Channels in a porous scaffold: a new player for vascularization

Mechanical strategies to promote vascularization for tissue engineering and regenerative medicine

Vascularization is a major challenge in the field of tissue engineering and regenerative medicine. Mechanical factors have been demonstrated to play a fundamental role in vasculogenesis and angiogenesis and can affect the architecture of the generated vascular network. Through the regulation of mechanical factors in engineered tissues, various mechanical strategies can be used to optimize the preformed vascular network and promote its rapid integration with host vessels. Optimization of the mechanical properties of scaffolds, including controlling scaffold stiffness, increasing surface roughness and anisotropic structure, and designing interconnected, hierarchical pore structures, is beneficial for the in vitro formation of vascular networks and the ingrowth of host blood vessels. The incorporation of hollow channels into scaffolds promotes the formation of patterned vascular networks. Dynamic stretching and perfusion can facilitate the formation and maturation of preformed vascular networks in vitro. Several indirect mechanical strategies provide sustained mechanical stimulation to engineered tissues in vivo, which further promotes the vascularization of implants within the body. Additionally, stiffness gradients, anisotropic substrates and hollow channels in scaffolds, as well as external cyclic stretch, boundary constraints and dynamic flow culture, can effectively regulate the alignment of vascular networks, thereby promoting better integration of prevascularized engineered tissues with host blood vessels. This review summarizes the influence and contribution of both scaffold-based and external stimulus-based mechanical strategies for vascularization in tissue engineering and elucidates the underlying mechanisms involved.

Gingerol-zinc complex loaded 3D-printed calcium phosphate for controlled release application

The therapeutic potential of natural medicines in treating bone disorders is well-established. Modifications in formulation or molecular structure can enhance their efficacy. Gingerol, an osteogenic active compound derived from ginger roots (Zingiber officinale), can form metal ion complexes. Zinc (Zn), a trace element that combats bacterial infections and promotes osteoblast proliferation, can be complexed with gingerol to form a G-Zn+2 complex. This study investigates a porous 3D-printed (3DP) calcium phosphate (CaP) scaffold loaded with the G-Zn+2 complex for drug release and cellular interactions. The scaffold is coated with polycaprolactone (PCL) to control the drug release. Diffusion-mediated kinetics results in 50% release of the G-Zn+2 complex over 6 weeks. The G-Zn+2 complex demonstrates cytotoxicity against MG-63 osteosarcoma cells, indicated by the formation of apoptotic bodies and ruptured cell morphology on the scaffolds. G-Zn+2 PCL-coated scaffolds show a 1.2 ± 0.1-fold increase in osteoblast cell viability, and an 11.6 ± 0.5% increase in  alkaline phosphatase compared to untreated scaffolds. Treated scaffolds also exhibit reduced bacterial colonization against Staphylococcus aureus bacteria, highlighting the antibacterial potential of the G-Zn+2 complex. The functionalized 3DP CaP scaffold with the G-Zn+2 complex shows significant potential for enhancing bone regeneration and preventing infections in low-load-bearing applications.

Enhanced Vascular-like Network Formation of Encapsulated HUVECs and ADSCs Coculture in Growth Factors Conjugated GelMA Hydrogels

Tissue engineering primarily aimed to alleviate the insufficiency of organ donations worldwide. Nonetheless, the survival of the engineered tissue is often compromised due to the complexity of the natural organ architectures, especially the vascular system inside the organ, which allows food-waste transfer. Thus, vascularization within the engineered tissue is of paramount importance. A critical aspect of this endeavor is the ability to replicate the intricacies of the extracellular matrix and promote the formation of functional vascular networks within engineered constructs. In this study, human adipose-derived stem cells (hADSCs) and human umbilical vein endothelial cells (HUVECs) were cocultured in different types of gelatin methacrylate (GelMA). In brief, pro-angiogenic signaling growth factors (GFs), vascular endothelial growth factor (VEGF165) and basic fibroblast growth factor (bFGF), were conjugated onto GelMA via an EDC/NHS coupling reaction. The GelMA hydrogels conjugated with VEGF165 (GelMA@VEGF165) and bFGF (GelMA@bFGF) showed marginal changes in the chemical and physical characteristics of the GelMA hydrogels. Moreover, the conjugation of these growth factors demonstrated improved cell viability and cell proliferation within the hydrogel construct. Additionally, vascular-like network formation was observed predominantly on GelMA@GrowthFactor (GelMA@GF) hydrogels, particularly on GelMA@bFGF. This study suggests that growth factor-conjugated GelMA hydrogels would be a promising biomaterial for 3D vascular tissue engineering.

Fused Deposition Modeling 3D-Printed Scaffolds for Bone Tissue Engineering Applications: A Review

The emergence of bone tissue engineering as a trend in regenerative medicine is forcing scientists to create highly functional materials and scaffold construction techniques. Bone tissue engineering uses 3D bio-printed scaffolds that allow and stimulate the attachment and proliferation of osteoinductive cells on their surfaces. Bone grafting is necessary to expedite the patient's condition because the natural healing process of bones is slow. Fused deposition modeling (FDM) is therefore suggested as a technique for the production process due to its simplicity, ability to create intricate components and movable forms, and low running costs. 3D-printed scaffolds can repair bone defects in vivo and in vitro. For 3D printing, various materials including metals, polymers, and ceramics are often employed but polymeric biofilaments are promising candidates for replacing non-biodegradable materials due to their adaptability and environment friendliness. This review paper majorly focuses on the fused deposition modeling approach for the fabrication of 3D scaffolds. In addition, it also provides information on biofilaments used in FDM 3D printing, applications, and commercial aspects of scaffolds in bone tissue engineering.

