Pore diameter of more than 100 microm is not requisite for bone ingrowth in rabbits

The Effect of Ti-6Al-4V Alloy Surface Structure on the Adhesion and Morphology of Unidirectional Freeze-Coated Gelatin

The modification of a metal implant surface with a biomimetic coating of bone-like anisotropic and graded porosity structures to improve its biological anchorage with the surrounding bone tissue at implanting, is still a high challenge. In this paper, we present an innovative way of a gelatin (GEL) dip-coating on Ti-6Al-4V substrates of different square-net surface textures by the unidirectional deep-freezing process at simultaneous cross-linking using carbodiimide chemistry. Different concentrations of GEL solution were used to study the changes in morphology, density, and mechanical properties of the coatings. In addition, the surface free energy and polarity of Ti-6Al-4V substrate surfaces and GEL solutions have been evaluated to assess the wetting properties at the substrate interfaces, and to describe the adhesion of GEL macromolecules with their surfaces, being supported by mechanical pull-out testing. The results indicate that the coating’s morphology depends primarily on the Ti-6Al-4V substrate’s surface texture and second, on the concentration of GEL, which further influences their adhesion properties, orientation, morphological arrangement, as well as compression strength. The substrate with a 300 × 300 μm2 texture resulted in a highly adhered GEL coating with ≥80% porosity, interconnected and well-aligned pores of 75–200 μm, required to stimulate the bone ingrowth, mechanically and histologically.

Comparison of two pore sizes of LAE442 scaffolds and their effect on degradation and osseointegration behavior in the rabbit model

The magnesium alloy LAE442 emerged as a possible bioresorbable bone substitute over a decade ago. In the present study, using the investment casting process, scaffolds of the Magnesium (Mg) alloy LAE442 with two different and defined pore sizes, which had on average a diameter of 400 μm (p400) and 500 μm (p500), were investigated to evaluate degradation and osseointegration in comparison to a ß-TCP control group. Open-pored scaffolds were implanted in both greater trochanter of rabbits. Ten scaffolds per time group (6, 12, 24, and 36 weeks) and type were analyzed by clinical, radiographic and μ-CT examinations (2D and 3D). None of the scaffolds caused adverse reactions. LAE442 p400 and p500 developed moderate gas accumulation due to the Mg associated in vivo corrosion, which decreased from week 20 for both pore sizes. After 36 weeks, p400 and p500 showed volume decreases of 15.9 and 11.1%, respectively, with homogeneous degradation, whereas ß-TCP lost 74.6% of its initial volume. Compared to p400, osseointegration for p500 was significantly better at week 2 postsurgery due to more frequent bone-scaffold contacts, higher number of trabeculae and higher bone volume in the surrounding area. No further significant differences between the two pore sizes became apparent. However, p500 was close to the values of ß-TCP in terms of bone volume and trabecular number in the scaffold environment, suggesting better osseointegration for the larger pore size.

Degradation behavior, cytotoxicity, hemolysis, and antibacterial properties of electro-deposited Zn-Cu metal foams as potential biodegradable bone implants

Zinc (Zn) alloys have attracted much attention for biomedical applications due to their biodegradability, biocompatibility, and biological functionalities. Zn alloy foams have high potential to be used as regenerative medical implants by virtue of their porous structure, which allows new bone tissue ingrowth, their low elastic modulus approximating that of natural bone, and their biodegradation, which eliminates the need for follow-up surgery to remove the implants after bone tissue healing. In this context, a biodegradable Zn-Cu foam was fabricated by electrochemical deposition on a foamed Cu template and given a subsequent diffusion heat treatment. The microstructure, mechanical properties, degradation behavior, toxicity, hemolysis percentages, and antibacterial effects of the Zn-Cu foams were assessed for biomedical applications. The Zn-Cu foams exhibited a yield strength of ~12.1 MPa, a plateau strength of 16.8 MPa, and a strain over 50% under compression tests. The corrosion rate of the Zn-Cu foams measured by electrochemical polarization testing was 0.18 mm/y. The Zn-Cu foams showed good blood compatibility with a hemolysis percentage of less than 5%. Cytotoxicity assessment indicated that a 100% concentration of the Zn-Cu foam extract showed clear cytotoxicity against MC3T3-E1 osteoblast cells, but a 12.5% concentration of the extract showed > 90% cell viability. Moreover, the Zn-Cu foams showed good antibacterial effects. STATEMENT OF SIGNIFICANCE: This work reportsa biodegradable Zn-Cu foam with high mechanical strength and ductility, suitable degradation rate, good antibacterial capacity, and good hemolysis property and biocompatibility. The Zn-Cu foam exhibited a yield strength of ~12.1 MPa, a plateau strength of 16.8 MPa, and a strain over 50% under compression tests. The corrosion rate of the Zn-Cu foam measured by electrochemical polarization testing was 0.18 mm/y in Hanks' Solutions. The Zn-Cu foam showed good blood compatibility with a hemolysis percentage of less than 5%. Cytotoxicity assessment indicated that a 12.5% concentration of the foam extract showed > 90% cell viability. Moreover, the Zn-Cu foam showed good antibacterial effects against S. aureus.

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.

Evaluation of Peri-Implant Bone Grafting Around Surface-Porous Dental Implants: An In Vivo Study in a Goat Model

Dental implants with surface-porous designs have been recently developed. Clinically, peri-implant bone grafting is expected to promote early osseointegration and bone ingrowth when applied with surface-porous dental implants in challenging conditions. The aim of this study was to comparatively analyze peri-implant bone healing around solid implants and surface-porous implants with and without peri-implant bone grafting, using biomechanical and histomorphometrical assessment in a goat iliac bone model. A total of 36 implants (4.1 mm wide, 11.5 mm long) divided into three groups, solid titanium implant (STI; n = 12), porous titanium implants (PTI; n = 12) and PTI with peri-implant bone grafting using biphasic calcium phosphate granules (PTI + BCP; n = 12), were placed bilaterally in the iliac crests of six goats. The goats were sacrificed seven weeks post-operatively and then subjected to biomechanical (n = 6 per group) and histomorphometrical (n = 6 per group) assessment. The biomechanical assessment revealed no significant differences between the three types of implants. Although the peri-implant bone-area (PIBA%) measured by histomorphometry (STI: 8.63 ± 3.93%, PTI: 9.89 ± 3.69%, PTI + BCP: 9.28 ± 2.61%) was similar for the three experimental groups, the percentage of new bone growth area (BGA%) inside the porous implant portion was significantly higher (p < 0.05) in the PTI group (10.67 ± 4.61%) compared to the PTI + BCP group (6.50 ± 6.53%). These data demonstrate that peri-implant bone grafting around surface-porous dental implants does not significantly accelerate early osseointegration and bone ingrowth.

