Additive manufacturing of an elastic poly(ester)urethane for cartilage tissue engineering

Advances in additive manufacturing for bone tissue engineering: materials, design strategies, and applications

The growing annual demand for bone grafts and artificial implants emphasizes the need for effective solutions to repair or replace injured bones. Additive manufacturing technology offers unique merits for advancing bone tissue engineering (BTE), enabling the creation of scaffolds and implants with customized shapes and designs, interconnected architecture, controlled mechanical properties and compositions, and broadening its range of applications. It overcomes the limitations of traditional manufacturing methods such as electrospinning, salt leaching, freeze drying, solvent casting etc. This review highlights additive manufacturing technologies and their applications in BTE, as well as materials and scaffold architectures to widen the potential of the biomedical sector. The selection of optimal printing methods for BTE requires careful consideration of the advantages and disadvantages against the needs for degradation, strength, and biocompatibility. Material extrusion and powder bed fusion techniques are the most widely used additive manufacturing processes in BTE. The comprehensive review also revealed that parametric designs such as triply periodic minimal surface (TPMS) and Voronoi hold better characteristics for their application in BTE. Voronoi designs exhibit exceptional randomness whereas TPMS structures feature high permeability with continuous surfaces. Topology optimized and gradient models exhibited superior physical and mechanical properties compared to uniform lattices. Future research should focus on the development of novel biomaterials, multi-material printing, assessing long-term impacts, and enhancing 3D printing technologies.

Collagen-based 3D printed poly (glycerol sebacate) composite scaffold with biomimicking mechanical properties for enhanced cartilage defect repair

Cartilage defect repair with optimal efficiency remains a significant challenge due to the limited self-repair capability of native tissues. The development of bioactive scaffolds with biomimicking mechanical properties and degradation rates matched with cartilage regeneration while simultaneously driving chondrogenesis, plays a crucial role in enhancing cartilage defect repair. To this end, a novel composite scaffold with hierarchical porosity was manufactured by incorporating a pro-chondrogenic collagen type I/II-hyaluronic acid (CI/II-HyA) matrix to a 3D-printed poly(glycerol sebacate) (PGS) framework. Based on the mechanical enforcement of PGS framework, the composite scaffold exhibited a compressive modulus of 167.0 kPa, similar to that of native cartilage, as well as excellent fatigue resistance, similar to that of native joint tissue. In vitro degradation tests demonstrated that the composite scaffold maintained structural, mass, and mechanical stability during the initial cartilage regeneration period of 4 weeks, while degraded linearly over time. In vitro biological tests with rat-derived mesenchymal stem cell (MSC) revealed that, the composite scaffold displayed increased cell loading efficiency and improved overall cell viability due to the incorporation of CI/II-HyA matrix. Additionally, it also sustained an effective and high-quality MSC chondrogenesis and abundant de-novo cartilage-like matrix deposition up to day 28. Overall, the biomimetic composite scaffold with sufficient mechanical support, matched degradation rate with cartilage regeneration, and effective chondrogenesis stimulation shows great potential to be an ideal candidate for enhancing cartilage defect repair.

A facile strategy for tuning the density of surface-grafted biomolecules for melt extrusion-based additive manufacturing applications

Melt extrusion-based additive manufacturing (ME-AM) is a promising technique to fabricate porous scaffolds for tissue engineering applications. However, most synthetic semicrystalline polymers do not possess the intrinsic biological activity required to control cell fate. Grafting of biomolecules on polymeric surfaces of AM scaffolds enhances the bioactivity of a construct; however, there are limited strategies available to control the surface density. Here, we report a strategy to tune the surface density of bioactive groups by blending a low molecular weight poly(ε-caprolactone)5k (PCL5k) containing orthogonally reactive azide groups with an unfunctionalized high molecular weight PCL75k at different ratios. Stable porous three-dimensional (3D) scaffolds were then fabricated using a high weight percentage (75 wt.%) of the low molecular weight PCL5k. As a proof-of-concept test, we prepared films of three different mass ratios of low and high molecular weight polymers with a thermopress and reacted with an alkynated fluorescent model compound on the surface, yielding a density of 201-561 pmol/cm2. Subsequently, a bone morphogenetic protein 2 (BMP-2)-derived peptide was grafted onto the films comprising different blend compositions, and the effect of peptide surface density on the osteogenic differentiation of human mesenchymal stromal cells (hMSCs) was assessed. After two weeks of culturing in a basic medium, cells expressed higher levels of BMP receptor II (BMPRII) on films with the conjugated peptide. In addition, we found that alkaline phosphatase activity was only significantly enhanced on films containing the highest peptide density (i.e., 561 pmol/cm2), indicating the importance of the surface density. Taken together, these results emphasize that the density of surface peptides on cell differentiation must be considered at the cell-material interface. Moreover, we have presented a viable strategy for ME-AM community that desires to tune the bulk and surface functionality via blending of (modified) polymers. Furthermore, the use of alkyne-azide "click" chemistry enables spatial control over bioconjugation of many tissue-specific moieties, making this approach a versatile strategy for tissue engineering applications.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42242-024-00286-2.