Accelerating Patterned Vascularization Using Granular Hydrogel Scaffolds and Surgical Micropuncture

Bulk hydrogel scaffolds are common in reconstructive surgery. They allow for the staged repair of soft tissue loss by providing a base for revascularization. Unfortunately, they are limited by both slow and random vascularization, which may manifest as treatment failure or suboptimal repair. Rapidly inducing patterned vascularization within biomaterials has profound translational implications for current clinical treatment paradigms and the scaleup of regenerative engineering platforms. To address this long-standing challenge, a novel microsurgical approach and granular hydrogel scaffold (GHS) technology are co-developed to hasten and pattern microvascular network formation. In surgical micropuncture (MP), targeted recipient blood vessels are perforated using a microneedle to accelerate cell extravasation and angiogenic outgrowth. By combining MP with an adjacent GHS with precisely tailored void space architecture, microvascular pattern formation as assessed by density, diameter, length, and intercapillary distance is rapidly guided. This work opens new translational opportunities for microvascular engineering, advancing reconstructive surgery, and regenerative medicine.

Sacrificial biomaterials in 3D fabrication of scaffolds for tissue engineering applications

Three-dimensional (3D) printing technology has progressed exceedingly in the area of tissue engineering. Despite the tremendous potential of 3D printing, building scaffolds with complex 3D structure, especially with soft materials, still exist as a challenge due to the low mechanical strength of the materials. Recently, sacrificial materials have emerged as a possible solution to address this issue, as they could serve as temporary support or templates to fabricate scaffolds with intricate geometries, porous structures, and interconnected channels without deformation or collapse. Here, we outline the various types of scaffold biomaterials with sacrificial materials, their pros and cons, and mechanisms behind the sacrificial material removal, compare the manufacturing methods such as salt leaching, electrospinning, injection-molding, bioprinting with advantages and disadvantages, and discuss how sacrificial materials could be applied in tissue-specific applications to achieve desired structures. We finally conclude with future challenges and potential research directions.

Hydrothermally treated coral scaffold promotes proliferation of mesenchymal stem cells and enhances segmental bone defect healing

Introduction: Synthetic hydroxyapatite (HAp) scaffolds have shown promising therapeutic outcomes in both animals and patients. In this study, we aim to evaluate the chemical and physical phenotype, biocompatibility, and bone repair effects of hydrothermally treated coral with natural coral and synthetic HAp. Methods: The phase composition, surface pattern, 3D structures, and porosity of the scaffolds were characterized, and cell viability, proliferation, and osteogenic differentiation of mesenchymal stem cells (MSCs) after seeding onto the scaffold were determined. The scaffolds were implanted into rats to assess their bone repair effects using micro-CT analysis, mechanical testing, and histological staining. Results: The results showed that the phase composition, porous structure, and porosity of hydrothermally treated coral were comparable to pure HAp scaffold. While only the natural coral happens to be dominantly calcium carbonate. Higher cell proliferation and osteogenic differentiation potential were observed in the hydrothermally treated coral scaffold compared to natural coral and pure HAp. Histological results also showed increased new bone formation in the hydrothermally treated coral group. Discussion: Overall, our study suggests that hydrothermal modification enhances the cytocompatibility and therapeutic capacity of coral without altering its physical properties, showing superior effectiveness in bone repair to synthetic HAp.

A multi-channel collagen conduit with aligned Schwann cells and endothelial cells for enhanced neuronal regeneration in spinal cord injury

Traumatic spinal cord injury (SCI) leads to Wallerian degeneration and the accompanying disruption of vasculature leads to ischemia, which damages motor and sensory function. Therefore, understanding the biological environment during regeneration is essential to promote neuronal regeneration and overcome this phenomenon. The band of Büngner is a structure of an aligned Schwann cell (SC) band that guides axon elongation providing a natural recovery environment. During axon elongation, SCs promote axon elongation while migrating along neovessels (endothelial cells [ECs]). To model this, we used extrusion 3D bioprinting to develop a multi-channel conduit (MCC) using collagen for the matrix region and sacrificial alginate to make the channel. The MCC was fabricated with a structure in which SCs and ECs were longitudinally aligned to mimic the sophisticated recovering SCI conditions. Also, we produced an MCC with different numbers of channels. The aligned SCs and ECs in the 9-channel conduit (9MCC-SE) were more biocompatible and led to more proliferation than the 5-channel conduit (5MCC-SE) in vitro. Also, the 9MCC-SE resulted in a greater healing effect than the 5MCC-SE with respect to neuronal regeneration, remyelination, inflammation, and angiogenesis in vivo. The above tissue recovery results led to motor function repair. Our results show that our 9MCC-SE model represents a new therapeutic strategy for SCI.

