Cell‐Tethered Ligands Modulate Bone Remodeling by Osteoblasts and Osteoclasts

The goals of the present study are to establish an in vitro co‐culture model of osteoblast and osteoclast function and to quantify the resulting bone remodeling. The bone is tissue engineered using well‐defined silk protein biomaterials in 2D and 3D formats in combination with human cells. Parathyroid hormone (PTH) and glucose‐dependent insulinotropic peptide (GIP) are selected because of their roles in bone remodeling for expression in tethered format on human mesenchymal stem cells (hMSCs). The cell‐modified biomaterial surfaces are reconstructed from scanning electron microscopy images into 3D models for quantitative measurement of surface characteristics. Increased calcium deposition and surface roughness are found in 3D surface models of silk protein films remodeled by co‐cultures containing tethered PTH, and decreased surface roughness is found for the films remodeled by tethered GIP co‐cultures. Increased surface roughness is not found in monocultures of hMSCs expressing tethered PTH, suggesting that osteoclast‐osteoblast interactions in the presence of PTH signaling are responsible for the increased mineralization. These data point towards the design of in vitro bone models in which osteoblast‐osteoclast interactions are mimicked for a better understanding of bone remodeling.     

A Novel Hydrogel Surface Grafted With Dual Functional Peptides for Sustaining Long‐Term Self‐Renewal of Human Induced Pluripotent Stem Cells and Manipulating Their Osteoblastic Maturation

Realizing the clinical potential of human induced pluripotent stem cells (hiPSCs) in bone regenerative medicine requires the development of safe and chemically defined biomaterials for expansion of hiPSCs followed by directing their lineage commitment to osteoblasts. In this study, novel multipurpose peptide‐presenting hydrogel surfaces are prepared on common tissue culture plates via carboxymethyl chitosan grafting and subsequent immobilization of two functional peptides allowing for in vitro feeder‐free culture, long‐term self‐renewal, and osteogenic induction of hiPSCs. After vitronectin (VN) peptide modification, the engineered surfaces facilitate adhesion, proliferation, colony formation, and the maintenance of pluripotency of hiPSCs up to passage 10 under fully defined conditions without Matrigel or protein coating. Further, this synthetic niche exhibits an appealing regulatory effect on the osteogenic conversion of hiPSCs to osteoblastic phenotype without an embryoid body formation step by co‐decoration of different ratios of VN and bone‐forming peptide. Such a well‐defined, xeno‐free 2D engineered microenvironment not only helps to accelerate the clinical development of hiPSCs, but also provides a safe and robust platform for the generation of osteoblast‐like cells or bone‐like tissues at different maturation levels. Thus, the strategy may hold great potential for application in cell therapy and bone tissue engineering.     

3D Molecularly Functionalized Cell‐Free Biomimetic Scaffolds for Osteochondral Regeneration

Clinically, cartilage damage is frequently accompanied with subchondral bone injuries caused by disease or trauma. However, the construction of biomimetic scaffolds to support both cartilage and subchondral bone regeneration remains a great challenge. Herein, a novel strategy is adopted to realize the simultaneous repair of osteochondral defects by employing a self‐assembling peptide hydrogel (SAPH) FEFEFKFK (F, phenylalanine; E, glutamic acid; K, lysine) to coat onto 3D‐printed polycaprolactone (PCL) scaffolds. Results show that the SAPH‐coated PCL scaffolds exhibit highly improved hydrophilicity and biomimetic extracellular matrix (ECM) structures compared to PCL scaffolds. In vitro experiments demonstrate that the SAPH‐coated PCL scaffolds promote the proliferation and osteogenic differentiation of rabbit bone mesenchymal stem cells (rBMSCs) and maintain the chondrocyte phenotypes. Furthermore, 3% SAPH‐coated PCL scaffolds significantly induce simultaneous regeneration of cartilage and subchondral bone after 8‐ and 12‐week implantation in vivo, respectively. Mechanistically, by virtue of the enhanced deposition of ECM in SAPH‐coated PCL scaffolds, SAPH with increased stiffness facilitates and remodels the microenvironment around osteochondral defects, which may favor simultaneous dual tissue regeneration. These findings indicate that the 3% SAPH provides efficient and reliable modification on PCL scaffolds and SAPH‐coated PCL scaffolds appear to be a promising biomaterial for osteochondral defect repair.     

