Additive manufacturing techniques for the production of tissue engineering constructs

Advanced fabrication approaches to controlled delivery systems for epilepsy treatment

INTRODUCTION

Epilepsy is a chronic brain disease characterized by unprovoked seizures, which can have severe consequences including loss of awareness and death. Currently, 30% of epileptic patients do not receive adequate seizure alleviation from oral routes of medication. Over the last decade, local drug delivery to the focal area of the brain where the seizure originates has emerged as a potential alternative and may be achieved through the fabrication of drug-loaded polymeric implants for controlled on-site delivery.

AREAS COVERED

This review presents an overview of the latest advanced fabrication techniques for controlled drug delivery systems for refractory epilepsy treatment. Recent advances in the different techniques are highlighted and the limitations of the respective techniques are discussed.

EXPERT OPINION

Advances in biofabrication technologies are expected to enable a new paradigm of local drug delivery systems through offering high versatility in controlling drug release profiles, personalized customization and multi-drug incorporation. Tackling some of the current issues with advanced fabrication methods, including adhering to GMP-standards and industrial scale-up, together with innovative solutions for complex designs will see to the maturation of these techniques and result in increased clinical research into implant-based epilepsy treatment.

ABBREVIATIONS

GMP: Good manufacturing process; DDS(s): Drug delivery system(s); 3D: Three-dimensional; AEDs: Anti-epileptic drugs; BBB: Blood brain barrier; PLA: Polylactic acid; PLGA: Poly(lactic-co-glycolic acid); PCL: poly(ɛ-caprolactone); ESE: Emulsification solvent evaporation; O/W: Oil-in-water; W/O/W: Water-in-oil-in-water; DZP: Diazepam; PHT: Phenytoin; PHBV: Poly(hydroxybutyrate-hydroxyvalerate); PEG: Polyethylene glycol; SWD: Spike-and-wave discharges; CAD: Computer aided design; FDM: Fused deposition modeling; ABS: Acrylonitrile butadiene styren; eEVA: Ethylene-vinyl acetate; GelMA: Gelatin methacrylate; PVA: Poly-vinyl alcohol; PDMS: Polydimethylsiloxane; SLA: Stereolithography; SLS: Selective laser sintering.

Tailoring the Interface of Biomaterials to Design Effective Scaffolds

Tissue engineering (TE) is a multidisciplinary science, which including principles from material science, biology and medicine aims to develop biological substitutes to restore damaged tissues and organs. A major challenge in TE is the choice of suitable biomaterial to fabricate a scaffold that mimics native extracellular matrix guiding resident stem cells to regenerate the functional tissue. Ideally, the biomaterial should be tailored in order that the final scaffold would be (i) biodegradable to be gradually replaced by regenerating new tissue, (ii) mechanically similar to the tissue to regenerate, (iii) porous to allow cell growth as nutrient, oxygen and waste transport and (iv) bioactive to promote cell adhesion and differentiation. With this perspective, this review discusses the options and challenges facing biomaterial selection when a scaffold has to be designed. We highlight the possibilities in the final mold the materials should assume and the most effective techniques for its fabrication depending on the target tissue, including the alternatives to ameliorate its bioactivity. Furthermore, particular attention has been given to the influence that all these aspects have on resident cells considering the frontiers of materiobiology. In addition, a focus on chitosan as a versatile biomaterial for TE scaffold fabrication has been done, highlighting its latest advances in the literature on bone, skin, cartilage and cornea TE.

Preparation of Polymeric and Composite Scaffolds by 3D Bioprinting

Over the recent years, the advent of 3D bioprinting technology has marked a milestone in osteochondral tissue engineering (TE) research. Nowadays, the traditional used techniques for osteochondral regeneration remain to be inefficient since they cannot mimic the complexity of joint anatomy and tissue heterogeneity of articular cartilage. These limitations seem to be solved with the use of 3D bioprinting which can reproduce the anisotropic extracellular matrix (ECM) and heterogeneity of this tissue. In this chapter, we present the most commonly used 3D bioprinting approaches and then discuss the main criteria that biomaterials must meet to be used as suitable bioinks, in terms of mechanical and biological properties. Finally, we highlight some of the challenges that this technology must overcome related to osteochondral bioprinting before its clinical implementation.