Resorbable GBR Scaffolds in Oral and Maxillofacial Tissue Engineering: Design, Fabrication, and Applications

Guided bone regeneration (GBR) is a promising technique in bone tissue engineering that aims to replace lost or injured bone using resorbable scaffolds. The promotion of osteoblast adhesion, migration, and proliferation is greatly aided by GBR materials, and surface changes are critical in imitating the natural bone structure to improve cellular responses. Moreover, the interactions between bioresponsive scaffolds, growth factors (GFs), immune cells, and stromal progenitor cells are essential in promoting bone regeneration. This literature review comprehensively discusses various aspects of resorbable scaffolds in bone tissue engineering, encompassing scaffold design, materials, fabrication techniques, and advanced manufacturing methods, including three-dimensional printing. In addition, this review explores surface modifications to replicate native bone structures and their impact on cellular responses. Moreover, the mechanisms of bone regeneration are described, providing information on how immune cells, GFs, and bioresponsive scaffolds orchestrate tissue healing. Practical applications in clinical settings are presented to underscore the importance of these principles in promoting tissue integration, healing, and regeneration. Furthermore, this literature review delves into emerging areas of metamaterials and artificial intelligence applications in tissue engineering and regenerative medicine. These interdisciplinary approaches hold immense promise for furthering bone tissue engineering and improving therapeutic outcomes, leading to enhanced patient well-being. The potential of combining material science, advanced manufacturing, and cellular biology is showcased as a pathway to advance bone tissue engineering, addressing a variety of clinical needs and challenges. By providing this comprehensive narrative, a detailed, up-to-date account of resorbable scaffolds' role in bone tissue engineering and their transformative potential is offered.

Natural bone inspired core-shell triple-layered gel/PCL/gel 3D printed scaffolds for bone tissue engineering

Despite technological advancements in bone tissue engineering, it is still a challenge to fabricate a scaffold with high bioactivity as well as high mechanical strength that can promote osteogenesis as well as bear load. Here we developed a 3D printed gel-polymer multi-layered hybrid scaffold. The innermost layer is porous gel-based framework made of gelatin/carboxymethyl-chitin/nano-hydroxyapatite and is cryogenically 3D printed. Further, the second and middle layer of micro-engineered polycaprolactone (PCL) is infused in the gel with controlled penetration and tuneable coating thickness. The PCL surface is further coated with a third and final thin layer of gel matrix used for the first layer. This triple-layered structure demonstrates compression strength and modulus of 13.07 ± 1.15 MPa and 21.8 ± 0.82 MPa, respectively, post 8 weeks degradation which is >3000% and >700% than gel scaffold. It also shows degradation of 6.84 ± 0.70% (83% reduction than gel scaffold) after 12 weeks and swelling of 69.09 ± 6.83% (81% reduction) as compared to gel scaffolds. Further, nearly 300%, 250%, 50%, and 440% increase in cellular attachment, proliferation, protein generation, and mineralization, respectively are achieved as compared to only PCL scaffolds. Thus, these hybrid scaffolds offer high mechanical strength, slow degradation rate, high bioactivity, and high osteoconductivity. These multifunctional scaffolds have potential for reconstructing non-load-bearing bone defects like sinus lift, jaw cysts, and moderate load-bearing like reconstructing hard palate, orbital palate, and other craniomaxillofacial bone defects.

3D-printed LEGO®-inspired titanium scaffolds for patient-specific regenerative medicine

Despite the recent advances in 3D-printing, it is often difficult to fabricate implants that optimally fit a defect size or shape. There are some approaches to resolve this issue, such as patient-specific implant/scaffold designs based on CT images of the patients, however, this process is labor-intensive and costly. Especially in developing countries, affordable treatment options are required, while still not excluding these patient groups from potential material and manufacturing advances. Here, a selective laser melting (SLM) 3D-printing strategy was used to fabricate a hierarchical, LEGO®-inspired Assemblable Titanium Scaffold (ATS) system, which can be manually assembled in any shape or size with ease. A surgeon can quickly create a scaffold that would fit to the defect right before the implantation during the surgery. Additionally, the direct inclusion of micro- and macroporous structures via 3D-printing, as well as a double acid-etched surface treatment (ST) in the ATS, ensure biocompatibility, sufficient nutrient flow, cell migration and enhanced osteogenesis. Three different structures were designed (non-porous:NP, semi-porous:SP, ultra-porous:UP), 3D-printed with the SLM technique and then surface treated for the ST groups. After analyzing characteristics of the ATS such as printing quality, surface roughness and interconnected porosity, mechanical testing and finite element analysis (FEA) demonstrated that individual and stacked ATS have sufficient mechanical properties to withstand loading in a physiological system. All ATS showed high cell viability, and the SP and UP groups demonstrated enhanced cell proliferation rates compared to the NP group. Furthermore, we also verified that cells were well-attached and spread on the porous structures and successful cell migration between the ATS units was seen in the case of assemblies. The UP and SP groups exhibited higher calcium deposition and RT-qPCR proved higher osteogenic gene expression compared to NP group. Finally, we demonstrate a number of possible medical applications that reveal the potential of the ATS through assembly.

Tailoring the multiscale mechanics of tunable decellularized extracellular matrix (dECM) for wound healing through immunomodulation

With the discovery of the pivotal role of macrophages in tissue regeneration through shaping the tissue immune microenvironment, various immunomodulatory strategies have been proposed to modify traditional biomaterials. Decellularized extracellular matrix (dECM) has been extensively used in the clinical treatment of tissue injury due to its favorable biocompatibility and similarity to the native tissue environment. However, most reported decellularization protocols may cause damage to the native structure of dECM, which undermines its inherent advantages and potential clinical applications. Here, we introduce a mechanically tunable dECM prepared by optimizing the freeze-thaw cycles. We demonstrated that the alteration in micromechanical properties of dECM resulting from the cyclic freeze-thaw process contributes to distinct macrophage-mediated host immune responses to the materials, which are recently recognized to play a pivotal role in determining the outcome of tissue regeneration. Our sequencing data further revealed that the immunomodulatory effect of dECM was induced via the mechnotrasduction pathways in macrophages. Next, we tested the dECM in a rat skin injury model and found an enhanced micromechanical property of dECM achieved with three freeze-thaw cycles significantly promoted the M2 polarization of macrophages, leading to superior wound healing. These findings suggest that the immunomodulatory property of dECM can be efficiently manipulated by tailoring its inherent micromechanical properties during the decellularization process. Therefore, our mechanics-immunomodulation-based strategy provides new insights into the development of advanced biomaterials for wound healing.