Honeycomb blocks composed of carbonate apatite, β-tricalcium phosphate, and hydroxyapatite for bone regeneration: effects of composition on biological responses

Synthetic scaffolds exhibiting bone repair ability equal to that of autogenous bone are required in the fields of orthopedics and dentistry. A suitable synthetic bone graft substitute should induce osteogenic differentiation of mesenchymal stem cells, osteogenesis, and angiogenesis. In this study, three types of honeycomb blocks (HCBs), composed of hydroxyapatite (HAp), β-tricalcium phosphate (TCP), and carbonate apatite (CO3Ap), were fabricated, and the effects of HCB composition on bone formation and maturation were investigated. The HC structure was selected to promote cell penetration and tissue ingrowth. HAp and β-TCP HCBs were fabricated by extrusion molding followed by sintering. The CO3Ap HCBs were fabricated by extrusion molding followed by sintering and dissolution-precipitation reactions. These HCBs had similar macroporous structures: all harbored uniformly distributed macropores (∼160 ​μm) that were regularly arrayed and penetrated the blocks unidirectionally. Moreover, the volumes of macropores were nearly equal (∼0.15 ​cm3/g). The compressive strengths of CO3Ap, HAp, and β-TCP HCBs were 22.8 ​± ​3.5, 34.2 ​± ​3.3, and 24.4 ​± ​2.4 ​MPa, respectively. Owing to the honeycomb-type macroporous structure, the compressive strengths of these HCBs were higher than those of commercial scaffolds with intricate three-dimensional or unidirectional macroporous structure. Notably, bone maturation was markedly faster in CO3Ap HCB grafting than in β-TCP and HAp HCB grafting, and the mature bone area percentages for CO3Ap HCBs at postsurgery weeks 4 and 12 were 14.3- and 4.3-fold higher and 7.5- and 1.4-fold higher than those for HAp and β-TCP HCBs, respectively. The differences in bone maturation and formation were probably caused by the disparity in concentrations of calcium ions surrounding the HCBs, which were dictated by the inherent material resorption behavior and mechanism; generally, CO3Ap is resorbed only by osteoclastic resorption, HAp is not resorbed, and β-TCP is rapidly dissolved even in the absence of osteoclasts. Besides the composition, the microporous structure of HC struts, inevitably generated during the formation of HCBs of various compositions, may contribute to the differences in bone maturation and formation.

The Design and In Vivo Testing of a Locally Stiffness-Matched Porous Scaffold

An increasing volume of work supports utilising the mechanobiology of bone for bone ingrowth into a porous scaffold. However, typically during in vivo testing of implants, the mechanical properties of the bone being replaced are not quantified. Consequently there remains inconsistencies in the literature regarding 'optimum' pore size and porosity for bone ingrowth. It is also difficult to compare ingrowth results between studies and to translate in vivo animal testing to human subjects without understanding the mechanical environment. This study presents a clinically applicable approach to determining local bone mechanical properties and design of a scaffold with similar properties. The performance of the scaffold was investigated in vivo in an ovine model. The density, modulus and strength of trabecular bone from the medial femoral condyle from ovine bones was characterised and power-law relationships were established. A porous titanium scaffold, intended to maintain bone mechanical homeostasis, was additively manufactured and implanted into the medial femoral condyle of 6 ewes. The stiffness of the scaffold varied throughout the heterogeneous structure and matched the stiffness variation of bone at the surgical site. Bone ingrowth into the scaffold was 10.73±2.97% after 6 weeks. Fine woven bone, in the interior of the scaffold, and intense formations of more developed woven bone overlaid with lamellar bone at the implant periphery were observed. The workflow presented will allow future in vivo testing to test specific bone strains on bone ingrowth in response to a scaffold and allow for better translation from in vivo testing to commercial implants.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

Porous Titanium for Biomedical Applications: Evaluation of the Conventional Powder Metallurgy Frontier and Space-Holder Technique

Titanium and its alloys are reference materials in biomedical applications because of their desirable properties. However, one of the most important concerns in long-term prostheses is bone resorption as a result of the stress-shielding phenomena. Development of porous titanium for implants with a low Young’s modulus has accomplished increasing scientific and technological attention. The aim of this study is to evaluate the viability, industrial implementation and potential technology transfer of different powder-metallurgy techniques to obtain porous titanium with stiffness values similar to that exhibited by cortical bone. Porous samples of commercial pure titanium grade-4 were obtained by following both conventional powder metallurgy (PM) and space-holder technique. The conventional PM frontier (Loose-Sintering) was evaluated. Additionally, the technical feasibility of two different space holders (NH4HCO3 and NaCl) was investigated. The microstructural and mechanical properties were assessed. Furthermore, the mechanical properties of titanium porous structures with porosities of 40% were studied by Finite Element Method (FEM) and compared with the experimental results. Some important findings are: (i) the optimal parameters for processing routes used to obtain low Young’s modulus values, retaining suitable mechanical strength; (ii) better mechanical response was obtained by using NH4HCO3 as space holder; and (iii) Ti matrix hardening when the interconnected porosity was 36–45% of total porosity. Finally, the advantages and limitations of the PM techniques employed, towards an industrial implementation, were discussed.