Hydrodynamically Guided Hierarchical Self‐Assembly of Peptide–Protein Bioinks

Effective integration of molecular self‐assembly and additive manufacturing would provide a technological leap in bioprinting. This article reports on a biofabrication system based on the hydrodynamically guided co‐assembly of peptide amphiphiles (PAs) with naturally occurring biomolecules and proteins to generate hierarchical constructs with tuneable molecular composition and structural control. The system takes advantage of droplet‐on‐demand inkjet printing to exploit interfacial fluid forces and guide molecular self‐assembly into aligned or disordered nanofibers, hydrogel structures of different geometries and sizes, surface topographies, and higher‐ordered constructs bound by molecular diffusion. PAs are designed to co‐assemble during printing in cell diluent conditions with a range of extracellular matrix (ECM) proteins and biomolecules including fibronectin, collagen, keratin, elastin‐like proteins, and hyaluronic acid. Using combinations of these molecules, NIH‐3T3 and adipose derived stem cells are bioprinted within complex structures while exhibiting high cell viability (>88%). By integrating self‐assembly with 3D‐bioprinting, the study introduces a novel biofabrication platform capable of encapsulating and spatially distributing multiple cell types within tuneable pericellular environments. In this way, the work demonstrates the potential of the approach to generate complex bioactive scaffolds for applications such as tissue engineering, in vitro models, and drug screening.     

Direct‐Write Assembly of Microperiodic Silk Fibroin Scaffolds for Tissue Engineering Applications

Three–dimensional, microperiodic scaffolds of regenerated silk fibroin have been fabricated for tissue engineering by direct ink writing. The ink, which consisted of silk fibroin solution from the Bombyx mori silkworm, was deposited in a layer‐by‐layer fashion through a fine nozzle to produce a 3D array of silk fibers of diameter 5 µm. The extruded fibers crystallized when deposited into a methanol‐rich reservoir, retaining a pore structure necessary for media transport. The rheological properties of the silk fibroin solutions were investigated and the crystallized silk fibers were characterized for structure and mechanical properties by infrared spectroscopy and nanoindentation, respectively. The scaffolds supported human bone marrow‐derived mesenchymal stem cell (hMSC) adhesion, and growth. Cells cultured under chondrogenic conditions on these scaffolds supported enhanced chondrogenic differentiation based on increased glucosaminoglycan production compared to standard pellet culture. Our results suggest that 3D silk fibroin scaffolds may find potential application as tissue engineering constructs due to the precise control of their scaffold architecture and their biocompatibility.     

Injection and Self‐Assembly of Bioinspired Stem Cell‐Laden Gelatin/Hyaluronic Acid Hybrid Microgels Promote Cartilage Repair In Vivo

In this paper, a novel bioinspired stem cell‐laden microgel and related in vivo cartilage repair strategy are proposed. In particular, herein the preparation of new stem cell‐laden microgels, which can be injected into the chondral defect site in a minimally invasive way, and more importantly, capable of in situ self‐assembly into 3D macroporous scaffold without external stimuli, is presented. Specifically, thiolated gelatin (Gel‐SH) and vinyl sulfonated hyaluronic acid (HA‐VS) are first synthesized, and then stem cell‐laden gelatin/hyaluronic acid hybrid microgels (Gel‐HA) are generated by mixing Gel‐SH, HA‐VS, and bone mesenchymal stem cells (BMSCs) together via droplet‐based microfluidic approach, followed by gelation through fast and efficient thiol‐Michael addition reaction. The encapsulated BMSCs show high viability, proliferation, and chondrogenic differentiation potential in the microgels. Moreover, the in vitro test proves that BMSC‐laden Gel‐HA microgels are injectable without sacrificing BMSC viability, and more importantly, can self‐assemble into cartilage‐like scaffolds via cell–cell interconnectivity. In vivo experiments further confirm that the self‐assembled microgels can inhibit vascularization and hypertrophy. The Gel‐HA microgels and relevant cartilage repair strategy, i.e., injecting BMSC‐laden microgels separately and reconstructing chondral defect structure by microgel self‐assembly, provides a simple and effective method for cartilage tissue engineering and regenerative medicine.     