Thiol-Ene Alginate Hydrogels as Versatile Bioinks for Bioprinting

Bioprinting is a powerful technique that allows precise and controlled 3D deposition of biomaterials in a predesigned, customizable, and reproducible manner. Cell-laden hydrogel ("bioink") bioprinting is especially advantageous for tissue engineering applications as multiple cells and biomaterial compositions can be selectively dispensed to create spatially well-defined architectures. Despite this promise, few hydrogel systems are easily available and suitable as bioinks, with even fewer systems allowing for molecular design of mechanical and biological properties. In this study, we report the development of a norbornene functionalized alginate system as a cell-laden bioink for extrusion-based bioprinting, with a rapid UV-induced thiol-ene cross-linking mechanism that avoids acrylate kinetic chain formation. The mechanical and swelling properties of the hydrogels are tunable by varying the concentration, length, and structure of dithiol PEG cross-linkers and can be further modified by postprinting secondary cross-linking with divalent ions such as calcium. The low concentrations of alginate needed (<2 wt %), coupled with their rapid in situ gelation, allow both the maintenance of high cell viability and the ability to fabricate large multilayer or multibioink constructs with identical bioprinting conditions. The modularity of this bioink platform design enables not only the rational design of materials properties but also the gel's biofunctionality (as shown via RGD attachment) for the expected tissue-engineering application. This modularity enables the creation of multizonal and multicellular constructs utilizing a chemically similar bioink platform. Such tailorable bioink platforms will enable increased complexity in 3D bioprinted constructs.

Current advances for bone regeneration based on tissue engineering strategies

Bone tissue engineering (BTE) is a rapidly developing strategy for repairing critical-sized bone defects to address the unmet need for bone augmentation and skeletal repair. Effective therapies for bone regeneration primarily require the coordinated combination of innovative scaffolds, seed cells, and biological factors. However, current techniques in bone tissue engineering have not yet reached valid translation into clinical applications because of several limitations, such as weaker osteogenic differentiation, inadequate vascularization of scaffolds, and inefficient growth factor delivery. Therefore, further standardized protocols and innovative measures are required to overcome these shortcomings and facilitate the clinical application of these techniques to enhance bone regeneration. Given the deficiency of comprehensive studies in the development in BTE, our review systematically introduces the new types of biomimetic and bifunctional scaffolds. We describe the cell sources, biology of seed cells, growth factors, vascular development, and the interactions of relevant molecules. Furthermore, we discuss the challenges and perspectives that may propel the direction of future clinical delivery in bone regeneration.

Numerical Evaluation and Prediction of Porous Implant Design and Flow Performance

Porous structure has been widely acknowledged as important factor for mass transfer and tissue regeneration. This study investigates effect of aimed-control design on mass transfer and tissue regeneration of porous implant with regular unit cell. Two shapes of unit cells (Octet truss, and Rhombic dodecahedron) were selected, which have similar symmetrical structure and are commonly used in practice. Through parametric design, porous scaffolds with the strut sizes of φ 0.5, 0.7, 0.9, and 1.1mm were created, respectively. Then using fluid flow simulation method, flow velocity, permeability, and shear stress which could reflect the properties of mass transfer and tissue regeneration were compared and evaluated, and the relationships between porous structure's physical parameters and flow performance were studied. Results demonstrated that unit cell shape and strut size greatly determine and influence other physical parameters and flow performances of porous implant. With the increasing of strut size, pore size and porosity linearly decrease, but the volume, surface area, and specific surface area increased. Importantly, implant with smaller strut size resulted in smaller flow velocity directly but greater permeability and more appropriate shear stress, which should be beneficial to cell attachment and proliferation. This study confirmed that porous implant with different unit cell shows different performances of mass transfer and tissue regeneration, and unit cell shape and strut size play vital roles in the control design. These findings could facilitate the quantitative assessment and optimization of the porous implant.

Engineering in-vitro stem cell-based vascularized bone models for drug screening and predictive toxicology

The production of veritable in-vitro models of bone tissue is essential to understand the biology of bone and its surrounding environment, to analyze the pathogenesis of bone diseases (e.g., osteoporosis, osteoarthritis, osteomyelitis, etc.), to develop effective therapeutic drug screening, and to test potential therapeutic strategies. Dysregulated interactions between vasculature and bone cells are often related to the aforementioned pathologies, underscoring the need for a bone model that contains engineered vasculature. Due to ethical restraints and limited prediction power of animal models, human stem cell-based tissue engineering has gained increasing relevance as a candidate approach to overcome the limitations of animals and to serve as preclinical models for drug testing. Since bone is a highly vascularized tissue, the concomitant development of vasculature and mineralized matrix requires a synergistic interaction between osteogenic and endothelial precursors. A number of experimental approaches have been used to achieve this goal, such as the combination of angiogenic factors and three-dimensional scaffolds, prevascularization strategies, and coculture systems. In this review, we present an overview of the current models and approaches to generate in-vitro stem cell-based vascularized bone, with emphasis on the main challenges of vasculature engineering. These challenges are related to the choice of biomaterials, scaffold fabrication techniques, and cells, as well as the type of culturing conditions required, and specifically the application of dynamic culture systems using bioreactors.