In Vitro Bioactivity and Cell Biocompatibility of a Hypereutectic Bioceramic

Two dense biphasic ceramics, with a hypereutectic composition of 30 wt % CaSiO3–70 wt % Ca3(PO4)2, were synthesized by a solid-state reaction of homogeneous pressed combinations of previously synthesized synthetic CaSiO3 and Ca3(PO4)2 powders. The objective was to produce a dense structure to generate large enough in situ pores for the ceramic to be used in tissue engineering. To develop such a structure, two grain sizes of CaSiO3 were used (63–100 µm and 100–150 μm) and some of their properties were studied in vitro, as they are relevant for tissue engineering. X-ray diffraction analysis, μ-Raman spectroscopy, diametrical compression test, and scanning electron microscopy with elemental mapping showed a coarse-grained homogeneous microstructure for the materials, which consisted of wollastonite (α-CaSiO3) and tricalcium phosphate (α-Ca3(PO4)2), with adequate mechanical properties for implantation. In vitro bioactivity was evaluated in simulated body fluid (SBF) by exploring a hydroxyapatite (HA)-like formation. The results showed that tricalcium phosphate grains dissolved more preferentially than those of wollastonite, but not fast enough to leave a pore before the surface was coated with an HA-like layer after soaking only for three days. Biocompatibility was evaluated by in vitro cell experiments, which showed cell proliferation, adhesion, and spreading on the ceramic surface. This ceramic is expected to be used as a bone graft substitute.

Mesenchymal stem cell-loaded porous tantalum integrated with biomimetic 3D collagen-based scaffold to repair large osteochondral defects in goats

Background

The body is unable to repair and regenerate large area bone defects. Moreover, the repair capacity of articular cartilage is very limited. There has long been a lack of effective treatments for osteochondral lesions. The engineered tissue with biphase synthetic for osteochondral repair has become one of the hot research fields over the past few years. In this study, an integrated biomanufacturing platform was constructed with bone marrow mesenchymal stem cells (BMSCs)/porous tantalum (pTa) associated with chondrocytes/collagen membranes (CM) to repair large osteochondral defects in load-bearing areas of goats.

Methods

Twenty-four goats with a large osteochondral defect in the femoral heads of the left hind legs were randomly divided into three groups: eight were treated with chondrocytes/CM-BMSCs/pTa, eight were treated with pure CM-pTa composite, and the other eight goats were untreated. The repair effect was assessed by X-ray, gross observation, and histomorphology for 16 weeks after the operation. In addition, the biocompatibility of chondrocytes/CM-BMSCs/pTa was observed by flow cytometry, CCK8, immunocytochemistry, and Q-PCR. The characteristics of the chondrocytes/CM-BMSCs/pTa were evaluated using both scanning electron microscopy and mechanical testing machine.

Results

The integrated repair material consists of pTa, injectable fibrin sealant, and CM promoted adhesion and growth of BMSCs and chondrocytes. pTa played an important role in promoting the differentiation of BMSCs into osteoblasts. Three-dimensional CM maintained the phenotype of chondrocytes successfully and expressed chondrogenic genes highly. The in vivo study showed that after 16 weeks from implantation, osteochondral defects in almost half of the femoral heads had been successfully repaired by BMSC-loaded pTa associated with biomimetic 3D collagen-based scaffold.

Conclusions

The chondrocytes/CM-BMSCs/pTa demonstrated significant therapeutic efficacy in goat models of large osteochondral defect. This provides a novel therapeutic strategy for large osteochondral lesions in load-bearing areas caused by severe injury, necrosis, infection, degeneration, and tumor resection with a high profile of safety, effectiveness, and simplicity.

Non-Auxetic Mechanical Metamaterials

The concept of "mechanical metamaterials" has become increasingly popular, since their macro-scale characteristics can be designed to exhibit unusual combinations of mechanical properties on the micro-scale. The advances in additive manufacturing (AM, three-dimensional printing) techniques have boosted the fabrication of these mechanical metamaterials by facilitating a precise control over their micro-architecture. Although mechanical metamaterials with negative Poisson's ratios (i.e., auxetic metamaterials) have received much attention before and have been reviewed multiple times, no comparable review exists for architected materials with positive Poisson's ratios. Therefore, this review will focus on the topology-property relationships of non-auxetic mechanical metamaterials in general and five topological designs in particular. These include the designs based on the diamond, cube, truncated cube, rhombic dodecahedron, and the truncated cuboctahedron unit cells. We reviewed the mechanical properties and fatigue behavior of these architected materials, while considering the effects of other factors such as those of the AM process. In addition, we systematically analyzed the experimental, computational, and analytical data and solutions available in the literature for the titanium alloy Ti-6Al-4V. Compression dominated lattices, such as the (truncated) cube, showed the highest mechanical properties. All of the proposed unit cells showed a normalized fatigue strength below that of solid titanium (i.e., 40% of the yield stress), in the range of 12⁻36% of their yield stress. The unit cells discussed in this review could potentially be applied in bone-mimicking porous structures.

Cancellous-Bone-like Porous Iron Scaffold Coated with Strontium Incorporated Octacalcium Phosphate Nanowhiskers for Bone Regeneration

The repair of large bone defects poses a grand challenge in tissue engineering. Thus, developing biocompatible scaffolds with mechanical and structural similarity to human cancellous bone is in great demand. Herein, we fabricated a three-dimensional (3D) porous iron (Fe) scaffold with interconnected pores via a template-assisted electrodeposition method. The porous Fe scaffold with a skeleton diameter of 143 μm had the porosity >90%, an average pore size of 345 μm, and a yield strength of 3.5 MPa. Such structure and mechanical strength were close to those of cancellous bone. In order to enhance the biocompatibility of the scaffold, strontium incorporated octacalcium phosphate (Sr-OCP) was coated on the skeletons of the porous Fe scaffold. The coated Sr-OCP was in the form of nanowhiskers with a mean diameter of 300 nm and length of 30 μm. Such Sr-OCP coating could effectively reduce the release rate of the Fe ions to a level which was safe for the human body. Both in vitro cytotoxicity tests by extraction method and direct contact assay confirmed that the Sr-OCP coating could promote the cell adhesion and substantially enhance the biocompatibility of the porous Fe scaffolds. Thus, the cancellous-bone-like porous structure with compatible mechanical properties and excellent biocompatibility enables the present Sr-OCP coated porous Fe scaffold to be a promising candidate for bone repair and regeneration.