Alginate-waterborne polyurethane 3D bioprinted scaffolds for articular cartilage tissue engineering

Articular cartilage defects comprise a spectrum of diseases associated with degeneration or damage of the connective tissue present in particular joints, presenting progressive osteoarthritis if left untreated. In vitro tissue regeneration is an innovative treatment for articular cartilage injuries that is attracting not only clinical attention, but also great interest in the development of novel biomaterials, since this procedure involves the formation of a neotissue with the help of material support. In this work, functional alginate and waterborne polyurethane (WBPU) scaffolds have been developed for articular cartilage regeneration using 3D bioprinting technology. The particular properties of alginate-WBPU blends, like mechanical strength, elasticity and moistening, mimic the original cartilage tissue characteristics, being ideal for this application. To fabricate the scaffolds, mature chondrocytes were loaded into different alginate-WBPU inks with rheological properties suitable for 3D bioprinting. Bioinks with high alginate content showed better 3D printing performance, as well as structural integrity and cell viability, being most suitable for scaffolds fabrication. After 28 days of in vitro cartilage formation experiments, scaffolds containing 3.2 and 6.4 % alginate resulted in the maintenance of cell number in the range of 104 chondrocytes/scaffold in differentiated phenotypes, capable of synthesizing specialized extracellular matrix (ECM) up to 6 μg of glycosaminoglycans (GAG) and thus, showing a potential application of these scaffolds for in vitro regeneration of articular cartilage tissue.

Hypotrochoidal scaffolds for cartilage regeneration

The main function of articular cartilage is to provide a low friction surface and protect the underlying subchondral bone. The extracellular matrix composition of articular cartilage mainly consists of glycosaminoglycans and collagen type II. Specifically, collagen type II fibers have an arch-like organization that can be mimicked with segments of a hypotrochoidal curve. In this study, a script was developed that allowed the fabrication of scaffolds with a hypotrochoidal design. This design was investigated and compared to a regular 0-90 woodpile design. The mechanical analyses revealed that the hypotrochoidal design had a lower component Young's modulus while the toughness and strain at yield were higher compared to the woodpile design. Fatigue tests showed that the hypotrochoidal design lost more energy per cycle due to the damping effect of the unique microarchitecture. In addition, data from cell culture under dynamic stimulation demonstrated that the collagen type II deposition was improved and collagen type X reduced in the hypotrochoidal design. Finally, Alcian blue staining revealed that the areas where the stress was higher during the stimulation produced more glycosaminoglycans. Our results highlight a new and simple scaffold design based on hypotrochoidal curves that could be used for cartilage tissue engineering.

Development, Physiochemical characterization, Mechanical and Finite element analysis of 3D printed Polylactide-β-TCP/α-Al2O3 composite

Herein, material extrusion (MEX) technique is utilized to develop 3D printed models based on reinforcing β-Ca3(PO4)2/α-Al2O3 composite in polylactide (PLA) matrix. β-Ca3(PO4)2/α-Al2O3 composite has been synthesized through co-precipitation method and the phase content of β-Ca3(PO4)2 and α-Al2O3 components are respectively determined as 64 and 36 wt%. The resultant β-Ca3(PO4)2/α-Al2O3 composite mixed with PLA at various weight ratios were extruded as filaments and subsequently 3D printed into definite shapes for the physiochemical, morphological and mechanical evaluation. 3D printed bodies that comprise 5 wt % β-Ca3(PO4)2/α-Al2O3 composite yielded an increasing tensile, compressive and flexural strength in the corresponding order of ∼15, ∼15 and 22% than 3D printed pure PLA. Further, the Representative volume element (RVE) unit cells developed based on the various investigated compositions of PLA-β-Ca3(PO4)2/α-Al2O3 were subjected to mechanical evaluation through Finite element analysis (FEA) under both static and dynamic loading conditions on ASTM standard specimens. The results from experimental and FEA analysis demonstrated good uniformity that confirmed the reinforcement of 5 wt % β-Ca3(PO4)2/α-Al2O3 in PLA matrix as an optimum combination to yield better mechanical strength.