Cell-Tissue Interaction: The Biomimetic Approach to Design Tissue Engineered Biomaterials

The advancement achieved in Tissue Engineering is based on a careful and in-depth study of cell-tissue interactions. The choice of a specific biomaterial in Tissue Engineering is fundamental, as it represents an interface for adherent cells in the creation of a microenvironment suitable for cell growth and differentiation. The knowledge of the biochemical and biophysical properties of the extracellular matrix is a useful tool for the optimization of polymeric scaffolds. This review aims to analyse the chemical, physical, and biological parameters on which are possible to act in Tissue Engineering for the optimization of polymeric scaffolds and the most recent progress presented in this field, including the novelty in the modification of the scaffolds' bulk and surface from a chemical and physical point of view to improve cell-biomaterial interaction. Moreover, we underline how understanding the impact of scaffolds on cell fate is of paramount importance for the successful advancement of Tissue Engineering. Finally, we conclude by reporting the future perspectives in this field in continuous development.

Hollow channels scaffold in bone regenerative: a review

Bone substitute materials have been extensively used for bone regeneration over the past 50 years. The development of novel materials, fabrication technologies and the incorporation and release of regenerative cytokines, growth factors, cells and antimicrobials has been driven by the rapid development in the field of additive manufacturing technology. There are still however, significant challenges that need addressing, including ways to better mediate the rapid vascularization of bone scaffolds to enhance subsequent regeneration and osteogenesis. Increasing construct porosity can accelerate the development of blood vessels in the scaffold, but doing so also weakens the constructs mechanical properties. A novel design for promoting rapid vascularization is to fabricate custom-made hollow channels as bone scaffolds. Summarized here are the current developments in hollow channels scaffold, including their biological attributes, physio-chemical properties, and effects on regeneration. An overview of recent developments in scaffold fabrication as they relate to hollow channel constructs and their structural features will be introduced with an emphasis on attributes that enhance new bone and vessel formation. Furthermore, the potential to enhance angiogenesis and osteogenesis by replicating the structure of real bone will be highlighted.

Closed-loop vasculature network design for bioprinting large, solid tissue scaffolds

Vascularization is an indispensable requirement for fabricating large solid tissues and organs. The natural vasculature derived from medical imaging modalities for large tissues and organs are highly complex and convoluted. However, the present bioprinting capabilities limit the fabrication of such complex natural vascular networks. Simplified bioprinted vascular networks, on the other hand, lack the capability to sustain large solid tissues. This work proposes a generalized and adaptable numerical model to design the vasculature by utilizing the tissue/organ anatomy. Starting with processing the patient's medical images, organ structure, tissue-specific cues, and key vasculature tethers are determined. An open-source abdomen magnetic resonance image dataset was used in this work. The extracted properties and cues are then used in a mathematical model for guiding the vascular network formation comprising arterial and venous networks. Next, the generated three-dimensional networks are used to simulate the nutrient transport and consumption within the organ over time and the regions deprived of the nutrients are identified. These regions provide cues to evolve and optimize the vasculature in an iterative manner to ensure the availability of the nutrient transport throughout the bioprinted scaffolds. The mass transport of six components of cell culture media-glucose, glycine, glutamine, riboflavin, human serum albumin, and oxygen was studied within the organ with designed vasculature. As the vascular structure underwent iterations, the organ regions deprived of these key components decreased significantly highlighting the increase in structural complexity and efficacy of the designed vasculature. The numerical method presented in this work offers a valuable tool for designing vascular scaffolds to guide the cell growth and maturation of the bioprinted tissues for faster regeneration post bioprinting.

Chitosan scaffolds with mesoporous hydroxyapatite and mesoporous bioactive glass

Bone regeneration is one of the most well-known fields in tissue regeneration. The major focus concerns polymeric/ceramic composite scaffolds. In this work, several composite scaffolds based on chitosan (CH), with low and high molecular weights, and different concentrations of ceramics like mesoporous bioactive glass (MBG), mesoporous hydroxyapatite (MHAp) and both MBG and MHAp (MC) were produced by lyophilization. The purpose is to identify the best combination regarding optimal morphology and properties. The tests of the scaffolds present a highly porous structure with interconnected pores. The compression modulus increases with ceramic concentration in the scaffolds. Furthermore, the 75%MBG (835 ± 160 kPa) and 50%MC (1070 ± 205 kPa) samples are the ones that mostly enhance increases in mechanical properties. The swelling capacity increases with MBG and MC, respectively, to 700% and 900% and decreases to 400% when MHAp concentration increases. All scaffolds are non-cytotoxic at 12.5 mg/mL. The CHL scaffolds improve cell adhesion and proliferation compared to CHH, and the MC scaffold samples, show better results than those produced with just MBG or MHAp. The composite scaffolds of chitosan with MBG and MHAp, have revealed to be the best combination due to their enhanced performance in bone tissue engineering.