Porous Titanium Scaffolds Fabricated by Metal Injection Moulding for Biomedical Applications

Biocompatible titanium scaffolds with up to 40% interconnected porosity were manufactured through the metal injection moulding process and the space holder technique. The mechanical properties of the manufactured scaffold showed a high level of compatibility with those of the cortical human bone. Sintering at 1250 °C produced scaffolds with 36% porosity and more than 90% interconnected pores, a compressive yield stress of 220 MPa and a Young's modulus of 7.80 GPa, all suitable for bone tissue engineering. Increasing the sintering temperature to 1300 °C increased the Young's modulus to 22.0 GPa due to reduced porosity, while reducing the sintering temperature to 1150 °C lowered the yield stress to 120 MPa, indicative of insufficient sintering. Electrochemical studies revealed that samples sintered at 1150 °C have a higher corrosion rate compared with those at a sintering temperature of 1250 °C. Overall, it was concluded that sintering at 1250 °C yielded the most desirable results.

Additive manufacturing technique-designed metallic porous implants for clinical application in orthopedics

Traditional metallic scaffold prostheses, as vastly applied implants in clinical orthopedic operations, have achieved great success in rebuilding limb function. However, mismatch of bone defects and additional coating requirements limit the long-term survival of traditional prostheses. Recently, additive manufacturing (AM) has opened up unprecedented possibilities for producing complicated structures in prosthesis shapes and microporous surface designs of customized prostheses, which can solve the drawback of traditional prostheses mentioned above. This review presents the most commonly used metallic additive manufacturing techniques, the microporous structure design of metallic scaffolds, and novel applications of customized prostheses in the orthopedic field. Challenges and future perspectives on AM fabricated scaffolds are also summarized.

[Surface modifications of implants. Part 1 : Material technical and biological principles]

The biological effects of implants in vivo are not only dependent on the implantation site and patient-specific factors but are also influenced by the physicochemical composition and the surface topography of the biomaterial. In cases of permanent implants applied to bone, primary stability, the promotion of rapid osteointegration and antimicrobial properties of the implant are strived for; however, surface modifications are also found on biomaterials which only temporarily come into contact with tissue. These include not only osteosynthesis materials, jig or implant templates but also surgical instruments. This article summarizes the relevant technical principles of materials for the assessment of implant surfaces. Besides technical material-specific and biological principles, different surface modifications for targeted clinical applications are presented. Furthermore, current developmental strategies are outlined.

Materials-Directed Differentiation of Mesenchymal Stem Cells for Tissue Engineering and Regeneration

Cell-based therapies are a promising alternative to grafts and organ transplantation for treating tissue loss or damage due to trauma, malfunction, or disease. Over the past two decades, mesenchymal stem cells (MSCs) have attracted much attention as a potential cell population for use in regenerative medicine. While the proliferative capacity and multilineage potential of MSCs provide an opportunity to generate clinically relevant numbers of transplantable cells, their use in tissue regenerative applications has met with relatively limited success to date apart from secreting paracrine-acting factors to modulate the defect microenvironment. Presently, there is significant effort to engineer the biophysical properties of biomaterials to direct MSC differentiation and further expand on the potential of MSCs in tissue engineering, regeneration, and repair. Biomaterials can dictate MSC differentiation by modulating features of the substrate including composition, mechanical properties, porosity, and topography. The purpose of this review is to highlight recent approaches for guiding MSC fate using biomaterials and provide a description of the underlying characteristics that promote differentiation toward a desired phenotype.

In vitro and in vivo evaluation of polylactic acid-based composite with tricalcium phosphate microsphere for enhanced biodegradability and osseointegration

A biodegradable polylactic acid composite containing tricalcium phosphate microsphere was fabricated. The composite exhibited enhanced biocompatibility and a well-interconnected porous structure that enabled tissue ingrowth after degradation. The tricalcium phosphate microspheres had an average size of 106 ± 43 μm and were incorporated into the polylactic acid matrix using a high-shear mixer. The resulting bioactivity and hydrophilicity were enhanced to levels comparable to those of a polylactic acid composite containing tricalcium phosphate powder, which is a well-known material used in the medical field. An accelerated 30-day degradation test in HCl revealed successful generation of an open porous structure with ∼98% interconnectivity in the polylactic acid–tricalcium phosphate microsphere composite, demonstrating the potential of this material to induce enhanced osseointegration in the later stage of bone regeneration. The early stage osseointegration was also evaluated by implanting the composite in vivo using a rabbit femoral defect model. After 16 weeks of implantation, the bone-to-implant contact ratio of the polylactic acid–tricalcium phosphate microsphere composite was enhanced owing to tissue ingrowth through the generated pores near the surface.

Titanium Fiber Plates for Bone Tissue Repair

Titanium plates are widely used in clinical settings because of their high bone affinity. However, owing to their high elastic modulus, these plates are not suitable for bone repair since their proximity to the bone surface for prolonged periods can cause stress shielding, leading to bone embrittlement. In contrast, titanium fiber plates prepared by molding titanium fibers into plates by simultaneously applying compression and shear stress at normal room temperature can have an elastic modulus similar to that of bone cortex, and stress shielding will not occur even when the plate lies flush against the bone's surface. Titanium fibers can form a porous structure suitable for cell adhesion and as a bone repair scaffold. A titanium fiber plate is combined with osteoblasts and shown that the titanium fiber plate is better able to facilitate bone tissue repair than the conventional titanium plate when implanted in rat bone defects. Capable of being used in close contact with bone for a long time, and even capable of promoting bone repair, titanium fiber plates have a wide range of applications, and are expected to make great contributions to clinical management of increasing bone diseases, including bone fracture repair and bone 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.