Biomimetic Structures: Biological Implications of Dipeptide‐Substituted Polyphosphazene–Polyester Blend Nanofiber Matrices for Load‐Bearing Bone Regeneration

Successful bone regeneration benefits from three‐dimensional (3D) bioresorbable scaffolds that mimic the hierarchical architecture and mechanical characteristics of native tissue extracellular matrix (ECM). A scaffold platform that integrates unique material chemistry with nanotopography while mimicking the 3D hierarchical bone architecture and bone mechanics is reported. A biocompatible dipeptide polyphosphazene‐polyester blend is electrospun to produce fibers in the diameter range of 50–500 nm to emulate dimensions of collagen fibrils present in the natural bone ECM. Various electrospinning and process parameters are optimized to produce blend nanofibers with good uniformity, appropriate mechanical strength, and suitable porosity. Biomimetic 3D scaffolds are created by orienting blend nanofiber matrices in a concentric manner with an open central cavity to replicate bone marrow cavity, as well as the lamellar structure of bone. This biomimicry results in scaffold stress–strain curve similar to that of native bone with a compressive modulus in the mid‐range of values for human trabecular bone. Blend nanofiber matrices support adhesion and proliferation of osteoblasts and show an elevated phenotype expression compared to polyester nanofibers. Furthermore, the 3D structure encourages osteoblast infiltration and ECM secretion, bridging the gaps of scaffold concentric walls during in vitro culture. The results also highlight the importance of in situ ECM secretion by cells in maintaining scaffold mechanical properties following scaffold degradation with time. This study for the first time demonstrates the feasibility of developing a mechanically competent nanofiber matrix via a biomimetic strategy and the advantages of polyphosphazene blends in promoting osteoblast phenotype progression for bone regeneration.     

Piezoelectric Nanotopography Induced Neuron‐Like Differentiation of Stem Cells

The biophysical characteristics of the extracellular matrix, such as nanotopography and bioelectricity, have a profound influence on cell proliferation, adhesion, differentiation, etc. Recognition of the function of a certain biophysical cue and fabrication of biomaterial scaffolds with specific properties would have important implications and significant applications in tissue engineering. Herein, nanotopographic and piezoelectric biomaterials are fabricated and the combination effect of and individual contribution to proliferation, adhesion, and neuron‐like differentiation of rat bone marrow‐derived mesenchymal stem cells (rbMSCs) are clarified via nanotopography and piezoelectricity. Piezoelectric polyvinylidene fluoride with nanostripe array structures is fabricated, which can generate a surface piezoelectric potential up to millivolt by cell movement and traction. The results reveal a more favorable effect on neuron‐like differentiation of rbMSCs from the combination of piezoelectricity and nanotopography rather than nanotopography alone, whereas nanotopography can increase cellular adhesion. This research provides a new insight into designing biomaterials for the potential application in neural tissue engineering.     

Biomimetic Elastomeric Bioactive Siloxane‐Based Hybrid Nanofibrous Scaffolds with miRNA Activation: A Joint Physico‐Chemical‐Biological Strategy for Promoting Bone Regeneration