Spatial-Controlled Coating of Pro-Angiogenic Proteins on 3D Porous Hydrogels Guides Endothelial Cell Behavior

In tissue engineering, the composition and the structural arrangement of molecular components within the extracellular matrix (ECM) determine the physical and biochemical features of a scaffold, which consequently modulate cell behavior and function. The microenvironment of the ECM plays a fundamental role in regulating angiogenesis. Numerous strategies in tissue engineering have attempted to control the spatial cues mimicking in vivo angiogenesis by using simplified systems. The aim of this study was to develop 3D porous crosslinked hydrogels with different spatial presentation of pro-angiogenic molecules to guide endothelial cell (EC) behavior. Hydrogels with pores and preformed microchannels were made with pharmaceutical-grade pullulan and dextran and functionalized with novel pro-angiogenic protein polymers (Caf1-YIGSR and Caf1-VEGF). Hydrogel functionalization was achieved by electrostatic interactions via incorporation of diethylaminoethyl (DEAE)-dextran. Spatial-controlled coating of hydrogels was realized through a combination of freeze-drying and physical absorption with Caf1 molecules. Cells in functionalized scaffolds survived, adhered, and proliferated over seven days. When incorporated alone, Caf1-YIGSR mainly induced cell adhesion and proliferation, whereas Caf1-VEGF promoted cell migration and sprouting. Most importantly, directed cell migration required the presence of both proteins in the microchannel and in the pores, highlighting the need for an adhesive substrate provided by Caf1-YIGSR for Caf1-VEGF to be effective. This study demonstrates the ability to guide EC behavior through spatial control of pro-angiogenic cues for the study of pro-angiogenic signals in 3D and to develop pro-angiogenic implantable materials.

Acellular Tissue-Engineered Vascular Grafts from Polymers: Methods, Achievements, Characterization, and Challenges

Extensive and permanent damage to the vasculature leading to different pathogenesis calls for developing innovative therapeutics, including drugs, medical devices, and cell therapies. Innovative strategies to engineer bioartificial/biomimetic vessels have been extensively exploited as an effective replacement for vessels that have seriously malfunctioned. However, further studies in polymer chemistry, additive manufacturing, and rapid prototyping are required to generate highly engineered vascular segments that can be effectively integrated into the existing vasculature of patients. One recently developed approach involves designing and fabricating acellular vessel equivalents from novel polymeric materials. This review aims to assess the design criteria, engineering factors, and innovative approaches for the fabrication and characterization of biomimetic macro- and micro-scale vessels. At the same time, the engineering correlation between the physical properties of the polymer and biological functionalities of multiscale acellular vascular segments are thoroughly elucidated. Moreover, several emerging characterization techniques for probing the mechanical properties of tissue-engineered vascular grafts are revealed. Finally, significant challenges to the clinical transformation of the highly promising engineered vessels derived from polymers are identified, and unique perspectives on future research directions are presented.

Cell-Seeded Biomaterial Scaffolds: The Urgent Need for Unanswered Accelerated Angiogenesis

One of the most arduous challenges in tissue engineering is neovascularization, without which there is a lack of nutrients delivered to a target tissue. Angiogenesis should be completed at an optimal density and within an appropriate period of time to prevent cell necrosis. Failure to meet this challenge brings about poor functionality for the tissue in comparison with the native tissue, extensively reducing cell viability. Prior studies devoted to angiogenesis have provided researchers with some biomaterial scaffolds and cell choices for angiogenesis. For example, while most current angiogenesis approaches require a variety of stimulatory factors ranging from biomechanical to biomolecular to cellular, some other promising stimulatory factors have been underdeveloped (such as electrical, topographical, and magnetic). When it comes to choosing biomaterial scaffolds in tissue engineering for angiogenesis, key traits rush to mind including biocompatibility, appropriate physical and mechanical properties (adhesion strength, shear stress, and malleability), as well as identifying the appropriate biomaterial in terms of stability and degradation profile, all of which may leave essential trace materials behind adversely influencing angiogenesis. Nevertheless, the selection of the best biomaterial and cells still remains an area of hot dispute as such previous studies have not sufficiently classified, integrated, or compared approaches. To address the aforementioned need, this review article summarizes a variety of natural and synthetic scaffolds including hydrogels that support angiogenesis. Furthermore, we review a variety of cell sources utilized for cell seeding and influential factors used for angiogenesis with a concentrated focus on biomechanical factors, with unique stimulatory factors. Lastly, we provide a bottom-to-up overview of angiogenic biomaterials and cell selection, highlighting parameters that need to be addressed in future studies.

Effects of Pore Interconnectivity on Bone Regeneration in Carbonate Apatite Blocks

Porous architecture in bone substitutes, notably the interconnectivity of pores, is a critical factor for bone ingrowth. However, controlling the pore interconnectivity while maintaining the microarchitecture has not yet been achieved using conventional methods, such as sintering. Herein, we fabricated a porous block using the crystal growth of calcium sulfate dihydrate, and controlled the pore interconnectivity by limiting the region of crystal growth. The calcium sulfate dihydrate blocks were transformed to bone apatite, carbonate apatite (CO₃Ap) through dissolution–precipitation reactions. Thus, CO₃Ap blocks with 15% and 30% interconnected pore volumes were obtained while maintaining the microarchitecture: they were designated as CO₃Ap-15 and CO₃Ap-30, respectively. At 4 weeks after implantation in a rabbit femur defect, new bone formed throughout CO₃Ap-30, whereas little bone was formed in the center region of CO₃Ap-15. At 12 weeks after implantation, a large portion of CO₃Ap-30 was replaced with new bone and the boundary with the host bone became blurred. In contrast, CO₃Ap-15 remained in the defect and the boundary with the host bone was still clear. Thus, the interconnected pores promote bone ingrowth, followed by replacement of the material with new bone. These findings provide a useful guide for designing bone substitutes for rapid bone regeneration.

