and its derivatives are considered as an ideal choice for the regeneration or reconstruction of osteochondral lesions and ligaments [25, 26]. The bioactivity of such materials facilitates cellular adhesion. However, the resistance to mechanical stress is low, so they are coupled with other support materials. A number of reports have been published using collagen for TE. Aravamudhan reported the synthesis of micro-nanostructured scaffolds based on cellulose and collagen. The results showed good adhesion of human osteoblasts to their surface and progressive calcium deposition [27]. Collagen I/ III hydrogel scaffold was prepared by Schneider et al. The group isolated human mesenchymal stem cells from bone marrow and used them for seeding. After stimulation, the cells showed comparable osteogenic gene expression and migration [28]. Another application of the collagen scaffold was reported by Lu H., who used it to deliver osteogenic differentiation factors to promote osteogenesis [29]. Chitin, chitosan and alginate are polysaccharides used in tissue regeneration and can be prepared by various methods. For example, chitosan scaffolds are prepared by freeze-drying, which produces porous material. Chitosan scaffolds are positively charged and can therefore interact with swellable proteins in tissues and provide good cell adhesion. Several applications have been reported so far, including the successful cultivation of human bone marrow MSC cells on porous chitosan scaffolds by Costa-Pinto et al. The results showed an increase in cell viability [30]. In addition, chitosan was also used as an injectable biomaterial for scaffold fabrication. The scaffold consisted of tricalcium phosphate, chitosan, and platelet-rich plasma and was used in vivo to test its ability to repair bone fractures in animals [31]. In short, naturally derived polymers have numerous advantages. However, the fabrication of scaffolds from biological materials is not very homogeneous and reproducible.
Synthetic polymers generally consist of a sequence of monomeric components. Structurally, they can be linear, branched or cross-linked and can be produced in various forms such as fibers, films, rods and viscous liquids. To date, several types of synthetic polymers such as polystyrene, polylactic acid (PLA), polyglycolic acid (PGA), polylactic acid-co-glycolic acid (PLGA), and polycaprolactone (PCL) have been reported. Biodegradable is the most important property to be considered in these biomaterials. Fortunately, these types of synthetic polymers degrade on the surface and produce non-toxic compounds. Therefore, these polymers are used in various applications, such as sutures, orthopedic screws and prostheses, and scaffold fabrication for TE. The combination of PLA and PEG for the fabrication of PLA-PEG-PLA scaffolds was reported by Eğri et al. The obtained material was able to release vascular endothelial growth factors in bone tissue lesions [32]. However, PLLA- and PGA-containing materials generate CO2 during hydrolysis, which decreases the pH and may lead to cell and tissue necrosis [33].
Metals
Metals are a popular choice for TE because of their excellent mechanical properties. However, they suffer from poor cell adhesion and the release of toxic ions during implantation, which limits their use [34]. Various metals, such as iron, cobalt and titanium, have been used for scaffold fabrication. In iron, carbon and chromium-based materials, carbon provides good mechanical properties, but iron causes corrosion in a biological environment. Cobalt alloys, on the other hand, are divided into two categories: Co/Cr/Mo alloys can be synthesised by casting or melting, while Co/Ni/Cr/Mo alloys are produced by forging. Higher concentrations of Cr and Mo increase the granule size and improve the mechanical properties. Titanium-based frameworks can be divided into three types: Alpha, Beta, and combinations of Alpha and Beta. The first type contains aluminium and gallium and has excellent mechanical properties such as strength, hardness and sliding resistance. Beta stabiliser alloys contain vanadium and niobium and have better ductility. The third category contains a mixture of stabilisers and is ductile, such as Ti 6Al 4 V, which is used in biomedical applications. Ti and TiO2 scaffolds were tested by Wohlfahrt for osteoinductivity and osteointegration in rabbits, with new bone observed after 4 weeks of implantation [35]. In another study, a scaffold was fabricated from Cr-Co-Mo membranes and implanted into the tibias of rabbits. The results showed that a significant number of cells were observed on the scaffold [36].
Composites
This type of framework is made by combining one, two, or even three materials, such as combining polymers with ceramics or metals to make composite scaffolds. The composites have higher biocompatibility, biodegradability and considerable mechanical strength so that they can be used for soft and hard tissue regeneration. A number of studies have been reported with promising results in which scaffolds were fabricated from the combination of polymers and ceramics. Recently, scaffolds made of coated polymer, metal or ceramic have been reported [37, 38].