Rapid and efficient disease‐induced or critical‐size bone regeneration remains a challenge in tissue engineering due to the lack of highly bioactive biomaterial scaffolds. Physical structures such as nanostructures, chemical components such as silicon elements, and biological factors such as genes have shown positive effects on bone regeneration. Herein, a bioactive photoluminescent elastomeric silicate‐based nanofibrous scaffold with sustained miRNA release is reported for promoting bone regeneration based on a joint physico‐chemical‐biological strategy. Bioactive nanofibrous scaffolds are fabricated by cospinning poly (ε‐caprolactone) (PCL), elastomeric poly (citrates‐siloxane) (PCS), and bioactive osteogenic miRNA nanocomplexes (denoted PPM nanofibrous scaffolds). The PPM scaffolds possess uniform nanostructures, significantly enhanced tensile stress (≈15 MPa) and modulus (≈32 MPa), improved hydrophilicity (30–60°), controlled biodegradation, and strong blue fluorescence. Bioactive miRNA complexes are efficiently loaded into the nanofibrous matrix and exhibit long‐term release for up to 70 h. The PPM scaffolds significantly promote the adhesion, proliferation, and osteoblast differentiation of bone marrow stem cells in vitro and enhanced rat cranial defect restoration (12 weeks) in vivo. This work reports an attractive joint physico‐chemical‐biological strategy for the design of novel cell/protein‐free bioactive scaffolds for synergistic tissue regeneration.     

Channeled β‐TCP Scaffolds Promoted Vascularization and Bone Augmentation in Mandible of Beagle Dogs

A major hindrance to successful alveolar bone augmentation and ridge preservation using synthetic scaffolds is insufficient vascularization in the implanted bone grafts. The slow ingrowth of host vasculature from the bone bed of alveolar bone to the top of the implanted bone grafts leads to limited bone formation in the upper layers of the implanted grafts, which hinders the subsequent implantation of titanium dental implants. In this study, macroporous beta‐tricalcium phosphate (β‐TCP) scaffolds with multiple vertical hollow channels are fabricated that play a similar role as blood vessels for nutrient diffusion and cell migration. The results show that the hollow channels accelerate the degradation rate of the β‐TCP scaffolds and the in vitro release of a bone forming peptide‐1, which significantly promote proliferation and osteogenesis of human bone mesenchymal stem cells on the channeled scaffolds, compared to nonchanneled scaffolds in vitro. More volume of newly formed bone tissues with more blood vessels are augmented in the channeled scaffolds when implanted in mandibular bone defects of beagle dogs. Channeled scaffolds significantly promote new bone formation and augment the height of the mandible. These findings indicate channeled scaffolds facilitate vascularization and bone formation and have great potential for vascularized bone augmentation.     

2D and 3D Hybrid Systems for Enhancement of Chondrogenic Differentiation of Tonsil‐Derived Mesenchymal Stem Cells

2D/3D hybrid cell culture systems are constructed by increasing the temperature of the thermogelling poly(ethylene glycol)‐poly(l ‐alanine) diblock copolymer (PEG‐l ‐PA) aqueous solution in which tonsil tissue‐derived mesenchymal stem cells and graphene oxide (GO) or reduced graphene oxide (rGO) are suspended, to 37 °C. The cells exhibit spherical cell morphologies in 2D/3D hybrid culture systems of GO/PEG‐l ‐PA and rGO/PEG‐l ‐PA by using the growth medium. The cell proliferations are 30%–50% higher in the rGO/PEG‐l ‐PA hybrid system than in the GO/PEG‐l ‐PA hybrid system. When chondrogenic culture media enriched with TGF‐β3 is used in the 2D/3D hybrid systems, cells extensively aggregate, and the expression of chondrogenic biomarkers of SOX 9, COL II A1, COL II, and COL X significantly increases in the GO/PEG‐l ‐PA 2D/3D hybrid system as compared with the PEG‐l ‐PA 3D systems and rGO/PEG‐l ‐PA 2D/3D hybrid system, suggesting that the GO/PEG‐l ‐PA 2D/3D hybrid system can be an excellent candidate as a chondrogenic differentiation platform of the stem cell. This paper also suggests that a 2D/3D hybrid system prepared by incorporating 2D materials with various surface biofunctionalities in the in situ forming 3D hydrogel matrix can be a new cell culture system.     