Vascularization in tissue engineering: The architecture cues of pores in scaffolds

Vascularization is a key event and also still a challenge in tissue engineering. Many efforts have been devoted to the development of vascularization based on cells, growth factors, and porous scaffolds in the past decades. Among these efforts, the architecture features of pores in scaffolds played important roles for vascularization, which have attracted increasing attention. It has been known that the open macro pores in scaffolds could facilitate cell migration, nutrient, and oxygen diffusion, which then could promote new tissue formation and vascularization. The pore parameters are the important factors affecting cells response and vessel formation. Thus, this review will give an overview of the current advances in the effects of pore parameters on vascularization in tissue engineering, mainly including pore size, interconnectivity, pore size distribution, pore shape (channel structure), and the micro/nano-surface topography of pores.

Controlled self-assembly of chemical gardens enables fabrication of heterogeneous chemobrionic materials

Chemical gardens are an example of a chemobrionic system that typically result in abiotic macro-, micro- and nano- material architectures, with formation driven by complex out-of-equilibrium reaction mechanisms. From a technological perspective, controlling chemobrionic processes may hold great promise for the creation of novel, compositionally diverse and ultimately, useful materials and devices. In this work, we engineer an innovative custom-built liquid exchange unit that enables us to control the formation of tubular chemical garden structures grown from the interface between calcium loaded hydrogel and phosphate solution. We show that systematic displacement of phosphate solution with water (H₂O) can halt self-assembly, precisely control tube height and purify structures in situ. Furthermore, we demonstrate the fabrication of a heterogeneous chemobrionic composite material composed of aligned, high-aspect ratio calcium phosphate channels running through an otherwise dense matrix of poly(2-hydroxyethyl methacrylate) (pHEMA). Given that the principles we derive can be broadly applied to potentially control various chemobrionic systems, this work paves the way for fabricating multifunctional materials that may hold great potential in a variety of application areas, such as regenerative medicine, catalysis and microfluidics.

Effects of Macro-/Micro-Channels on Vascularization and Immune Response of Tissue Engineering Scaffolds

Although the use of porous scaffolds in tissue engineering has been relatively successful, there are still many limitations that need to be addressed, such as low vascularization, low oxygen and nutrient levels, and immune-induced inflammation. As a result, the current porous scaffolds are insufficient when treating large defects. This paper analyzed scientific research pertaining to the effects of macro-/micro-channels on the cell recruitment, vascularization, and immune response of tissue engineering scaffolds. Most of the studies contained either cell culturing experimentation or experimentation on small animals such as rats and mice. The sacrificial template method, template casting method, and 3D printing method were the most common methods in the fabrication of channeled scaffolds. Some studies combine the sacrificial and 3D printing methods to design and create their scaffold with channels. The overall results from these studies showed that the incorporation of channels within scaffolds greatly increased vascularization, reduced immune response, and was much more beneficial for cell and growth factor recruitment compared with control groups that contained no channels. More research on the effect of micro-/macro-channels on vascularization or immune response in animal models is necessary in the future in order to achieve clinical translation.

Freeze-Casting with 3D-Printed Templates Creates Anisotropic Microchannels and Patterned Macrochannels within Biomimetic Nanofiber Aerogels for Rapid Cellular Infiltration

A new approach is described for fabricating 3D poly(ε-caprolactone) (PCL)/gelatin (1:1) nanofiber aerogels with patterned macrochannels and anisotropic microchannels by freeze-casting with 3D-printed sacrificial templates. Single layer or multiple layers of macrochannels are formed through an inverse replica of 3D-printed templates. Aligned microchannels formed by partially anisotropic freezing act as interconnected pores between templated macrochannels. The resulting macro-/microchannels within nanofiber aerogels significantly increase preosteoblast infiltration in vitro. The conjugation of vascular endothelial growth factor (VEGF)-mimicking QK peptide to PCL/gelatin/gelatin methacryloyl (1:0.5:0.5) nanofiber aerogels with patterned macrochannels promotes the formation of a microvascular network of seeded human microvascular endothelial cells. Moreover, nanofiber aerogels with patterned macrochannels and anisotropic microchannels show significantly enhanced cellular infiltration rates and host tissue integration compared to aerogels without macrochannels following subcutaneous implantation in rats. Taken together, this novel class of nanofiber aerogels holds great potential in biomedical applications including tissue repair and regeneration, wound healing, and 3D tissue/disease modeling.

Advances in removing mass transport limitations for more physiologically relevant in vitro 3D cell constructs

Spheroids and organoids are promising models for biomedical applications ranging from human disease modeling to drug discovery. A main goal of these 3D cell-based platforms is to recapitulate important physiological parameters of their in vivo organ counterparts. One way to achieve improved biomimetic architectures and functions is to culture cells at higher density and larger total numbers. However, poor nutrient and waste transport lead to low stability, survival, and functionality over extended periods of time, presenting outstanding challenges in this field. Fortunately, important improvements in culture strategies have enhanced the survival and function of cells within engineered microtissues/organs. Here, we first discuss the challenges of growing large spheroids/organoids with a focus on mass transport limitations, then highlight recent tools and methodologies that are available for producing and sustaining functional 3D in vitro models. This information points toward the fact that there is a critical need for the continued development of novel cell culture strategies that address mass transport in a physiologically relevant human setting to generate long-lasting and large-sized spheroids/organoids.