Influence of Porosity and Pore-Size Distribution in Ti6Al4 V Foam on Physicomechanical Properties, Osteogenesis, and Quantitative Validation of Bone Ingrowth by Micro-Computed Tomography

Cementless fixation for orthopedic implants aims to obviate challenges associated with bone cement, providing long-term stability of bone prostheses after implantation. The application of porous titanium and its alloy-based implants is emerging for load-bearing applications due to their high specific strength, low stiffness, corrosion resistance, and superior osteoconductivity. In this study, coagulant-assisted foaming was utilized for the fabrication of porous Ti₆Al₄ V using egg-white foam. Samples with three different porosities of 68.3%, 75.4%, and 83.1% and average pore sizes of 92, 178, and 297 μm, respectively, were prepared and subsequently characterized for mechanical properties, osteogenesis, and tissue ingrowth. A microstructure-mechanical properties relationship study revealed that an increase of porosity from 68.3 to 83.1% increased the average pore size from 92 to 297 μm with the subsequent reduction of compresive strength by 85% and modulus by 90%. Samples with 75.4% porosity and a 178 μm average pore size produced signifcant osteogenic effects on human mesenchymal stem cells, which was further supported by immunocytochemistry and real-time polymerase chain reaction data. Quantitative assessment of bone ingrowth by micro-computed tomography revealed that there was an approximately 52% higher bone formation and more than 90% higher bone penetration at the center of femoral defects in rabbit when implanted with Ti₆Al₄ V foam (75.4% porosity) compared to the empty defects after 12 weeks. Hematoxylin and eosin (H&E) and Masson trichrome (MT) staining along with energy-dispersive X-ray mapping on the sections obtained from the retrieved bone samples support bone ingrowth into the implanted region.

Designing of PLA scaffolds for bone tissue replacement fabricated by ordinary commercial 3D printer

BACKGROUND

The primary objective of Tissue engineering is a regeneration or replacement of tissues or organs damaged by disease, injury, or congenital anomalies. At present, Tissue engineering repairs damaged tissues and organs with artificial supporting structures called scaffolds. These are used for attachment and subsequent growth of appropriate cells. During the cell growth gradual biodegradation of the scaffold occurs and the final product is a new tissue with the desired shape and properties. In recent years, research workplaces are focused on developing scaffold by bio-fabrication techniques to achieve fast, precise and cheap automatic manufacturing of these structures. Most promising techniques seem to be Rapid prototyping due to its high level of precision and controlling. However, this technique is still to solve various issues before it is easily used for scaffold fabrication. In this article we tested printing of clinically applicable scaffolds with use of commercially available devices and materials. Research presented in this article is in general focused on "scaffolding" on a field of bone tissue replacement.

RESULTS

Commercially available 3D printer and Polylactic acid were used to create originally designed and possibly suitable scaffold structures for bone tissue engineering. We tested printing of scaffolds with different geometrical structures. Based on the osteosarcoma cells proliferation experiment and mechanical testing of designed scaffold samples, it will be stated that it is likely not necessary to keep the recommended porosity of the scaffold for bone tissue replacement at about 90%, and it will also be clarified why this fact eliminates mechanical properties issue. Moreover, it is demonstrated that the size of an individual pore could be double the size of the recommended range between 0.2-0.35 mm without affecting the cell proliferation.

CONCLUSION

Rapid prototyping technique based on Fused deposition modelling was used for the fabrication of designed scaffold structures. All the experiments were performed in order to show how to possibly solve certain limitations and issues that are currently reported by research workplaces on the field of scaffold bio-fabrication. These results should provide new valuable knowledge for further research.

The influence of direct laser metal sintering implants on the early stages of osseointegration in diabetic mini-pigs

Background

High failure rates of oral implants have been reported in diabetic patients due to the disruption of osseointegration. The aim of this study was to investigate whether direct laser metal sintering (DLMS) could improve osseointegration in diabetic animal models.

Methods

Surface characterizations were carried out on two types of implants. Cell morphology and the osteogenic-related gene expression of MG63 cells were observed under conditions of DLMS and microarc oxidation (MAO). A diabetes model in mini-pigs was established by intravenous injection of streptozotocin (150 mg/kg), and a total of 36 implants were inserted into the mandibular region. Micro-computed tomography (micro-CT) and histologic evaluations were performed 3 and 6 months after implantation.

Results

The Ra (the average of the absolute height of all points) of MAO surface was 2.3±0.3 µm while the DLMS surface showed the Ra of 27.4±1.1 µm. The cells on DLMS implants spread out more podia than those on MAO implants through cell morphology analysis. Osteogenic-related gene expression was also dramatically increased in the DLMS group. Obvious improvement was observed in the micro-CT and Van Gieson staining analyses of DLMS implants compared with MAO at 3 months, although this difference disappeared by 6 months. DLMS implants showed a higher bone–implant contact percentage (33.2%±11.2%) at 3 months compared with MAO group (18.9%±7.3%) while similar results were showed at 6 months between DLMS group (42.8%±10.1%) and MAO group (38.3%±10.8%).

Conclusion

The three-dimensional environment of implant surfaces with highly porous and fully interconnected channel and pore architectures can improve cell spreading and accelerate the progress of osseointegration in diabetic mini-pigs.

Metallic powder-bed based 3D printing of cellular scaffolds for orthopaedic implants: A state-of-the-art review on manufacturing, topological design, mechanical properties and biocompatibility

Metallic cellular scaffold is one of the best choices for orthopaedic implants as a replacement of human body parts, which could improve life quality and increase longevity for the people needed. Unlike conventional methods of making cellular scaffolds, three-dimensional (3D) printing or additive manufacturing opens up new possibilities to fabricate those customisable intricate designs with highly interconnected pores. In the past decade, metallic powder-bed based 3D printing methods emerged and the techniques are becoming increasingly mature recently, where selective laser melting (SLM) and selective electron beam melting (SEBM) are the two representatives. Due to the advantages of good dimensional accuracy, high build resolution, clean build environment, saving materials, high customisability, etc., SLM and SEBM show huge potential in direct customisable manufacturing of metallic cellular scaffolds for orthopaedic implants. Ti-6Al-4V to date is still considered to be the optimal materials for producing orthopaedic implants due to its best combination of biocompatibility, corrosion resistance and mechanical properties. This paper presents a state-of-the-art overview mainly on manufacturing, topological design, mechanical properties and biocompatibility of cellular Ti-6Al-4V scaffolds via SLM and SEBM methods. Current manufacturing limitations, topological shortcomings, uncertainty of biocompatible test were sufficiently discussed herein. Future perspectives and recommendations were given at the end.