Tumor‐Microenvironment‐Adaptive Nanoparticles Codeliver Paclitaxel and siRNA to Inhibit Growth and Lung Metastasis of Breast Cancer

Prolonged circulation, specific and effective uptake by tumor cells, and rapid intracellular drug release are three main factors for the drug delivery systems to win the battle against metastatic breast cancer. In this work, a tumor microenvironment‐adaptive nanoparticle co‐loading paclitaxel (PTX) and the anti‐metastasis siRNA targeting Twist is prepared. The nanoparticle consists of a pH‐sensitive core, a cationic shell, and a matrix metalloproteinase (MMP)‐cleavable polyethylene glycol (PEG) corona conjugated via a peptide linker. PEG will be cut away by MMPs at the tumor site, which endows the nanoparticle with smaller particle size and higher positive charge, leading to more efficient cellular uptake in tumor cells and higher intra‐tumor accumulation of both PTX and siRNA in the 4T1 tumor‐bearing mice models compared to the nanoparticles with irremovable PEG. In addition, acid‐triggered drug release in endo/lysosomes is achieved through the pH‐sensitive core. As a result, the MMP/pH dual‐sensitive nanoparticles significantly inhibit tumor growth and pulmonary metastasis. Therefore, this tumor‐microenvironment‐adaptive nanoparticle can be a promising codelivery vector for effective therapy of metastatic breast cancer due to simultaneously satisfying the requirements of long circulating time, efficient tumor cell targeting, and fast intracellular drug release.     

Black‐Phosphorus‐Incorporated Hydrogel as a Conductive and Biodegradable Platform for Enhancement of the Neural Differentiation of Mesenchymal Stem Cells

Conductive hydrogel scaffolds have important applications for electroactive tissue repairs. However, the development of conductive hydrogel scaffolds tends to incorporate nonbiodegradable conductive nanomaterials that will remain in the human body as foreign matters. Herein, a biodegradable conductive hybrid hydrogel is demonstrated based on the integration of black phosphorus (BP) nanosheets into the hydrogel matrix. To address the challenge of applying BP nanosheets in tissue engineering due to its intrinsic instability, a polydopamine (PDA) modification method is developed to improve the stability. Moreover, PDA modification also enhances interfacial bonding between pristine BP nanosheets and the hydrogel matrix. The incorporation of polydopamine‐modified black phosphorous (BP@PDA) nanosheets into the gelatin methacryloyl (GelMA) hydrogels significantly enhances the electrical conductivity of the hydrogels and improves the cell migration of mesenchymal stem cells (MSCs) within the 3D scaffolds. On the basis of the gene expression and protein level assessments, the BP@PDA‐incorporated GelMA scaffold can significantly promote the differentiation of MSCs into neural‐like cells under the synergistic electrical stimulation. This strategy of integrating biodegradable conductive BP nanomaterials within a biocompatible hydrogel provides a new insight into the design of biomaterials for broad applications in tissue engineering of electroactive tissues, such as neural, cardiac, and skeletal muscle tissues.     

A Non‐Mulberry Silk Fibroin Protein Based 3D In Vitro Tumor Model for Evaluation of Anticancer Drug Activity

The limitations of clinical chemotherapy are credited primarily to drug resistance. Effective development and screening of new drugs require appropriate in vitro tumor models that resemble the in vivo situation to evaluate drug efficiency and to decrease the use of experimental animals. 3D in vitro model systems that are able to mimic in vivo microenvironments are now highly sought after in cancer research. Here, the characteristics of breast cancer cell line MDA‐MB‐231 cells on 3D, and 2D Antheraea mylitta silk matrices and tissue culture plates are compared. After long term culture of breast cancer cells in the silk scaffold, the engineered tumor construct shows different zones of cell proliferation, such as an avascular tumor. Silk fibroin matrix 3D tumor models are studied for the evaluation of various anticancer drugs. The cytotoxic effects of three different drugs (Paclitaxel, Celecoxib, and ZD6474) at different concentrations are evaluated for MDA‐MB‐231 grown on 2D films as well as on a 3D fibroin scaffold. Higher drug concentrations are required to achieve a comparable reduction in cell viability and invasive potential in 3D culture. Combinatorial treatment of drugs at IC₅₀ concentrations result in up to 84% death of cancer cells. The results indicate that 3D in vitro tumor models may be better systems to evaluate cancer treatment strategies.     