Induction of scaffold angiogenesis by recipient vasculature precision micropuncture

The success of engineered tissues continues to be limited by time to vascularization and perfusion. Here, we studied the effects of precision injury to a recipient macrovasculature in promoting neovessel formation in an adjacently placed scaffold. Segmental 60 μm diameter micropunctures (MP) were created in the recipient rat femoral artery and vein followed by coverage with a simple collagen scaffold. Scaffolds were harvested at 24, 48, 72, and 96 h post-implantation for detailed analysis. Those placed on top of an MP segment showed an earlier and more robust cellular infiltration, including both endothelial cells (CD31) and macrophages (F4/80), compared to internal non-micropunctured control limbs (p < 0.05). At the 96-hour timepoint, MP scaffolds demonstrated an increase in physiologic perfusion (p < 0.003) and a 2.5-fold increase in capillary network formation (p < 0.001). These were attributed to an overall upsurge in small vessel quantity. Furthermore, MP positioned scaffolds demonstrated significant increases in many modulators of angiogenesis, including VEGFR2 and Tie-2 despite a decrease in HIF-1α at all timepoints. This study highlights a novel microsurgical approach that can be used to rapidly vascularize or inosculate contiguously placed scaffolds and grafts. Thereby, offering an easily translatable route towards the creation of thicker and more clinically relevant engineered tissues.

Synthetic ECM: Bioactive Synthetic Hydrogels for 3D Tissue Engineering

The specific microenvironment that cells reside in fundamentally impacts their broader function in tissues and organs. At its core, this microenvironment is composed of precise arrangements of cells that encourage homotypic and heterotypic cell-cell interactions, biochemical signaling through soluble factors like cytokines, hormones, and autocrine, endocrine, or paracrine secretions, and the local extracellular matrix (ECM) that provides physical support and mechanobiological stimuli, and further regulates biochemical signaling through cell-ECM interactions like adhesions and growth factor sequestering. Each cue provided in the microenvironment dictates cellular behavior and, thus, overall potential to perform tissue and organ specific function. It follows that in order to recapitulate physiological cell responses and develop constructs capable of replacing damaged tissue, we must engineer the cellular microenvironment very carefully. Many great strides have been made toward this goal using various three-dimensional (3D) tissue culture scaffolds and specific media conditions. Among the various 3D biomimetic scaffolds, synthetic hydrogels have emerged as a highly tunable and tissue-like biomaterial well-suited for implantable tissue-engineered constructs. Because many synthetic hydrogel materials are inherently bioinert, they minimize unintentional cell responses and thus are good candidates for long-term implantable grafts, patches, and organs. This review will provide an overview of commonly used biomaterials for forming synthetic hydrogels for tissue engineering applications and techniques for modifying them to with bioactive properties to elicit the desired cell responses.

Fluidic embedding of additional macroporosity in alginate-gelatin composite structure for biomimetic application

Biomimetic characteristics of hydrogel scaffold are tuned in this study utilizing the synergy of alginate, gelatin, and microfluidically embedded voids. Superposition of alginate and gelatin polymer networks results in additional rigidity, which can be tuned by introduction of voids, and thereby allowing faster release of pore pressure through movement of aqueous phase through the pore network. More importantly, voids enabled the cells to penetrate from the surface of seeding into the depth of the scaffold and proliferate there, as demonstrated for MDA MB 231 breast cancer cells. The uniform voids, generated by the microfluidic device, self-align creating uniform macroporosity within the gel structure, get readily filled by the media due to hydrophilicity, and extend the characteristics of composite uniformly across the entire scaffold.

Novel Extrusion-Microdrilling Approach to Fabricate Calcium Phosphate-Based Bioceramic Scaffolds Enabling Fast Bone Regeneration

This study proposes a novel approach, termed extrusion-microdrilling, to fabricate three-dimensional (3D) interconnected bioceramic scaffolds with channel-like macropores for bone regeneration. The extrusion-microdrilling method is characterized by ease of use, high efficiency, structural flexibility, and precision. The 3D interconnected β-tricalcium phosphate bioceramic (EM-TCP) scaffolds prepared by this method showed channel-like square macropores (∼650 μm) by extrusion and channel-like round macropores (∼570 μm) by microdrilling as well as copious micropores. By incorporating a strontium-containing phosphate-based glass (SrPG), the obtained calcium phosphate-based bioceramic (EM-TCP/SrPG) scaffolds had noticeably higher compressive strength, lower porosity, and smaller macropore size, tremendously enhanced in vitro proliferation and osteogenic differentiation of mouse bone marrow stromal cells, and suppressed in vitro osteoclastic activities of RAW264.7 cells, as compared with the EM-TCP scaffolds. In vivo assessment results indicated that at postoperative week 6, new vessels and a large percentage of new bone tissues (24-25%) were formed throughout the interconnected macropores of EM-TCP and EM-TCP/SrPG, which were implanted in the femoral defects of rabbits; the bone formation of the EM-TCP group was comparable to that of the EM-TCP/SrPG group. At 12 weeks postimplantation, the bone formation percentage of EM-TCP was slightly reduced, while that of EM-TCP/SrPG with a slower degradation rate was pronouncedly increased. This work provides a new strategy to fabricate interconnected bioceramic scaffolds allowing for fast bone regeneration, and the EM-TCP/SrPG scaffolds are promising for efficiently repairing bone defects.

Chemobrionic structures in tissue engineering: self-assembling calcium phosphate tubes as cellular scaffolds

A diverse range of complex patterns and mineralised hierarchical microstructures can be derived from chemobrionic systems. In this work, we explore chemobrionic calcium phosphate tubes as cellular scaffolds.