Additively Manufactured Scaffolds for Bone Tissue Engineering and the Prediction of their Mechanical Behavior: A Review

Additive manufacturing (AM), nowadays commonly known as 3D printing, is a revolutionary materials processing technology, particularly suitable for the production of low-volume parts with high shape complexities and often with multiple functions. As such, it holds great promise for the fabrication of patient-specific implants. In recent years, remarkable progress has been made in implementing AM in the bio-fabrication field. This paper presents an overview on the state-of-the-art AM technology for bone tissue engineering (BTE) scaffolds, with a particular focus on the AM scaffolds made of metallic biomaterials. It starts with a brief description of architecture design strategies to meet the biological and mechanical property requirements of scaffolds. Then, it summarizes the working principles, advantages and limitations of each of AM methods suitable for creating porous structures and manufacturing scaffolds from powdered materials. It elaborates on the finite-element (FE) analysis applied to predict the mechanical behavior of AM scaffolds, as well as the effect of the architectural design of porous structure on its mechanical properties. The review ends up with the authors’ view on the current challenges and further research directions.

Electrophoretic deposition of chitosan/gelatin coatings with controlled porous surface topography to enhance initial osteoblast adhesive responses

Electrophoretically deposited (EPD) coatings have often been employed recently for the addition of different new chemical compositions to classic chitosan coatings to improve the biocompatibility and therapeutic potential of coated implants. However, little attention has been paid to enhance the cell response to EPD coatings via integrating the effects of chemical components and surface topography. Here, we fabricated EPD chitosan/gelatin (CS/G) coatings with controlled porous surface topography by controlling bubble generation in the EPD process via changing the gelatin content in solution from 0, 0.01, 0.1, and 1 to 10 mg ml^-1. The pure chitosan coating surface was characterized by homogeneous large pores of 500 μm. After 0.01 mg ml^-1 gelatin was added, 180 μm small pores appeared on the walls of large pores. As the gelatin content increased to 0.1 mg ml^-1, a number of small pores increased noticeably. When the gelatin content reached 1 mg ml^-1, large pores disappeared, and the coating displayed homogeneous small pores. 10 mg ml^-1 gelatin concentration led to coatings consisting of small pores with not integral and continuous structures. The initial osteoblastic responses, including cell adherence progress, spreading area, proliferation rate, and focal adhesion-related gene expression, gradually improved from 0 to 0.01, 0.1, and 1 mg ml^-1 gelatin content, but decreased from 1 to 10 mg ml^-1. All these results indicated that the initial cell responses to coatings reached a peak when it was 1 mg ml^-1 gelatin and they had homogeneous small pores, which might contribute to the synergistic effects of the porous surface structure and components. This work would be beneficial for expanding the potential application of EPD coatings.

Ectopic osteogenesis and angiogenesis regulated by porous architecture of hydroxyapatite scaffolds with similar interconnecting structure in vivo

The macro-pore sizes of porous scaffold play a key role for regulating ectopic osteogenesis and angiogenesis but many researches ignored the influence of interconnection between macro-pores with different sizes. In order to accurately reveal the relationship between ectopic osteogenesis and macro-pore sizes in dorsal muscle and abdominal cavities of dogs, hydroxyapatite (HA) scaffolds with three different macro-pore sizes of 500–650, 750–900 and 1100–1250 µm were prepared via sugar spheres-leaching process, which also had similar interconnecting structure determined by keeping the d/s ratio of interconnecting window diameter to macro-pore size constant. The permeability test showed that the seepage flow of fluid through the porous scaffolds increased with the increase of macro-pore sizes. The cell growth in three scaffolds was not affected by the macro-pore sizes. The in vivo ectopic implantation results indicated that the macro-pore sizes of HA scaffolds with the similar interconnecting structure have impact not only the speed of osteogenesis and angiogenesis but also the space distribution of newly formed bone. The scaffold with macro-pore sizes of 750–900 µm exhibited much faster angiogenesis and osteogenesis, and much more uniformly distribution of new bone than those with other macro-pore sizes. This work illustrates the importance of a suitable macro-pore sizes in HA scaffolds with the similar interconnecting structure which provides the environment for ectopic osteogenesis and angiogenesis.

Proliferation and osteogenic differentiation of mesenchymal stromal cells in a novel porous hydroxyapatite scaffold

AIM

To compare the effect of bovine bone derived porous hydroxyapatite (BDHA) scaffold on proliferation and osteogenic differentiation of human bone marrow-derived mesenchymal stromal cells (hMSCs) compared with commercial hydroxyapatite (CHA) scaffold.

MATERIALS AND METHODS

The porosity and pore size were analyzed using micro-CT. The biocompatibility was demonstrated by alamar blue assay, and cell attachment through SEM and Hoechst staining. The osteogenic differentiation was demonstrated using biochemical assay and osteogenic gene expression.

RESULTS

BDHA and CHA scaffolds showed porosity of 76.6 ± 0.6 and 64.3 ± 0.3% and pore size diameter of 0.04-0.25 and 0.1-2.6 mm, respectively. hMSCs proliferation, ALP activity, osteocalcin secretion and osteogenic gene expression are comparable in both the scaffolds.

CONCLUSION

These results demonstrated that BDHA is biocompatible, supports cell adhesion and promotes proliferation and osteogenic differentiation.

Periodontal Wound Healing by Transplantation of Jaw Bone Marrow-Derived Mesenchymal Stem Cells in Chitosan/Anorganic Bovine Bone Carrier Into One-Wall Infrabony Defects in Beagles

BACKGROUND

This study aims to evaluate the performance of chitosan/anorganic bovine bone (C/ABB) scaffold seeded with human jaw bone marrow-derived mesenchymal stem cells (hJBMMSCs) in supporting the healing/repair of 1-wall critical-size periodontal defects.