Osseointegration of porous apatite-wollastonite and poly(lactic acid) composite structures created using 3D printing techniques

A novel apatite-wollastonite/poly(lactic acid) (AW/PLA) composite structure, which matches cortical and cancellous bone properties has been produced and evaluated in vitro and in vivo. The composites structure has been produced using an innovative combination of 3D printed polymer and ceramic macrostructures, thermally bonded to create a hybrid composite structure. In vitro cell assays demonstrated that the AW structure alone, PLA structure alone, and AW/PLA composite were all biocompatible, with the AW structure supporting the proliferation and osteogenic differentiation of rat bone marrow stromal cells. Within a rat calvarial defect model the AW material showed excellent osseointegration with the formation of new bone, and vascularisation of the porous AW structure, both when the AW was implanted alone and when it was part of the AW/PLA composite structure. However, the AW/PLA structure showed the largest amount of the newly formed bone in vivo, an effect which is considered to be a result of the presence of the osteoinductive AW structure stimulating bone growth in the larger pores of the adjacent PLA structure. The layered AW/PLA structure showed no signs of delamination in any of the in vitro or in vivo studies, a result which is attributed to good initial bonding between polymer and ceramic, slow resorption rates of the two materials, and excellent osseointegration. It is concluded that macro-scale composites offer an alternative route to the fabrication of bioactive bone implants which can provide a match to both cortical and cancellous bone properties over millimetre length scales.

Tissue engineering with gellan gum

Engineering complex tissues for research and clinical applications relies on high-performance biomaterials that are amenable to biofabrication, maintain mechanical integrity, support specific cell behaviours, and, ultimately, biodegrade. In most cases, complex tissues will need to be fabricated from not one, but many biomaterials, which collectively fulfill these demanding requirements. Gellan gum is an anionic polysaccharide with potential to fill several key roles in engineered tissues, particularly after modification and blending. This review focuses on the present state of research into gellan gum, from its origins, purification and modification, through processing and biofabrication options, to its performance as a cell scaffold for both soft tissue and load bearing applications. Overall, we find gellan gum to be a highly versatile backbone material for tissue engineering research, upon which a broad array of form and functionality can be built.

Proposal of a Novel Natural Biomaterial, the Scleral Ossicle, for the Development of Vascularized Bone Tissue In Vitro

Recovering of significant skeletal defects could be partially abortive due to the perturbations that affect the regenerative process when defects reach a critical size, thus resulting in a non-healed bone. The current standard treatments include allografting, autografting, and other bone implant techniques. However, although they are commonly used in orthopedic surgery, these treatments have some limitations concerning their costs and their side effects such as potential infections or malunions. On this account, the need for suitable constructs to fill the gap in wide fractures is still urgent. As an innovative solution, scleral ossicles (SOs) can be put forward as natural scaffolds for bone repair. SOs are peculiar bony plates forming a ring at the scleral-corneal border of the eyeball of lower vertebrates. In the preliminary phases of the study, these ossicles were structurally and functionally characterized. The morphological characterization was performed by SEM analysis, MicroCT analysis and optical profilometry. Then, UV sterilization was carried out to obtain a clean support, without neither contaminations nor modifications of the bone architecture. Subsequently, the SO biocompatibility was tested in culture with different cell lines, focusing the attention to the differentiation capability of endothelial and osteoblastic cells on the SO surface. The results obtained by the above mentioned analysis strongly suggest that SOs can be used as bio-scaffolds for functionalization processes, useful in regenerative medicine.

Repositioning Titanium: An In Vitro Evaluation of Laser-Generated Microporous, Microrough Titanium Templates As a Potential Bridging Interface for Enhanced Osseointegration and Durability of Implants

Although titanium alloys remain the preferred biomaterials for the manufacture of biomedical implants today, such devices can fail within 15 years of implantation due to inadequate osseointegration. Furthermore, wear debris toxicity due to alloy metal ion release has been found to cause side-effects including neurotoxicity and chronic inflammation. Titanium, with its known biocompatibility, corrosion resistance, and high elastic modulus, could if harnessed in the form of a superficial scaffold or bridging device, resolve such issues. A novel three-dimensional culture approach was used to investigate the potential osteoinductive and osseointegrative capabilities of a laser-generated microporous, microrough medical grade IV titanium template on human skeletal stem cells (SSCs). Human SSCs seeded on a rough 90-µm pore surface of ethylene oxide-sterilized templates were observed to be strongly adherent, and to display early osteogenic differentiation, despite their inverted culture in basal conditions over 21 days. Limited cellular migration across the template surface highlighted the importance of high surface wettability in maximizing cell adhesion, spreading and cell-biomaterial interaction, while restricted cell ingrowth within the conical-shaped pores underlined the crucial role of pore geometry and size in determining the extent of osseointegration of an implant device. The overall findings indicate that titanium only devices, with appropriate optimizations to porosity and surface wettability, could yet play a major role in improving the long-term efficacy, durability, and safety of future implant technology.