Hydrogels
Hydrogels are hydrophilic polymers with functional groups, such as carboxyl, amide, amino, and hydroxyl. In general, hydrogels are synthesised either by chemical or physical interactions. Interestingly, they are excellent water absorbers and swell without dissolving. Such materials were first described by Wichterle and Lim, who synthesised a poly(2-hydroxyethyl methacrylate) and used it in contact lenses [39]. Hydrogels are broadly classified into three main groups based on their origin: natural: derived from polypeptides and polysaccharides, synthetic: produced by polymerization, and semi-synthetic [40]. Usually, they have a soft and rubbery structure similar to ECM. In addition, hydrogels have been explored for TE scaffold fabrication and used as injectable hydrogels with cells. The mechanism for cartilage- and bone TE applications is sketched in Fig. (3).
Fig. (3)) Schematic representation for injectable hydrogels applied in cartilage- and bone tissue engineering.The most important factors in the synthesis of hydrogels are the determination and control of crosslink density, pore size, and structural interconnectivity for cellular functions. This type of material can be easily modified on its surface by various biological molecules such as growth factors. Recently, researchers have been developing smart hydrogels that interact and change their structure depending on the microenvironment, pH or temperature. For example, Pasqui et al. reported a natural cellulose-hydroxyapatite hybrid hydrogel used for bone TE. The hydrogel was mixed with microcrystals of HA and successfully applied in vitro [41]. Synthetic hydrogels offer the possibility of modifying mechanical and biological properties, such as the rate of degradation. For example, biodegradable oligo [poly(ethylene glycol) fumarate] was synthesised by Kinard to deliver demineralized bone matrix (DBM) into a bone defect in rats. The results showed that the higher the DBM concentration, the faster the degradation rate [42]. Summary of biomaterials for scaffold production in Table 1.
Methodologies for Scaffold Production
Scaffold fabrication has become an active area of research over the last two decades. To date, several techniques have been described for the development of scaffolds. These techniques are mainly focused on the fabrication of porous scaffolds for cell seeding [43-45]. In general, these fabrication techniques are classified into three categories: (I) porogen-based techniques, (II) woven or nonwoven fiber fabrication techniques, and (II) rapid prototyping [46]. The first two techniques are largely used for scaffold fabrication. In the first category, the biomaterial is combined with porogens (CO2, H2O or paraffin), and the combined mixture is then processed by casting or extrusion. Finally, the porogen is removed from the biomaterial by sublimation, evaporation or leaching to obtain a porous material. Porogens based techniques include gas foaming, freeze drying and solvent casting [47-49]. Nonwoven fibers are produced by spinning. Fibers produced by spinning are called nonwoven fibers. Spinning processes include electrospinning, wet spinning, microfludic spinning, melt spinning and bio-spinning. Owven fibers are produced by weaving, a textile technique in which two different sets of warp or weft threads are interwoven at right angles to form a fabric with controlled strength, porosity, morphology, and geometry, and by knotting, in which a fabric is formed by interlacing yarns or threads in a series of interconnected loops [50]. It is worth noting that the scaffold produced by these techniques has numerous advantages. For example, any type of biomaterial can be used for fabrication with precise architectural designs. Moreover, the scaffold structure can be combined with different materials that provide better mechanical support. However, some of these approaches are time-consuming and suffer from limited cell penetration into the scaffolds. One of the techniques to overcome these drawbacks is decellularized ECM from allogeneic or xenogeneic tissues for cell colonization. This method has been applied to various tissues in TE [51, 52]. This approach uses different strategies to decellularize ECM, such as physical, chemical and enzymatic methods [53, 54]. Decellularized ECM is useful in many other applications, such as small intestine submucosa, skin, and other body tissues [51, 55-57]. These natural scaffolds are biocompatible and can provide promis-ing results when combined with growth factors [58]. However, incomplete removal of cellular components during implantation may trigger immune reactions [59]. In general, some basic requirements should be considered before the fabrication of scaffolds, such as (I) biocompatibility, to facilitate cell adhesion, proliferation and migration, (II) bioactivity, the ability of the biomaterial to interact with the microenvironment, and (III) biodegradability, which allows cells to produce their own extracellular matrix [60-62].
Scaffold Design and Properties Relationship
Structural and mechanical properties are of great importance in the development of scaffolds for TE. These properties ensure that the ECM has good strength at the anatomical site so that it can perform its functions. The precise structure of the scaffold facilitates cell survival, adhesion, proliferation, differentiation, vascularization, and specific gene expression [63, 64]. In addition, the engineered structure should support a physiological load with appropriate porosity to allow cellular functions and avoid cell colonisation. However, fabricating a scaffold with a porous structure and mechanically strong properties is a challenging task. For example, an appropriate pore size allows cell penetration and migration into the scaffold and cell attachment. The size of the macropores of the scaffold in TE depends on the host tissues and is usually > 50 nm. For example, the ideal pore size for hepatocyte and fibroblast growth is 20 microns, and for soft tissue, healing is 20-150 microns. A higher pore size between 200 and 400 microns is used for bone TE.