METHODS

Physical properties of the C/ABB scaffold were compared with those of the chitosan scaffold. hJBMMSCs were obtained from healthy human alveolar bone during the extraction of third molar impacted teeth. One-wall (7 × 4 mm) infrabony defects were surgically created at the bilateral mandibular third premolars and first molars in six beagles. The defects were randomly assigned to six groups and implanted with different scaffolds: 1) chitosan (C) scaffold; 2) C scaffold with hJBMMSCs (C + cell); 3) C/ABB scaffold (C/ABB); 4) C/ABB scaffold with hJBMMSCs (C/ABB + cell); 5) ABB scaffold (ABB); and 6) open flap debridement (control). The animals were euthanized 8 weeks after surgery for histologic analysis.

RESULTS

The C/ABB scaffold had a porous structure and increased compressive strength. Both C/ABB and C/ABB + cell exhibited the newly formed cellular mixed-fiber cementum, woven/lamellar bone, and periodontal ligament. Cementum formation was significantly greater in group C/ABB + cell than in group C/ABB (2.64 ± 0.50 mm versus 0.91 ± 0.55 mm, P <0.05). For new bone (NB) height, group C/ABB + cell and C/ABB showed mean ± SD values of 2.83 ± 0.29 mm and 2.65 ± 0.52 mm and for NB area 8.89 ± 1.65 mm and 8.73 ± 1.94 mm(2), respectively. For NB (height and area), there was no significant difference between the two groups.

CONCLUSIONS

The combination of hJBMMSCs and C/ABB scaffolds could promote periodontal repair. Future studies are expected to further optimize the combination and lead to an ideal periodontal regeneration.

Long-term osseointegration of 3D printed CoCr constructs with an interconnected open-pore architecture prepared by electron beam melting

In orthopaedic surgery, cobalt chromium (CoCr) based alloys are used extensively for their high strength and wear properties, but with concerns over stress shielding and bone resorption due to the high stiffness of CoCr. The structural stiffness, principally related to the bulk and the elastic modulus of the material, may be lowered by appropriate design modifications, to reduce the stiffness mismatch between metal/alloy implants and the adjacent bone. Here, 3D printed CoCr and Ti6Al4V implants of similar macro-geometry and interconnected open-pore architecture prepared by electron beam melting (EBM) were evaluated following 26week implantation in adult sheep femora. Despite higher total bone-implant contact for Ti6Al4V (39±4%) than CoCr (27±4%), bone formation patterns were similar, e.g., densification around the implant, and gradual ingrowth into the porous network, with more bone in the outer half (periphery) than the inner half (centre). Raman spectroscopy revealed no major differences in mineral crystallinity, the apatite-to-collagen ratio, or the carbonate-to-phosphate ratio. Energy dispersive X-ray spectroscopy showed similar Ca/P ratio of the interfacial tissue adjacent to both materials. Osteocytes made direct contact with CoCr and Ti6Al4V. While osteocyte density and distribution in the new-formed bone were largely similar for the two alloys, higher osteocyte density was observed at the periphery of the porous network for CoCr, attributable to slower remodelling and a different biomechanical environment. The results demonstrate the possibility to achieve bone ingrowth into open-pore CoCr constructs, and attest to the potential for fabricating customised osseointegrated CoCr implants for load-bearing applications.

STATEMENT OF SIGNIFICANCE

Although cobalt chromium (CoCr) based alloys are used extensively in orthopaedic surgery, stress shielding due to the high stiffness of CoCr is of concern. To reduce the stiffness mismatch between CoCr and bone, CoCr and Ti6Al4V implants having an interconnected open-pore architecture were prepared by electron beam melting (EBM). After six months of submerged healing in sheep, both alloys showed similar patterns of bone formation, with densification around the implant and gradual ingrowth into the porous network. The molecular and elemental composition of the interfacial tissue was similar for both alloys. Osteocytes made direct contact with both alloys, with similar overall osteocyte density and distribution. The work attests to the potential for achieving osseointegration of EBM manufactured porous CoCr implants.

Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review

One of the critical issues in orthopaedic regenerative medicine is the design of bone scaffolds and implants that replicate the biomechanical properties of the host bones. Porous metals have found themselves to be suitable candidates for repairing or replacing the damaged bones since their stiffness and porosity can be adjusted on demands. Another advantage of porous metals lies in their open space for the in-growth of bone tissue, hence accelerating the osseointegration process. The fabrication of porous metals has been extensively explored over decades, however only limited controls over the internal architecture can be achieved by the conventional processes. Recent advances in additive manufacturing have provided unprecedented opportunities for producing complex structures to meet the increasing demands for implants with customized mechanical performance. At the same time, topology optimization techniques have been developed to enable the internal architecture of porous metals to be designed to achieve specified mechanical properties at will. Thus implants designed via the topology optimization approach and produced by additive manufacturing are of great interest. This paper reviews the state-of-the-art of topological design and manufacturing processes of various types of porous metals, in particular for titanium alloys, biodegradable metals and shape memory alloys. This review also identifies the limitations of current techniques and addresses the directions for future investigations.

Influence of a non-biodegradable porous structure on bone repair

Pore and interconnection size are two key structural parameters for bone tissue engineering scaffolds.