Direct Writing Electrospinning of Scaffolds with Multidimensional Fiber Architecture for Hierarchical Tissue Engineering

Nanofibrous structures have long been used as scaffolds for tissue engineering (TE) applications, due to their favorable characteristics, such as high porosity, flexibility, high cell attachment and enhanced proliferation, and overall resemblance to native extracellular matrix (ECM). Such scaffolds can be easily produced at a low cost via electrospinning (ESP), but generally cannot be fabricated with a regular and/or complex geometry, characterized by macropores and uniform thickness. We present here a novel technique for direct writing (DW) with solution ESP to produce complex three-dimensional (3D) multiscale and ultrathin (∼1 μm) fibrous scaffolds with desirable patterns and geometries. This technique was simply achieved via manipulating technological conditions, such as spinning solution, ambient conditions, and processing parameters. Three different regimes in fiber morphologies were observed, including bundle with dispersed fibers, bundle with a core of aligned fibers, and single fibers. The transition between these regimes depended on tip to collector distance (Wd) and applied voltage (V), which could be simplified as the ratio V/Wd. Using this technique, a scaffold mimicking the zonal organization of articular cartilage was further fabricated as a proof of concept, demonstrating the ability to better mimic native tissue organization. The DW scaffolds directed tissue organization and fibril matrix orientation in a zone-dependent way. Comparative expression of chondrogenic markers revealed a substantial upregulation of Sox9 and aggrecan (ACAN) on these structures compared to conventional electrospun meshes. Our novel method provides a simple way to produce customized 3D ultrathin fibrous scaffolds, with great potential for TE applications, in particular those for which anisotropy is of importance.

[Application progress of digital technology in auricle reconstruction]

Objective

To review the application progress of digital technology in auricle reconstruction.

Methods

The recently published literature concerning the application of digital technology in auricle reconstruction was extensively consulted, the main technology and its specific application areas were reviewed.

Results

Application of digital technology represented by three-dimensional (3D) data acquisition, 3D reconstruction, and 3D printing is an important developing trend of auricle reconstruction. It can precisely guide auricle reconstruction through fabricating digital ear model, auricular guide plate, and costal cartilage imaging.

Conclusion

Digital technology can improve effectiveness and decrease surgical trauma in auricle reconstruction. 3D bioprinting of ear cartilage future has bright prospect and needs to be further researched.

Cell-instructive high-resolution micropatterned polylactic acid surfaces

Micro and nanoscale topographical structuring of biomaterial surfaces has been a valuable tool for influencing cell behavior, including cell attachment, proliferation and differentiation. However, most fabrication techniques for surface patterning of implantable biomaterials suffer from a limited resolution, not allowing controlled generation of sub-cellular three-dimensional features. Here, a direct laser lithography technique based on two-photon absorption was used to construct several patterns varying in size between 500 nm and 15 μm. Through replication via an intermediate mold, the patterns were transferred into polylactic acid (PLA), a widely used biomedical polymer, while retaining the original geometry. An osteoblast-like cell line, MG-63 was used for characterizing the morphological response to the topographical patterns. The results indicated that semi-continuous (dashed) lines, with a height of 1 μm were able to induce cell elongation in the direction of the lines. However, when dashes with a height of 0.5 μm were combined with perpendicularly crossing continuous lines (rails) with a height of 8 μm, the contact guidance effect of the dashes was lost and elongation of the cells was observed in the direction of the larger features. A second pattern, consisting of different arrays of pillars showed that, depending on the pillar height, the cells were either able to spread over the pattern or were confined between the pattern features. These differences in the ability of cells to spread further resulted in the formation of tension forces through stress fibers and displacement of vimentin. The method for high-resolution micropatterning of PLA as presented here can also be applied to other biomedical polymers, making it useful both for fundamental studies and for designing new biomaterials with improved functionality.