Analysis of N-glycosylation protein of Kashin-Beck disease chondrocytes derived from induced pluripotent stem cells based on label-free strategies with LC-MS/MS

We aimed to compare N-glycosylation proteins in Kashin-Beck disease (KBD) chondrocytes and normal chondrocytes derived from induced pluripotent stem cells (iPSCs). KBD and normal iPSCs were reprogrammed from human KBD and normal dermal fibroblasts, respectively. Subsequently, chondrocytes were differentiated from KBD and normal iPSCs separately. Immunofluorescence was utilized to assay the protein markers of iPSCs and chondrocytes. Differential N-glycosylation proteins were screened using label-free strategies with LC-MS/MS. Bioinformatics analyses were utilized to interpret the functions of differential N-glycosylation proteins. Immunofluorescence staining revealed that both KBD-iPSCs and normal-iPSCs strongly expressed pluripotency markers OCT4 and NANOG. Meanwhile, chondrocyte markers collagen II and SOX9 are presented in KBD-iPSC-chondrocytes and normal-iPSC-chondrocytes. We obtained 87 differential N-glycosylation sites which corresponded to 68 differential proteins, which were constructed into 1 cluster. We obtained collagen type I trimer and 9 other biological processes; polysaccharide binding and 9 other molecular functions; regulation of transcription by RNA polymerase II and 9 other cellular components from GO; the Pl3K-Akt signaling pathway and 9 other KEGG pathways; peroxisome and 7 other subcellular locations; and integrin alpha chain, C-terminal cytoplasmic region, conserved site and 9 other classifications of domain annotations, and 2 networks. FGFR3 and LRP1 are expressed at higher levels in KBD-iPSC-chondrocytes, while the expressions of COL2A1, TIMP1, UNC5B, NOG, LEPR, and ITGA1 were down-regulated in KBD-iPSC-chondrocytes. The differential expressions of these N-glycosylation proteins may lead to the abnormal function of KBD chondrocytes.

Manufacturing of scaffolds with interconnected internal open porosity and surface roughness

Manufacturing of three-dimensional scaffolds with multiple levels of porosity are an advantage in tissue regeneration approaches to influence cell behavior. Three-dimensional scaffolds with surface roughness and intra-filament open porosity were successfully fabricated by additive manufacturing combined with chemical foaming and porogen leaching without the need of toxic solvents. The decomposition of sodium citrate, a chemical blowing agent, generated pores within the scaffold filaments, which were interconnected and opened to the external environment by leaching of a water-soluble sacrificial phase, as confirmed by micro-CT and buoyancy measurements. The additional porosity did not result in lower elastic modulus, but in higher strain at maximum load, i.e. scaffold ductility. Human mesenchymal stromal cells cultured for 24 h adhered in greater numbers on these scaffolds when compared to plain additive-manufactured ones, irrespectively of the scaffold pre-treatment method. Additionally, they showed a more spread and random morphology, which is known to influence cell fate. Cells cultured for a longer period exhibited enhanced metabolic activity while secreting higher osteogenic markers after 7 days in culture. STATEMENT OF SIGNIFICANCE: Inspired by the function of hierarchical cellular structures in natural materials, this work elucidates the development of scaffolds with multiscale porosity by combining in-situ foaming and additive manufacturing, and successive porogen leaching. The resulting scaffolds displayed enhanced mechanical toughness and multiscale pore network interconnectivity, combined with early differentiation of adult mesenchymal stromal cells into the osteogenic lineage.