Computational fluid dynamic analysis of bioprinted self-supporting perfused tissue models

Natural tissues are incorporated with vasculature, which is further integrated with a cardiovascular system responsible for driving perfusion of nutrient‐rich oxygenated blood through the vasculature to support cell metabolism within most cell‐dense tissues. Since scaffold‐free biofabricated tissues being developed into clinical implants, research models, and pharmaceutical testing platforms should similarly exhibit perfused tissue‐like structures, we generated a generalizable biofabrication method resulting in self‐supporting perfused (SSuPer) tissue constructs incorporated with perfusible microchannels and integrated with the modular FABRICA perfusion bioreactor. As proof of concept, we perfused an MLO‐A5 osteoblast‐based SSuPer tissue in the FABRICA. Although our resulting SSuPer tissue replicated vascularization and perfusion observed in situ, supported its own weight, and stained positively for mineral using Von Kossa staining, our in vitro results indicated that computational fluid dynamics (CFD) should be used to drive future construct design and flow application before further tissue biofabrication and perfusion. We built a CFD model of the SSuPer tissue integrated in the FABRICA and analyzed flow characteristics (net force, pressure distribution, shear stress, and oxygen distribution) through five SSuPer tissue microchannel patterns in two flow directions and at increasing flow rates. Important flow parameters include flow direction, fully developed flow, and tissue microchannel diameters matched and aligned with bioreactor flow channels. We observed that the SSuPer tissue platform is capable of providing direct perfusion to tissue constructs and proper culture conditions (oxygenation, with controllable shear and flow rates), indicating that our approach can be used to biofabricate tissue representing primary tissues and that we can model the system in silico.

Mechanical performance of highly permeable laser melted Ti6Al4V bone scaffolds

Critically engineered stiffness and strength of a scaffold are crucial for managing maladapted stress concentration and reducing stress shielding. At the same time, suitable porosity and permeability are key to facilitate biological activities associated with bone growth and nutrient delivery. A systematic balance of all these parameters are required for the development of an effective bone scaffold. Traditionally, the approach has been to study each of these parameters in isolation without considering their interdependence to achieve specific properties at a certain porosity. The purpose of this study is to undertake a holistic investigation considering the stiffness, strength, permeability, and stress concentration of six scaffold architectures featuring a 68.46-90.98% porosity. With an initial target of a tibial host segment, the permeability was characterised using Computational Fluid Dynamics (CFD) in conjunction with Darcy's law. Following this, Ashby's criterion, experimental tests, and Finite Element Method (FEM) were employed to study the mechanical behaviour and their interdependencies under uniaxial compression. The FE model was validated and further extended to study the influence of stress concentration on both the stiffness and strength of the scaffolds. The results showed that the pore shape can influence permeability, stiffness, strength, and the stress concentration factor of Ti6Al4V bone scaffolds. Furthermore, the numerical results demonstrate the effect to which structural performance of highly porous scaffolds deviate, as a result of the Selective Laser Melting (SLM) process. In addition, the study demonstrates that stiffness and strength of bone scaffold at a targeted porosity is linked to the pore shape and the associated stress concentration allowing to exploit the design freedom associated with SLM.

Embossed Membranes with Vascular Patterns Guide Vascularization in a 3D Tissue Model

The vascularization of three-dimensional (3D) tissue constructs is necessary for transporting nutrients and oxygen to the component cells. In this study, a vacuum forming method was applied to emboss a vascular pattern on an electrospun membrane so that guided vascular structures could develop within the construct. Two- or six-layer constructs of electrospun membranes seeded with endothelial cells and pericytes were stacked and subcutaneously implanted into mice. Blood vessel formation in the implanted constructs with six alternating layers of flat membranes and membranes embossed with a blood vessel pattern was observed after two weeks of implantation. The formation of blood vessels was observed along the embossed blood vessel pattern in the structure of the embossed membrane laminated at four weeks and eight weeks. Vascular endothelial growth factor (VEGF) and angiopoietin 1 (Ang-1) were highly expressed in the vascularized structures. Therefore, we demonstrated that a structure capable of producing a desired blood vessel shape with electrospun membranes embossed with a blood vessel pattern can be manufactured, and that a variety of structures can be manufactured using electrospun membranes in the tissue engineering era.

Hierarchical microchannel architecture in chitosan/bioactive glass scaffolds via electrophoretic deposition positive-replica

One of the main challenges in the design of scaffolds for cortical bone regeneration is mimicking the highly oriented, hierarchical structure of the native tissue in an efficient, simple, and consistent way. As a possible solution to this challenge, positive replica based on electrophoretic deposition (EPD) was here evaluated as a technique to produce organic/inorganic scaffolds with oriented micro-porosities mimicking Haversian canals diameter and spacing. Two different sizes of 45S5 bioactive glass (BG) powders were chosen as inclusions and loaded in a chitosan matrix via EPD on micro-patterned cathodes. Self-standing chitosan scaffolds, with a homogeneous dispersion of BG particles and regularly-oriented micro-channels (ϕ = 380 ± 50 μm, inter-channel spacing = 600 ± 40 μm), were obtained. In vitro analysis in simulated body fluid (SBF) revealed the ability to induce a deposition of a homogenous layer of hydroxyapatite (HA), with Ca/P nucleation reactions appearing kinetically favored by smaller BG particles. Cell interaction with hybrid scaffolds was evaluated in vitro with bone osteosarcoma cells (SAOS-2). The osteoconductive potential of EPD structures was assessed by evaluating cells proliferation, viability and scaffold colonization. Results indicate that EPD is a simple yet extremely effective technique to prepare composite micro-patterned structures and can represent a platform for the development of a new generation of bone scaffolds. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2019.