3D bioprinting and the current applications in tissue engineering

Bioprinting as an enabling technology for tissue engineering possesses the promises to fabricate highly mimicked tissue or organs with digital control. As one of the biofabrication approaches, bioprinting has the advantages of high throughput and precise control of both scaffold and cells. Therefore, this technology is not only ideal for translational medicine but also for basic research applications. Bioprinting has already been widely applied to construct functional tissues such as vasculature, muscle, cartilage, and bone. In this review, the authors introduce the most popular techniques currently applied in bioprinting, as well as the various bioprinting processes. In addition, the composition of bioink including scaffolds and cells are described. Furthermore, the most current applications in organ and tissue bioprinting are introduced. The authors also discuss the challenges we are currently facing and the great potential of bioprinting. This technology has the capacity not only in complex tissue structure fabrication based on the converted medical images, but also as an efficient tool for drug discovery and preclinical testing. One of the most promising future advances of bioprinting is to develop a standard medical device with the capacity of treating patients directly on the repairing site, which requires the development of automation and robotic technology, as well as our further understanding of biomaterials and stem cell biology to integrate various printing mechanisms for multi-phasic tissue engineering.

Design and fabrication of novel polymeric biodegradable stents for small caliber blood vessels by computer-aided wet-spinning

Biodegradable stents have emerged as one of the most promising approaches in obstructive cardiovascular disease treatment due to their potential in providing mechanical support while it is needed and then leaving behind only the healed natural vessel. The aim of this study was to develop polymeric biodegradable stents for application in small caliber blood vessels. Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHHx), a renewable microbial aliphatic polyester, and poly(ε-caprolactone), a synthetic polyester approved by the US Food and Drug Administration for different biomedical applications, were investigated as suitable polymers for stent development. A novel manufacturing approach based on computer-aided wet-spinning of a polymeric solution was developed to fabricate polymeric stents. By tuning the fabrication parameters, it was possible to develop stents with different morphological characteristics (e.g. pore size and wall thickness). Thermal analysis results suggested that material processing did not cause changes in the molecular structure of the polymers. PHBHHx stents demonstrated great radial elasticity while PCL stents showed higher axial and radial mechanical strength. The developed stents resulted able to sustain proliferation of human umbilical vein endothelial cells within two weeks of in vitro culture and they showed excellent results in terms of thromboresistivity when in contact with human blood.

Additive Manufacturing of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/poly(ε-caprolactone) Blend Scaffolds for Tissue Engineering

Additive manufacturing of scaffolds made of a polyhydroxyalkanoate blended with another biocompatible polymer represents a cost-effective strategy for combining the advantages of the two blend components in order to develop tailored tissue engineering approaches. The aim of this study was the development of novel poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/ poly(ε-caprolactone) (PHBHHx/PCL) blend scaffolds for tissue engineering by means of computer-aided wet-spinning, a hybrid additive manufacturing technique suitable for processing polyhydroxyalkanoates dissolved in organic solvents. The experimental conditions for processing tetrahydrofuran solutions containing the two polymers at different concentrations (PHBHHx/PCL weight ratio of 3:1, 2:1 or 1:1) were optimized in order to manufacture scaffolds with predefined geometry and internal porous architecture. PHBHHx/PCL scaffolds with a 3D interconnected network of macropores and a local microporosity of the polymeric matrix, as a consequence of the phase inversion process governing material solidification, were successfully fabricated. As shown by scanning electron microscopy, thermogravimetric, differential scanning calorimetric and uniaxial compressive analyses, blend composition significantly influenced the scaffold morphological, thermal and mechanical properties. In vitro biological characterization showed that the developed scaffolds were able to sustain the adhesion and proliferation of MC3T3-E1 murine preosteoblast cells. The additive manufacturing approach developed in this study, based on a polymeric solution processing method avoiding possible material degradation related to thermal treatments, could represent a powerful tool for the development of customized PHBHHx-based blend scaffolds for tissue engineering.

The Use of Finite Element Analyses to Design and Fabricate Three-Dimensional Scaffolds for Skeletal Tissue Engineering

Computational modeling has been increasingly applied to the field of tissue engineering and regenerative medicine. Where in early days computational models were used to better understand the biomechanical requirements of targeted tissues to be regenerated, recently, more and more models are formulated to combine such biomechanical requirements with cell fate predictions to aid in the design of functional three-dimensional scaffolds. In this review, we highlight how computational modeling has been used to understand the mechanisms behind tissue formation and can be used for more rational and biomimetic scaffold-based tissue regeneration strategies. With a particular focus on musculoskeletal tissues, we discuss recent models attempting to predict cell activity in relation to specific mechanical and physical stimuli that can be applied to them through porous three-dimensional scaffolds. In doing so, we review the most common scaffold fabrication methods, with a critical view on those technologies that offer better properties to be more easily combined with computational modeling. Finally, we discuss how modeling, and in particular finite element analysis, can be used to optimize the design of scaffolds for skeletal tissue regeneration.