Electrospun fibers coated with nanostructured biomimetic hydroxyapatite: A new platform for regeneration at the bone interfaces

Reconstruction of gradient organic/inorganic tissues is a challenging task in orthopaedics. Indeed, to mimic tissue characteristics and stimulate bone regeneration at the interface, it is necessary to reproduce both the mineral and organic components of the tissue ECM, as well as the micro/nano-fibrous morphology. To address this goal, we propose here novel biomimetic patches obtained by the combination of electrospinning and nanostructured bone apatite. In particular, we deposited apatite on the electrospun fibers by Ionized Jet Deposition, a plasma-assisted technique that allows conformal deposition and the preservation in the coating of the target's stoichiometry. The damage to the substrate and fibrous morphology is a polymer-dependent aspect, that can be avoided by properly selecting the substrate composition and deposition parameters. In fact, all the tested polymers (poly(l-lactide), poly(D,l-lactide-co-glycolide, poly(ε-caprolactone), collagen) were effectively coated, and the morphological and thermal characterization revealed that poly(ε-caprolactone) suffered the least damage. The coating of collagen fibers, on the other hand, destroyed the fiber morphology and it could only be performed when collagen is blended with a more resistant synthetic polymer in the nanofibers. Due to the biomimetic composition and multiscale morphology from micro to nano, the poly(ε-caprolactone)-collagen biomimetic patches coated with bone apatite supported MSCs adhesion, patch colonization and early differentiation, while allowing optimal viability. The biomimetic coating allowed better scaffold colonization, promoting cell spreading on the fibers.

Rational design of biodegradable thermoplastic polyurethanes for tissue repair

As a type of elastomeric polymers, non-degradable polyurethanes (PUs) have a long history of being used in clinics, whereas biodegradable PUs have been developed in recent decades, primarily for tissue repair and regeneration. Biodegradable thermoplastic (linear) PUs are soft and elastic polymeric biomaterials with high mechanical strength, which mimics the mechanical properties of soft and elastic tissues. Therefore, biodegradable thermoplastic polyurethanes are promising scaffolding materials for soft and elastic tissue repair and regeneration. Generally, PUs are synthesized by linking three types of changeable blocks: diisocyanates, diols, and chain extenders. Alternating the combination of these three blocks can finely tailor the physio-chemical properties and generate new functional PUs. These PUs have excellent processing flexibilities and can be fabricated into three-dimensional (3D) constructs using conventional and/or advanced technologies, which is a great advantage compared with cross-linked thermoset elastomers. Additionally, they can be combined with biomolecules to incorporate desired bioactivities to broaden their biomedical applications. In this review, we comprehensively summarized the synthesis, structures, and properties of biodegradable thermoplastic PUs, and introduced their multiple applications in tissue repair and regeneration. A whole picture of their design and applications along with discussions and perspectives of future directions would provide theoretical and technical supports to inspire new PU development and novel applications.

Investigation of Collagen-Incorporated Sodium Alginate Bioprinting Hydrogel for Tissue Engineering

Tissue engineering is a promising area that is aimed at tissue regeneration and wound repair. Sodium alginate (SA) has been widely used as one of the most biocompatible materials for tissue engineering. The cost-efficiency and rapid gel ability made SA attractive in would healing and regeneration area. To improve printability and elasticity, many hydrogel-based bioinks were developed by mixing SA with other natural or synthetic polymers. In this paper, composite SA/COL bioink was used for the bioprinting of artificial cartilage tissue mimicries. The results showed that the concentration of both SA and COL has significant effects on filament diameter and merging. A higher concentration of the bioink solution led to better printing fidelity and less deformation. Overall, a higher SA concentration and a lower COL concentration contributed to a lower shrinkage ratio after crosslinking. In summary, the SA/COL composite bioink has favorable rheological properties and this study provided material composition optimization for future bioprinting of engineered tissues.

Functionalized Hydrogels for Cartilage Repair: The Value of Secretome-Instructive Signaling

Cartilage repair has been a challenge in the medical field for many years. Although treatments that alleviate pain and injury are available, none can effectively regenerate the cartilage. Currently, regenerative medicine and tissue engineering are among the developed strategies to treat cartilage injury. The use of stem cells, associated or not with scaffolds, has shown potential in cartilage regeneration. However, it is currently known that the effect of stem cells occurs mainly through the secretion of paracrine factors that act on local cells. In this review, we will address the use of the secretome—a set of bioactive factors (soluble factors and extracellular vesicles) secreted by the cells—of mesenchymal stem cells as a treatment for cartilage regeneration. We will also discuss methodologies for priming the secretome to enhance the chondroregenerative potential. In addition, considering the difficulty of delivering therapies to the injured cartilage site, we will address works that use hydrogels functionalized with growth factors and secretome components. We aim to show that secretome-functionalized hydrogels can be an exciting approach to cell-free cartilage repair therapy.