Autologous Versatile Vesicles‐Incorporated Biomimetic Extracellular Matrix Induces Biomineralization

Restoring extracellular matrix (ECM) with supportive and osteoinductive abilities is of great significance for bone tissue regeneration. Current approaches involving cell‐based scaffolds or nanoparticle‐modified biomimic‐ECM have been met with additional biosafety concerns. Herein, the natural biomineralization process is first analyzed and is found that mesenchymal stem cells‐derived extracellular vesicles (EVs) from early and late stages of osteoinduction play different roles during the mineralization process. The functional EVs hierarchically with blood‐derived autohydrogel (AH) are then incorporated to form an osteoinductive biomimetic extracellular matrix (BECM). The alkaline phosphatase‐rich EVs are released from the outer layers to induce osteoblast differentiation during early stages. Thereafter, as the degradation of AH occurred, calcium/phosphorus (Ca/P)‐rich EVs are liberated to promote the nucleation of extracellular mineral crystals. Additionally, BECM contains considerable collagen fibrils that provide additional nucleation sites for crystallites deposition, thus reaching self‐mineralization in situ. In conclusion, this research provides a promising, versatile mineralization‐instructive platform to tackle the challenges faced in bone‐tissue engineering.     

Osteoconductive Performance of Carbon Nanotube Scaffolds Homogeneously Mineralized by Flow‐Through Electrodeposition

The treatment of bone lesions, including fractures, tumor resection and osteoporosis, is a common clinical practice where bone healing and repair are pursued. It is widely accepted that calcium phosphate‐based materials improve integration of biomaterials with surrounding bone tissue and further serve as a template for proper function of bone‐forming cells. Within this context, mineralization on preformed substrates appears as an interesting and successful alternative for mineral surface functionalization. However, mineralization of “true” 3D scaffolds –in which the magnitude of the third dimension is within the same scale as the other two– is by no means a trivial issue because of the difficulty to obtain a homogeneous mineral layer deposited on the entire internal surface of the scaffold. Herein, a “flow‐through” electrodeposition process is applied for mineralization of 3D scaffolds composed of multiwall carbon nanotubes and chitosan. It is demonstrated that, irrespective of the experimental conditions used for electrodeposition (e.g., time, temperature and voltages), the continuous feed of salts provided by the use of a flow‐through configuration is the main issue if one desires to coat the entire internal structure of 3D scaffolds with a homogeneous mineral layer. Finally, mineralized scaffolds not only showed a remarkable biocompatibility when tested with human osteoblast cells, but also enhanced osteoblast terminal differentiation (as early as 7 days in calcifying media).     

A Bifunctional Biomaterial with Photothermal Effect for Tumor Therapy and Bone Regeneration

Malignant bone tumor is one of the major bone diseases. The treatment of such a bone disease typically requires the removal of bone tumor and regeneration of tumor‐initiated bone defects simultaneously. To address this issue, it is required that implanted biomaterials should combine the bifunctions of both therapy and regeneration. In this work, a bifunctional graphene oxide (GO)‐modified β‐tricalcium phosphate (GO‐TCP) composite scaffold combining a high photothermal effect with significantly improved bone‐forming ability is prepared by 3D‐printing and surface‐modification strategies. The prepared GO‐TCP scaffolds exhibit excellent photothermal effects under the irradiation of 808 nm near infrared laser (NIR) even at an ultralow power density of 0.36 W cm^?2, while no photothermal effects are observed for pure β‐TCP scaffolds. The photothermal temperature of GO‐TCP scaffolds can be effectively modulated in the range of 40–90 °C by controlling the used GO concentrations, surface‐modification times, and power densities of NIR. The distinct photothermal effect of GO‐TCP scaffolds induces more than 90% of cell death for osteosarcoma cells (MG‐63) in vitro, and further effectively inhibits tumor growth in mice. Meanwhile, the prepared GO‐TCP scaffolds possess the improved capability to stimulate the osteogenic differentiation of rabbit bone mesenchymal stem cells (rBMSCs) by upregulating bone‐related gene expression, and significantly promote new bone formation in the bone defects of rabbits as compared to pure β‐TCP scaffolds. These results successfully demonstrate that the prepared GO‐TCP scaffolds have bifunctional properties of photothermal therapy and bone regeneration, which is believed to pave the way to design and fabricate novel implanting biomaterials in combination of therapy and regeneration functions.     