Bone Regenerative Medicine in Oral and Maxillofacial Region Using a Three-Dimensional Printer<sup/>

Bone grafts currently used for the treatment of large bone defect or asymmetry in oral and maxillofacial region include autologous, allogeneic, and artificial bones. Although artificial bone is free from the concerns of donor site morbidity, limitation of volume, disease transmission, and ethical issues, it lacks osteogenic and osteoinductive activities. In addition, molding of the artificial bone is an issue especially when it is used for the augmentation of bone as onlay grafts. To solve this problem, additive manufacturing techniques have been applied to fabricate bones which have outer shapes conformed to patients' bones. We developed a custom-made artificial bone called a computed tomography (CT)-bone. Efficacy of CT-bone was proven in a clinical research and clinical trial, showing good manipulability, stability, and patient satisfaction. However, low replacement rate of artificial bones by endogenous bones remain an unsolved issue. Loading of cells and growth factors will improve the bone replacement by inducing osteogenic and osteoinductive activities. In addition, the three-dimensional bioprinting technique will facilitate bone regeneration by placing cells and biological substances into appropriate sites.

Airborne particle emission of a commercial 3D printer: the effect of filament material and printing temperature

The knowledge of exposure to the airborne particle emitted from three-dimensional (3D) printing activities is becoming a crucial issue due to the relevant spreading of such devices in recent years. To this end, a low-cost desktop 3D printer based on fused deposition modeling (FDM) principle was used. Particle number, alveolar-deposited surface area, and mass concentrations were measured continuously during printing processes to evaluate particle emission rates (ERs) and factors. Particle number distribution measurements were also performed to characterize the size of the emitted particles. Ten different materials and different extrusion temperatures were considered in the survey. Results showed that all the investigated materials emit particles in the ultrafine range (with a mode in the 10-30-nm range), whereas no emission of super-micron particles was detected for all the materials under investigation. The emission was affected strongly by the extrusion temperature. In fact, the ERs increase as the extrusion temperature increases. Emission rates up to 1×10¹²  particles min^-1 were calculated. Such high ERs were estimated to cause large alveolar surface area dose in workers when 3D activities run. In fact, a 40-min-long 3D printing was found to cause doses up to 200 mm² .

The use of nanoscaffolds and dendrimers in tissue engineering

To avoid tissue rejection during organ transplantation, research has focused on the use of tissue engineering to regenerate required tissues or organs for patients. The biomedical applications of hyperbranched, multivalent, structurally uniform, biocompatible dendrimers in tissue engineering include the mimicking of natural extracellular matrices (ECMs) in the 3D microenvironment. Dendrimers are unimolecular architects that can incorporate a variety of biological and/or chemical substances in a 3D architecture to actively support the scaffold microenvironment during cell growth. Here, we review the use of dendritic delivery systems in tissue engineering. We discuss the available literature, highlighting the 3D architecture and preparation of these nanoscaffolds, and also review challenges to, and advances in, the use dendrimers in tissue engineering. Advances in the manufacturing of dendritic nanoparticles and scaffold architectures have resulted in the successful incorporation of dendritic scaffolds in tissue engineering.

Recent cell printing systems for tissue engineering

Three-dimensional (3D) printing in tissue engineering has been studied for the bio mimicry of the structures of human tissues and organs. Now, it is being applied to 3D cell printing, which can position cells and biomaterials, such as growth factors, at desired positions in the 3D space. However, there are some challenges of 3D cell printing, such as cell damage during the printing process and the inability to produce a porous 3D shape owing to the embedding of cells in the hydrogel-based printing ink, which should be biocompatible, biodegradable, and non-toxic, etc. Therefore, researchers have been studying ways to balance or enhance the post-print cell viability and the print-ability of 3D cell printing technologies by accommodating several mechanical, electrical, and chemical based systems. In this mini-review, several common 3D cell printing methods and their modified applications are introduced for overcoming deficiencies of the cell printing process.

Polylactides in additive biomanufacturing

New advanced manufacturing technologies under the alias of additive biomanufacturing allow the design and fabrication of a range of products from pre-operative models, cutting guides and medical devices to scaffolds. The process of printing in 3 dimensions of cells, extracellular matrix (ECM) and biomaterials (bioinks, powders, etc.) to generate in vitro and/or in vivo tissue analogue structures has been termed bioprinting. To further advance in additive biomanufacturing, there are many aspects that we can learn from the wider additive manufacturing (AM) industry, which have progressed tremendously since its introduction into the manufacturing sector. First, this review gives an overview of additive manufacturing and both industry and academia efforts in addressing specific challenges in the AM technologies to drive toward AM-enabled industrial revolution. After which, considerations of poly(lactides) as a biomaterial in additive biomanufacturing are discussed. Challenges in wider additive biomanufacturing field are discussed in terms of (a) biomaterials; (b) computer-aided design, engineering and manufacturing; (c) AM and additive biomanufacturing printers hardware; and (d) system integration. Finally, the outlook for additive biomanufacturing was discussed.