Osteoblast functions in functionally graded Ti-6Al-4 V mesh structures

We describe here the combined efforts of engineering and biological sciences as a systemic approach to fundamentally elucidate osteoblast functions in functionally graded Ti-6Al-4 V mesh structures in relation to uniform/monolithic mesh arrays. First, the interconnecting porous architecture of functionally graded mesh arrays was conducive to cellular functions including attachment, proliferation, and mineralization. The underlying reason is that the graded fabricated structure with cells seeded from the large pore size side provided a channel for efficient transfer of nutrients to other end of the structure (small pore size), leading to the generation of mineralized extracellular matrix by differentiating pre-osteoblasts. Second, a comparative and parametric study indicated that gradient mesh structure had a pronounced effect on cell adhesion and mineralization, and strongly influenced the proliferation phase. High intensity and near-uniform distribution of proteins (actin and vinculin) on struts of the gradient mesh structure (cells seeded from large pore side) implied signal transduction during cell adhesion and was responsible for superior cellular activity, in comparison to the uniform mesh structure and non-porous titanium alloy. Cells adhered to the mesh struts by forming a sheet, bridging the pores through numerous cytoplasmic extensions, in the case of porous mesh structures. Intercellular interaction in porous structures provided a pathway for cells to communicate and mature to a differentiated phenotype. Furthermore, the capability of cells to migrate through the interconnecting porous architecture on mesh structures led to colonization of the entire structure. Cells were embedded layer-by-layer in the extracellular matrix as the matrix mineralized. The outcomes of the study are expected to address challenges associated with the treatment of segmental bone defects and bone-remodeling through favorable modulation of cellular response. Moreover, the study provides a foundation for a new branch of functionally graded materials with interconnected porous architecture.

Interplay between self-assembled structure of bone morphogenetic protein-2 (BMP-2) and osteoblast functions in three-dimensional titanium alloy scaffolds: Stimulation of osteogenic activity

Three-dimensional cellular scaffolds are receiving significant attention in bone tissue engineering to treat segmental bone defects. However, there are indications of lack of significant osteoinductive ability of three-dimensional cellular scaffolds. In this regard, the objective of the study is to elucidate the interplay between bone morphogenetic protein (BMP-2) and osteoblast functions on 3D mesh structures with different porosities and pore size that were fabricated by electron beam melting. Self-assembled dendritic microstructure with interconnected cellular-type morphology of BMP-2 on 3D scaffolds stimulated osteoblast functions including adhesion, proliferation, and mineralization, with prominent effect on 2-mm mesh. Furthermore, immunofluorescence studies demonstrated higher density and viability of osteoblasts on lower porosity mesh structure (2 mm) as compared to 3- and 4-mm mesh structures. Enhanced filopodia cellular extensions with extensive cell spreading was observed on BMP-2 treated mesh structures, a behavior that is attributed to the unique self-assembled structure of BMP-2 that effectively communicates with the cells. The study underscores the potential of BMP-2 in imparting osteoinductive capability to the 3D printed scaffolds.

Heidelberg-mCT-Analyzer: a novel method for standardized microcomputed-tomography-guided evaluation of scaffold properties in bone and tissue research

Bone tissue engineering and bone scaffold development represent two challenging fields in orthopaedic research. Micro-computed tomography (mCT) allows non-invasive measurement of these scaffolds' properties in vivo. However, the lack of standardized mCT analysis protocols and, therefore, the protocols' user-dependency make interpretation of the reported results difficult. To overcome these issues in scaffold research, we introduce the Heidelberg-mCT-Analyzer. For evaluation of our technique, we built 10 bone-inducing scaffolds, which underwent mCT acquisition before ectopic implantation (T0) in mice, and at explantation eight weeks thereafter (T1). The scaffolds' three-dimensional reconstructions were automatically segmented using fuzzy clustering with fully automatic level-setting. The scaffold itself and its pores were then evaluated for T0 and T1. Analysing the scaffolds' characteristic parameter set with our quantification method showed bone formation over time. We were able to demonstrate that our algorithm obtained the same results for basic scaffold parameters (e.g. scaffold volume, pore number and pore volume) as other established analysis methods. Furthermore, our algorithm was able to analyse more complex parameters, such as pore size range, tissue mineral density and scaffold surface. Our imaging and post-processing strategy enables standardized and user-independent analysis of scaffold properties, and therefore is able to improve the quantitative evaluations of scaffold-associated bone tissue-engineering projects.

Effect of pore size on bone ingrowth into porous titanium implants fabricated by additive manufacturing: An in vivo experiment

Selective laser melting (SLM) is an additive manufacturing technique with the ability to produce metallic scaffolds with accurately controlled pore size, porosity, and interconnectivity for orthopedic applications. However, the optimal pore structure of porous titanium manufactured by SLM remains unclear. In this study, we evaluated the effect of pore size with constant porosity on in vivo bone ingrowth in rabbits into porous titanium implants manufactured by SLM. Three porous titanium implants (with an intended porosity of 65% and pore sizes of 300, 600, and 900μm, designated the P300, P600, and P900 implants, respectively) were manufactured by SLM. A diamond lattice was adapted as the basic structure. Their porous structures were evaluated and verified using microfocus X-ray computed tomography. Their bone-implant fixation ability was evaluated by their implantation as porous-surfaced titanium plates into the cortical bone of the rabbit tibia. Bone ingrowth was evaluated by their implantation as cylindrical porous titanium implants into the cancellous bone of the rabbit femur for 2, 4, and 8weeks. The average pore sizes of the P300, P600, and P900 implants were 309, 632, and 956μm, respectively. The P600 implant demonstrated a significantly higher fixation ability at 2weeks than the other implants. After 4weeks, all models had sufficiently high fixation ability in a detaching test. Bone ingrowth into the P300 implant was lower than into the other implants at 4weeks. Because of its appropriate mechanical strength, high fixation ability, and rapid bone ingrowth, our results indicate that the pore structure of the P600 implant is a suitable porous structure for orthopedic implants manufactured by SLM.

Hydrogels for Engineering of Perfusable Vascular Networks

Hydrogels are commonly used biomaterials for tissue engineering. With their high-water content, good biocompatibility and biodegradability they resemble the natural extracellular environment and have been widely used as scaffolds for 3D cell culture and studies of cell biology. The possible size of such hydrogel constructs with embedded cells is limited by the cellular demand for oxygen and nutrients. For the fabrication of large and complex tissue constructs, vascular structures become necessary within the hydrogels to supply the encapsulated cells. In this review, we discuss the types of hydrogels that are currently used for the fabrication of constructs with embedded vascular networks, the key properties of hydrogels needed for this purpose and current techniques to engineer perfusable vascular structures into these hydrogels. We then discuss directions for future research aimed at engineering of vascularized tissue for implantation.