Adequate mechanical properties are equally important in the fabrication of scaffolds as they facilitate the remodelling process and serve as support [65]. These properties depend on the type of bond or forces that hold the atoms together in the scaffold architecture. The rigidity of the scaffold is also an important parameter measured by Young's modulus. It is worth noting that the cellular response to scaffold stiffness is controlled by the activation of ion channels or protein unfolding mechanisms. Therefore, stiffness affects cell proliferation and differentiation. For example, it has been reported that the higher the stiffness of the free-floating collagen matrix, the greater the proliferation of dermal fibroblasts [64]. The general properties and in vivo, in vitro application characteristics are shown in Fig. (4).
Current Scaffold Fabrication Technologies
Tissues are a 3-dimensional (3D) like entity with variable sizes. This 3D structure allows them to perform physiological functions such as organizing biological processes, facilitating mechanical properties, and connecting with other body organs. The main goal of scaffolds is to achieve this development in 3D cells by supporting and ensuring mechanical properties in the process of tissue regeneration [66]. Current scaffold fabrication methods should mainly address two main aspects: 1) At the microscale level, the environment should facilitate cellular functions and survival. 2) At the macroscale, the tissue design should allow the organization of multicellular processes, facilitate nutrient transport, and have better mechanical properties. A suitable technique is the fabrication of scaffolds using 3D technology, which can be either conventional or rapid prototyping (RP) [67]. Conventionally fabricated scaffolds use porous polymer structures to support cell adhesion, but suffer from heterogeneous structures and difficulties in maintaining 3D structure at micro and macro levels [68]. In short, combining clinical knowledge and 3D fabrication techniques to produce a tailored scaffold that accelerates healing and improves scaffold properties [69].
Fig. (4)) Illustration of Tissue engineering process using biomaterial scaffold.Conventional approaches to scaffold fabrication have been applied in various fields, such as drug delivery and 3D cell culture at TE [70]. The scaffold is fabricated using traditional methods such as casting/particle leaching, which control the shape and pore size. Generally, in this method, the solvent is mixed with salt particles that have a certain particle size to dissolve the polymer solution. Then the solvent is evaporated, and the remaining matrix is dissolved in water. In this way, the salt is leached out, and a porosity of 50% and 90% is achieved. This technique is best suited for thin-walled 3D specimens [71, 72]. The method is simple, cheap and, most importantly, adjustable pore size can be obtained [73]. There are numerous reports on the application of this method. For example, a porous PLA/MD- HAP /PEO nanocomposite was prepared and used in bone engineering. PLA was mixed with different concentrations of NH4HCO3 and Progenia. The combination of natural polymers has also been reported [74, 75]. The addition of bioactive material into the scaffold structure has also been extensively described [76, 77]. Among its numerous advantages, this technique is time-consuming and uses toxic solvents [76]. In freeze-drying or lyophilization, the synthetic polymer is dissolved and cooled below the freezing point, causing the solvent to evaporate by sublimation and form pores [78, 79]. This technique does not require high temperatures and allows controllable pore size by simply changing the freezing method [80, 81]. Various types of scaffold fabrication materials have been produced using this technique, such as chitosan nanoparticles [78, 82]. This technique is popular partly because it avoids the use of toxic solvents. However, it requires high energy consumption, is a lengthy process, and causes irregular pore size [83]. To overcome these drawbacks, G'eraldine et al. suggested lowering the temperature to -70°C and performing an additional annealing step [84]. Thermally-induced phase separation (TIPS) is a conventional method that operates at lower temperatures to force phase separation. The method has been used in the preparation of thermoplastic crystalline polymer scaffolds [85, 86]. Incorporation of proteins into the scaffold prepared using TIPS has been used, for example, for drug delivery and the preparation of microspheres for TE [87, 88]. The gas foam method uses inert gasses, such as CO2 or N2, to pressurize the polymer with water or fluoroform until it is saturated and a sponge-like structure is formed. The pore size ranges from 30 to 700 microns with porosity up to 85%. However, the method suffers from closed pores. Harris reported an improved method, and the results showed good cell adhesion [89]. In the electrospinning method, scaffolds of nanofibrous material are fabricated using electricity [90]. The technique is complicated and high voltage is applied to obtain the desired scaffold. However, this technique does not succeed in obtaining a homogeneous distribution of pores. Examples of this method include the preparation of spider silk proteins and collagen [91]. Similarly Sarhan et al. [92] successfully incorporated antimicrobial molecules into the construction of nanofibers and used them in wound dressings. Surprisingly, the fabricated scaffold was active against a range of bacteria. In addition, chitosan-based nanofibers have been produced by electrospinning [93, 94]. Although electrospinning is a popular method for fabricating scaffolds, it lacks homogeneous pore distribution and pore size.