Lipid Coated Microbubbles and Low Intensity Pulsed Ultrasound Enhance Chondrogenesis of Human Mesenchymal Stem Cells in 3D Printed Scaffolds

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters-excitation intensity, frequency and duty cycle-to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Microstructured chitosan/poly(γ-glutamic acid) polyelectrolyte complex hydrogels by computer-aided wet-spinning for biomedical three-dimensional scaffolds

The application of additive manufacturing principles to hydrogel processing represents a powerful route to develop porous three-dimensional constructs with a variety of potential biomedical applications, such as scaffolds for tissue engineering and three-dimensional in vitro tissue models. The aim of this study was to develop novel porous hydrogels based on a microstructured polyelectrolyte complex between chitosan and poly(γ-glutamic acid) by applying a computer-aided wet-spinning technique. The developed fabrication process was used to build up three-dimensional porous hydrogels by collecting microstructured layers made of chitosan/poly(γ-glutamic acid) on top of the other. Microstructured polyelectrolyte complex hydrogels were characterized and compared to chitosan/poly(γ-glutamic acid) porous hydrogels with similar composition prepared by conventional freeze-drying technique. Fourier transform infrared analysis confirmed the formation of an electrostatic interaction between the two processed polymers in all the developed chitosan/poly(γ-glutamic acid) hydrogels. The composition of the porous constructs as well as the employed processing techniques had a significant influence on the resulting swelling, thermal, and mechanical properties. In particular, the combination of the ionic interaction between chitosan/poly(γ-glutamic acid) and the defined internal microarchitecture of microstructured polyelectrolyte complex hydrogels provided a significant improvement of the compressive mechanical properties. Preliminary in vitro biological investigations revealed that microstructured polyelectrolyte complex hydrogels were suitable for the adhesion and proliferation of Balb/3T3 clone A31 mouse embryo fibroblasts. The encouraging results in terms of cytocompatibility and stability of the microstructure in aqueous solutions due to the ionic crosslinking suggest the investigation of the developed microstructured polyelectrolyte complex hydrogels as suitable scaffolds for three-dimensional cells’ culture.

Bioprinting Organotypic Hydrogels with Improved Mesenchymal Stem Cell Remodeling and Mineralization Properties for Bone Tissue Engineering

3D-manufactured hydrogels with precise contours and biological adhesion motifs are interesting candidates in the regenerative medicine field for the culture and differentiation of human bone-marrow-derived mesenchymal stem cells (MSCs). 3D-bioprinting is a powerful technique to approach one step closer the native organization of cells. This study investigates the effect of the incorporation of collagen type I in 3D-bioprinted polysaccharide-based hydrogels to the modulation of cell morphology, osteogenic remodeling potential, and mineralization. By combining thermo-responsive agarose hydrogels with collagen type I, the mechanical stiffness and printing contours of printed constructs can be improved compared to pure collagen hydrogels which are typically used as standard materials for MSC osteogenic differentiation. The results presented here show that MSC not only survive the 3D-bioprinting process but also maintain the mesenchymal phenotype, as proved by live/dead staining and immunocytochemistry (vimentin positive, CD34 negative). Increased solids concentrations of collagen in the hydrogel blend induce changes in cell morphology, namely, by enhancing cell spreading, that ultimately contribute to enhanced and directed MSC osteogenic differentiation. 3D-bioprinted agarose-collagen hydrogels with high-collagen ratio are therefore feasible for MSC osteogenic differentiation, contrarily to low-collagen blends, as proved by two-photon microscopy, Alizarin Red staining, and real-time polymerase chain reaction.