Microphysiological Systems as Enabling Tools for Modeling Complexity in the Tumor Microenvironment and Accelerating Cancer Drug Development

Tumor cell heterogeneity with distinct phenotypes, genotypes, and epigenetic states as well as the complex tumor microenvironment is major challenges for cancer diagnosis and treatment. There have been substantial advances in our knowledge of tumor biology and in the capabilities of available biological analysis tools; however, the absence of physiologically relevant in vitro testing platforms limits our ability to gain an in‐depth understanding of the role of the tumor microenvironment in cancer pathology. In this review, recent advances in engineered tumor microenvironments to advance cancer research and drug discovery are presented, including tumor spheroids, microfluidic chips, paper scaffolds, hydrogel‐based engineered tissues, 3D bioprinted scaffolds, and multiscale topography. Furthermore, how these technologies address the specific characteristics of the native tumor microenvironment is described. Through the comparison of these biomimetic 3D tumor models to conventional 2D culture models, the validity and physiological relevance of these platforms for fundamental in vitro studies of the tumor biology, as well as their potential use in drug screening applications, is also discussed.     

Direct 3D Printed Biomimetic Scaffolds Based on Hydrogel Microparticles for Cell Spheroid Growth

Biocompatible hydrogel inks with shear‐thinning, appropriate yield strength, and fast self‐healing are desired for 3D bioprinting. However, the lack of ideal 3D bioprinting inks with outstanding printability and high structural fidelity, as well as cell‐compatibility, has hindered the progress of extrusion‐based 3D bioprinting for tissue engineering. In this study, novel self‐healable pre‐cross‐linked hydrogel microparticles (pcHμPs) of chitosan methacrylate (CHMA) and polyvinyl alcohol (PVA) hybrid hydrogels are developed and used as bioinks for extrusion‐based 3D printing of scaffolds with high fidelity and biocompatibility. The pcHμPs display excellent shear thinning when injected through a syringe and subsequently self‐heal into gels as shear forces are removed. Numerical simulations indicate that the pcHμPs experience a plug flow in the nozzle with minimal disturbance, which favors a steady and continuous printing. Moreover, the pcHμPs show a self‐supportive yield strength (540 Pa), which is critical for the fidelity of printed constructs. A series of biomimetic constructs with very high aspect ratio and delicate fine structures are directly printed by using the pcHμP ink. The 3D printed scaffolds support the growth of bone‐marrow‐derived mesenchymal stem cells and formation of cell spheroids, which are most important for tissue engineering.     

Thermodynamically Controlled Self‐Assembly of Hierarchically Staggered Architecture as an Osteoinductive Alternative to Bone Autografts

Osteoinductive synthetic biomaterials for replacing autografts can be developed by mimicking bone hierarchy and surface topography for host cell recruitment and differentiation. Until now, it has been challenging to reproduce a bone‐like staggered hierarchical structure since the energy change underlying synthetic pathways in vitro is essentially different from that of the natural process in vivo. Herein, a bone‐like hierarchically staggered architecture is reproduced under thermodynamic control involving two steps: fabrication of a high‐energy polyacrylic acid‐calcium intermediate and selective mineralization in collagenous gap regions driven by an energetically downhill process. The intermediate energy interval could easily be adjusted to determine different mineralization modes, with distinct morphologies and biofunctions. Similar to bone autografts, the staggered architecture offers a bone‐specific microenvironment for stem cell recruitment and multidifferentiation in vitro, and induces neo‐bone formation with bone marrow blood vessels by host stem cell homing in vivo. This work provides a novel perspective for an in vitro simulating biological mineralization process and proof of concept for the clinical application of smart biomaterials.