Use of Polyesters in Fused Deposition Modeling for Biomedical Applications

In recent years, 3D printing techniques experienced a growing interest in several sectors, including the biomedical one. Their main advantage resides in the possibility to obtain complex and personalized structures in a cost-effective way impossible to achieve with traditional production methods. This is especially true for Fused Deposition Modeling (FDM), one of the most diffused 3D printing methods. The easy customization of the final products' geometry, composition and physico-chemical properties is particularly interesting for the increasingly personalized approach adopted in modern medicine. Thermoplastic polymers are the preferred choice for FDM applications, and a wide selection of biocompatible and biodegradable materials is available to this aim. Moreover, these polymers can also be easily modified before and after printing to better suit the body environment and the mechanical properties of biological tissues. This review focuses on the use of thermoplastic aliphatic polyesters for FDM applications in the biomedical field. In detail, the use of poly(ε-caprolactone), poly(lactic acid), poly(lactic-co-glycolic acid), poly(hydroxyalkanoate)s, thermo-plastic poly(ester urethane)s and their blends has been thoroughly surveyed, with particular attention to their main features, applicability and workability. The state-of-the-art is presented and current challenges in integrating the additive manufacturing technology in the medical practice are discussed. This article is protected by copyright. All rights reserved.

CHOLECALCIFEROL AS BIOACTIVE PLASTICIZER OF HIGH Mw PDLLA SCAFFOLDS FOR BONE REGENERATION

Synthetic thermoplastic polymers are a widespread choice as material candidates for scaffolds for tissue engineering (TE), thanks to their ease of processing and tunable properties with respect to biological polymers. These features made them largely employed in melt-extrusion based additive manufacturing (AM), with particular application in hard-tissue engineering. In this field, high molecular weight (Mw) polymers ensuring entanglement network strength are often favorable candidates as scaffold materials because of their enhanced mechanical properties compared to lower Mw grades. However, this is accompanied by high viscosities once processed in molten conditions, which requires driving forces not always accessible technically or compatible with often chemically non-stabilized biomedical grades. When possible, this is circumvented by increasing the operating temperature, which often results in polymer chain scission and consequent degradation of properties. Additionally, synthetic polymers are mostly considered bioinert compared to biological materials and additional processing steps are often required to make them favorable for tissue regeneration. In this study, we report the plasticization of a common thermoplastic polymer with cholecalciferol, the metabolically inactive form of vitamin D3. Plasticization of the polymer allowed us to reduce its melt viscosity, and therefore the energy requirements (mechanical (torque) and heat (temperature)) for extrusion, limiting ultimately polymer degradation. Additionally, we evaluated the effect of cholecalciferol, which is more easily available than its active counterpart, on the osteogenic differentiation of mesenchymal stromal cells (hMSCs). Results indicated that cholecalciferol supported osteogenic differentiation more than the osteogenic culture medium, suggesting that hMSCs possess the enzymatic toolbox for Vitamin D3 (VD3) metabolism.

Polymeric Hydrogels for Controlled Drug Delivery to Treat Arthritis

Rheumatoid arthritis (RA) and osteoarthritis (OA) are disabling musculoskeletal disorders that affect joints and cartilage and may lead to bone degeneration. Conventional delivery of anti-arthritic agents is limited due to short intra-articular half-life and toxicities. Innovations in polymer chemistry have led to advancements in hydrogel technology, offering a versatile drug delivery platform exhibiting tissue-like properties with tunable drug loading and high residence time properties This review discusses the advantages and drawbacks of polymeric materials along with their modifications as well as their applications for fabricating hydrogels loaded with therapeutic agents (small molecule drugs, immunotherapeutic agents, and cells). Emphasis is given to the biological potentialities of hydrogel hybrid systems/micro-and nanotechnology-integrated hydrogels as promising tools. Applications for facile tuning of therapeutic drug loading, maintaining long-term release, and consequently improving therapeutic outcome and patient compliance in arthritis are detailed. This review also suggests the advantages, challenges, and future perspectives of hydrogels loaded with anti-arthritic agents with high therapeutic potential that may alter the landscape of currently available arthritis treatment modalities.

Ear reconstruction research using animal models: The effect of fat grafting on costal cartilage stents

OBJECT

For congenital microtia patients with a depressed mastoid area, it is unclear whether autologous fat grafting to fill the depressed area of the cheek will affect the survival of the subsequent grafted costal cartilage stent. An animal model was used for in vivo research to provide guidance for clinical applications.