Highly Concentrated Alginate-Gellan Gum Composites for 3D Plotting of Complex Tissue Engineering Scaffolds

In tissue engineering, additive manufacturing (AM) technologies have brought considerable progress as they allow the fabrication of three-dimensional (3D) structures with defined architecture. 3D plotting is a versatile, extrusion-based AM technology suitable for processing a wide range of biomaterials including hydrogels. In this study, composites of highly concentrated alginate and gellan gum were prepared in order to combine the excellent printing properties of alginate with the favorable gelling characteristics of gellan gum. Mixtures of 16.7 wt % alginate and 2 or 3 wt % gellan gum were found applicable for 3D plotting. Characterization of the resulting composite scaffolds revealed an increased stiffness in the wet state (15%⁻20% higher Young's modulus) and significantly lower volume swelling in cell culture medium compared to pure alginate scaffolds (~10% vs. ~23%). Cytocompatibility experiments with human mesenchymal stem cells (hMSC) revealed that cell attachment was improved-the seeding efficiency was ~2.5⁻3.5 times higher on the composites than on pure alginate. Additionally, the composites were shown to support hMSC proliferation and early osteogenic differentiation. In conclusion, print fidelity of highly concentrated alginate-gellan gum composites was comparable to those of pure alginate; after plotting and crosslinking, the scaffolds possessed improved qualities regarding shape fidelity, mechanical strength, and initial cell attachment making them attractive for tissue engineering applications.

Methods of Monitoring Cell Fate and Tissue Growth in Three-Dimensional Scaffold-Based Strategies for In Vitro Tissue Engineering

In the field of tissue engineering, there is a need for methods that allow assessing the performance of tissue-engineered constructs noninvasively in vitro and in vivo. To date, histological analysis is the golden standard to retrieve information on tissue growth, cellular distribution, and cell fate on tissue-engineered constructs after in vitro cell culture or on explanted specimens after in vivo applications. Yet, many advances have been made to optimize imaging techniques for monitoring tissue-engineered constructs with a sub-mm or μm resolution. Many imaging modalities have first been developed for clinical applications, in which a high penetration depth has been often more important than lateral resolution. In this study, we have reviewed the current state of the art in several imaging approaches that have shown to be promising in monitoring cell fate and tissue growth upon in vitro culture. Depending on the aimed tissue type and scaffold properties, some imaging methods are more applicable than others. Optical methods are mostly suited for transparent materials such as hydrogels, whereas magnetic resonance-based methods are mostly applied to obtain contrast between hard and soft tissues regardless of their transparency. Overall, this review shows that the field of imaging in scaffold-based tissue engineering is developing at a fast pace and has the potential to overcome the limitations of destructive endpoint analysis.

Levofloxacin-loaded star poly(ε-caprolactone) scaffolds by additive manufacturing

The employment of a tissue engineering scaffold able to release an antimicrobial agent with a controlled kinetics represents an effective tool for the treatment of infected tissue defects as well as for the prevention of scaffolds implantation-related infectious complications. This research activity was aimed at the development of additively manufactured star poly(ε-caprolactone) (*PCL) scaffolds loaded with levofloxacin, investigated as antimicrobial fluoroquinolone model. For this purpose a computer-aided wet-spinning technique allowing functionalizing the scaffold during the fabrication process was explored. Scaffolds with customized composition, microstructure and anatomical external shape were developed by optimizing the processing parameters. Morphological, thermal and mechanical characterization showed that drug loading did not compromise the fabrication process and the final performance of the scaffolds. The developed *PCL scaffolds showed a sustained in vitro release of the loaded antibiotic for 5 weeks. The proposed computer-aided wet-spinning technique appears well suited for the fabrication of anatomical scaffolds endowed with levofloxacin-releasing properties to be tested in vivo for the regeneration of long bone critical size defects in a rabbit model.

Tailored star poly (ε-caprolactone) wet-spun scaffolds for in vivo regeneration of long bone critical size defects

One of the most challenging requirements of a successful bone tissue engineering approach is the development of scaffolds specifically tailored to individual tissue defects. Besides materials chemistry, well-defined scaffold’s structural features at the micro- and macro-levels are needed for optimal bone in-growth. In this study, polymeric fibrous scaffolds with a controlled internal network of pores and modelled on the anatomical shape and dimensions of a critical size bone defect in a rabbit’s radius model were developed by employing a computer-aided wet-spinning technique. The tailored scaffolds made of star poly(ε-caprolactone) or star poly(ε-caprolactone)–hydroxyapatite composite material were implanted into 20-mm segmental defects created in radial diaphysis of New Zealand white rabbits. Bone regeneration and tissue response were assessed by X-rays and histological analysis at 4, 8 and 12 weeks after surgery. No signs of macroscopic and microscopic inflammatory reactions were detected, and the developed scaffolds showed a good ability to support and promote the bone regeneration process. However, no significant differences in osteoconductivity were observed between star poly(ε-caprolactone) and star poly(ε-caprolactone)–hydroxyapatite scaffolds. Long-term study on implanted star poly(ε-caprolactone) scaffolds confirmed the presence of signs of bone regeneration and remodelling, particularly evident at 24 weeks.