On the other hand, rapid prototyping technologies are often used in scaffold development. These methods are also known as solid free-form fabrication (SFF). These techniques are combined with computer-aided design (CAD) to obtain precise spatial control over the polymer structure during scaffold fabrication [95, 96]. In addition, these techniques allow the fabrication of customizable and patient-specific scaffolds. These methods mainly include stereolithography, fused deposition modelling (FDM), selective laser sintering (SLS), and 3D printing (3DP) [71, 97]. The stereolithography method creates solid 3D objects by successively printing a smooth, thin layer of ultraviolet material. The components work in flux to create a 3D scaffold structure without wasting starting materials. For example, a scaffold based on poly(D, L- lactate) or poly(D,L-lactide-co-e- caprolactone) resin was fabricated using the stereolithographic method [98]. Other researchers used gelatin methacrylate to fabricate precisely designed scaffolds [98, 99], and significant results have been reported proving that this method is suitable for TE. However, the method has its limitations as it requires a large number of monomers and polymerization treatment [100]. This method has great potential for many other applications such as biosensors, drug development and energy generation [100].
Hutmacher et al. reported a nonwoven scaffold [101]. The scaffold was made up of polyester to facilitate the growth of cells. Similarly, the fused deposition modeling(FDM) method was used to fabricate bioresorbable poly(e-caprolactone) to create porosity in the scaffold [101, 102]. The results for the scaffold fabricated were biocompatible, biodegradable, and better conductive in bone repair experiments and TE [102]. The SLS method is based on a laser that delivers energy to sinter powdered material in thin layers. Different types of materials such as polymers, metals or ceramics can be produced [103, 104]. PLLA-based biomaterial for TE scaffolds has also been reported [105]. 3DP uses a computer to create a complex 3D structure. Different biomaterials can be printed with a specific design for TE [106]. Previously, various scaffolds based on 3DP have been reported, such as poly(dopamine) and poly(lactic acid) scaffolds for bone TE [107]. A better 3DP scaffold was prepared using cold atmospheric plasma [108]. The proposed method showed better nanoscale roughness and chemical composition. Another study reported high-quality ceramic scaffolds fabricated by controlling the extrusion pressure [109]. The results showed that the frameworks were uniform. In addition, a low-cost scaffold based on bioactive (polyphosphazene, polyetheretherketone/hydroxyapatite (PEEK / HA) composites was prepared for bone tissue engineering [110]. Recently, an advanced method for 3DP, called bioprinting, has been developed. This technique is based on the assembly of relevant biological materials, such as cells or tissues [111]. This method can print biomaterials directly into a 3D structural scaffold without solvents. Bioprinting can be either acellular (without cells) or with cells, with the acellular method being superior due to its flexibility and lower fabrication requirements. To date, several printing methods have been developed, including autonomous self-assembly, biomimicry, mini-tissue building blocks, inkjet printing, microextrusion, and laser-assisted printing, with the latter three methods most commonly used for biomaterial printing [112, 113]. Inkjet bioprinting is a non-contact method that uses material jet techniques, such as thermal ink, acoustic waves, and electrohydrodynamic techniques [114]. The method has the advantage of being fast, inexpensive, and has cell viability of 70-90% [114]. However, it suffers from limited material selectivity and clogging [115]. Microextrusion is a temperature-controlled material handling and delivery system and a stage that moves in 3D. The technique consists of a fibre optic light source, a piezoelectric humidifier, and a video camera. The method produces continuous beads of material instead of liquid droplets [116]. The laser-assisted bioprinting method is nozzle-free and compatible with biomaterials of a certain viscosity (1-300 mPa/s) [117]. To date, considerable results have been achieved in the field of in vivo bioprinting [118, 119]. A summary of current scaffold fabrication methods can be found in Table 2.
CONCLUSION AND FUTURE DIRECTIONS
In recent decades, TE has become an active area of research that has led to the discovery of novel biomaterials for scaffold fabrication. Considerable success has been achieved in both in vitro and in vivo applications. Recently, research in the field of TE has greatly expanded. In particular, promising results have been achieved in the regeneration of various organs of the body, with the greatest success in skin, bladder, respiratory tract and bone. Some products have already been launched in various countries. One of the biggest challenges in the field of TE and scaffold fabrication is the lack of vascularity, so more attention needs to be paid to this research area. Introducing artificial microvasculature into the scaffolds before implantation could help to overcome the above-mentioned drawbacks, and this is only possible through multidisciplinary research approaches. Undoubtedly, TE offers many opportunities to improve human health, which is only possible if different fields are combined.
CONSENT FOR PUBLICATION
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CONFLICT OF INTEREST
The author declares no conflict of interest, financial or otherwise.
ACKNOWLEDGEMENT
Declared none.