METHODS

Autologous costal cartilage was implanted in nude mice. Fat samples were collected at different time points and histological examination performed to analyze the activity of chondrocytes and the deposition of the chondrocyte matrix.

RESULTS

This nude mouse fat transplantation model study showed that there were statistical differences in chondrocyte viability between the fat filling group and the control group, but there was no statistical difference in the effect on collagen content.

CONCLUSION

Transplanting fat reduces the viability of chondrocytes, but has little effect on collagen matrix deposition.

Articular cartilage and osteochondral tissue engineering techniques: Recent advances and challenges

In spite of the considerable achievements in the field of regenerative medicine in the past several decades, osteochondral defect regeneration remains a challenging issue among diseases in the musculoskeletal system because of the spatial complexity of osteochondral units in composition, structure and functions. In order to repair the hierarchical tissue involving different layers of articular cartilage, cartilage-bone interface and subchondral bone, traditional clinical treatments including palliative and reparative methods have showed certain improvement in pain relief and defect filling. It is the development of tissue engineering that has provided more promising results in regenerating neo-tissues with comparable compositional, structural and functional characteristics to the native osteochondral tissues. Here in this review, some basic knowledge of the osteochondral units including the anatomical structure and composition, the defect classification and clinical treatments will be first introduced. Then we will highlight the recent progress in osteochondral tissue engineering from perspectives of scaffold design, cell encapsulation and signaling factor incorporation including bioreactor application. Clinical products for osteochondral defect repair will be analyzed and summarized later. Moreover, we will discuss the current obstacles and future directions to regenerate the damaged osteochondral tissues.

Bioprinting Via a Dual-Gel Bioink Based on Poly(Vinyl Alcohol) and Solubilized Extracellular Matrix towards Cartilage Engineering

Various hydrogel systems have been developed as biomaterial inks for bioprinting, including natural and synthetic polymers. However, the available biomaterial inks, which allow printability, cell viability, and user-defined customization, remains limited. Incorporation of biological extracellular matrix materials into tunable synthetic polymers can merge the benefits of both systems towards versatile materials for biofabrication. The aim of this study was to develop novel, cell compatible dual-component biomaterial inks and bioinks based on poly(vinyl alcohol) (PVA) and solubilized decellularized cartilage matrix (SDCM) hydrogels that can be utilized for cartilage bioprinting. In a first approach, PVA was modified with amine groups (PVA-A), and mixed with SDCM. The printability of the PVA-A/SDCM formulations cross-linked by genipin was evaluated. On the second approach, the PVA was functionalized with cis-5-norbornene-endo-2,3-dicarboxylic anhydride (PVA-Nb) to allow an ultrafast light-curing thiol-ene cross-linking. Comprehensive experiments were conducted to evaluate the influence of the SDCM ratio in mechanical properties, water uptake, swelling, cell viability, and printability of the PVA-based formulations. The studies performed with the PVA-A/SDCM formulations cross-linked by genipin showed printability, but poor shape retention due to slow cross-linking kinetics. On the other hand, the PVA-Nb/SDCM showed good printability. The results showed that incorporation of SDCM into PVA-Nb reduces the compression modulus, enhance cell viability, and bioprintability and modulate the swelling ratio of the resulted hydrogels. Results indicated that PVA-Nb hydrogels containing SDCM could be considered as versatile bioinks for cartilage bioprinting.

Biomaterials for protein delivery for complex tissue healing responses

Tissue repair requires a complex cascade of events mediated by a variety of cells, proteins, and matrix molecules; however, the healing cascade can be easily disrupted by numerous factors, resulting in impaired tissue regeneration. Recent advances in biomaterials for tissue regeneration have increased the ability to tailor the delivery of proteins and other biomolecules to injury sites to restore normal healing cascades and stimulate robust tissue repair. In this review, we discuss the evolution of the field toward creating biomaterials that precisely control protein delivery to stimulate tissue regeneration, with a focus on addressing complex and dynamic injury environments. We highlight biomaterials that leverage different mechanisms to deliver and present proteins involved in healing cascades, tissue targeting and mimicking strategies, materials that can be triggered by environmental cues, and integrated strategies that combine multiple biomaterial properties to improve protein delivery. Improvements in biomaterial design to address complex injury environments will expand our understanding of both normal and aberrant tissue repair processes and ultimately provide a better standard of patient care.

3D additive manufactured composite scaffolds with antibiotic-loaded lamellar fillers for bone infection prevention and tissue regeneration

Bone infections following open bone fracture or implant surgery remain a challenge in the orthopedics field. In order to avoid high doses of systemic drug administration, optimized local antibiotic release from scaffolds is required. 3D additive manufactured (AM) scaffolds made with biodegradable polymers are ideal to support bone healing in non-union scenarios and can be given antimicrobial properties by the incorporation of antibiotics. In this study, ciprofloxacin and gentamicin intercalated in the interlamellar spaces of magnesium aluminum layered double hydroxides (MgAl) and α-zirconium phosphates (ZrP), respectively, are dispersed within a thermoplastic polymer by melt compounding and subsequently processed via high temperature melt extrusion AM (~190 °C) into 3D scaffolds. The inorganic fillers enable a sustained antibiotics release through the polymer matrix, controlled by antibiotics counterions exchange or pH conditions. Importantly, both antibiotics retain their functionality after the manufacturing process at high temperatures, as verified by their activity against both Gram + and Gram - bacterial strains. Moreover, scaffolds loaded with filler-antibiotic do not impair human mesenchymal stromal cells osteogenic differentiation, allowing matrix mineralization and the expression of relevant osteogenic markers. Overall, these results suggest the possibility of fabricating dual functionality 3D scaffolds via high temperature melt extrusion for bone regeneration and infection prevention.

A Preliminary Evaluation of the Pro-Chondrogenic Potential of 3D-Bioprinted Poly(ester Urea) Scaffolds

Degeneration of articular cartilage (AC) is a common healthcare issue that can result in significantly impaired function and mobility for affected patients. The avascular nature of the tissue strongly burdens its regenerative capacity contributing to the development of more serious conditions such as osteoarthritis. Recent advances in bioprinting have prompted the development of alternative tissue engineering therapies for the generation of AC. Particular interest has been dedicated to scaffold-based strategies where 3D substrates are used to guide cellular function and tissue ingrowth. Despite its extensive use in bioprinting, the application of polycaprolactone (PCL) in AC is, however, restricted by properties that inhibit pro-chondrogenic cell phenotypes. This study proposes the use of a new bioprintable poly(ester urea) (PEU) material as an alternative to PCL for the generation of an in vitro model of early chondrogenesis. The polymer was successfully printed into 3D constructs displaying adequate substrate stiffness and increased hydrophilicity compared to PCL. Human chondrocytes cultured on the scaffolds exhibited higher cell viability and improved chondrogenic phenotype with upregulation of genes associated with type II collagen and aggrecan synthesis. Bioprinted PEU scaffolds could, therefore, provide a potential platform for the fabrication of bespoke, pro-chondrogenic tissue engineering constructs.

Additive manufactured, highly resilient, elastic, and biodegradable poly(ester)urethane scaffolds with chondroinductive properties for cartilage tissue engineering

Articular cartilage was thought to be one of the first tissues to be successfully engineered. Despite the avascular and non-innervated nature of the tissue, the cells within articular cartilage – chondrocytes – account for a complex phenotype that is difficult to be maintained in vitro. The use of bone marrow–derived stromal cells (BMSCs) has emerged as a potential solution to this issue. Differentiation of BMSCs toward stable and non-hypertrophic chondrogenic phenotypes has also proved to be challenging. Moreover, hyaline cartilage presents a set of mechanical properties – relatively high Young's modulus, elasticity, and resilience – that are difficult to reproduce. Here, we report on the use of additive manufactured biodegradable poly(ester)urethane (PEU) scaffolds of two different structures (500 μm pore size and 90° or 60° deposition angle) that can support the loads applied onto the knee while being highly resilient, with a permanent deformation lower than 1% after 10 compression-relaxation cycles. Moreover, these scaffolds appear to promote BMSC differentiation, as shown by the deposition of glycosaminoglycans and collagens (in particular collagen II). At gene level, BMSCs showed an upregulation of chondrogenic markers, such as collagen II and the Sox trio, to higher or similar levels than that of traditional pellet cultures, with a collagen II/collagen I relative expression of 2–3, depending on the structure of the scaffold. Moreover, scaffolds with different pore architectures influenced the differentiation process and the final BMSC phenotype. These data suggest that additive manufactured PEU scaffolds could be good candidates for cartilage tissue regeneration in combination with microfracture interventions.