The Development of Lung Tissue Engineering: From Biomaterials to Multicellular Systems.

The challenge of the treatment of end-stage lung disease poses an urgent clinical demand for lung tissue engineering. Over the past few years, various lung tissue-engineered constructs are developed for lung tissue regeneration and respiratory pathology study. In this review, an overview of recent achievements in the field of lung tissue engineering is proposed. The introduction of lung structure and lung injury are stated briefly at first. After that, the lung tissue-engineered constructs are categorized into three types: acellular, monocellular, and multicellular systems. The different bioengineered constructs included in each system that can be applied to the reconstruction of the trachea, airway epithelium, alveoli, and even whole lung are described in detail, followed by the highlight of relevant representative research. Finally, the challenges and future directions of biomaterials, manufacturing technologies, and cells involved in lung tissue engineering are discussed. Overall, this review can provide referable ideas for the realization of functional lung regeneration and permanent lung substitution.

Xonotlite Nanowire-Containing Bioactive Scaffolds for the Therapy of Defective Adipose Tissue in Breast Cancer.

Considering the challenge in the treatment of severe breast tumor patients, xonotlite nanowire-containing bioactive scaffolds (Fe3O4-CS-GelMA) were fabricated by the 3D-printing technique for the therapy of injured adipose tissue after surgery. Importantly, benefiting from the excellent magnetothermal performance of Fe3O4 microspheres, Fe3O4-CS-GelMA scaffolds could effectively kill tumor cells in vitro and suppress breast cancer in vivo under an alternating magnetic field, and the tumor did not recur in 2 weeks. In addition, attributed to the released bioactive inorganic ions, Fe3O4-CS-GelMA composite scaffolds could effectively promote the expression of adipogenesis-related genes and proteins of adipose-derived stem cells (ADSCs) via the PI3K-AKT signaling pathway in vitro. Furthermore, Fe3O4-CS-GelMA scaffolds with ADSCs could obviously stimulate the formation of adipose in vivo, compared with that of pure GelMA without inorganic components. Therefore, this study offers a promising strategy for the therapy of breast tumors after the surgical excision of breast carcinoma.

3D multicellular micropatterning biomaterials for hair regeneration and vascularization.

Hair loss caused by the abnormal functions of hair follicles in skin can seriously impact the quality of an individual's life. The development of sophisticated skin tissue-engineered constructs is required to enable the function recovery of hair follicles. However, effective hair regrowth in skin substitutes still remains a great challenge. In this study, a 3D multicellular micropattern was successfully fabricated by arranging the hair follicle-related cells orderly distributed in the interval of vascular-cell networks via bioprinting technology. By combining the stable biomimetic micropattern structure and the bio-inducing substrate incorporated with magnesium silicate (MS) nanomaterials, the 3D multicellular micropattern possessed significant follicular potential and angiogenic capacity in vitro. Furthermore, the 3D multicellular micropattern with MS incorporation contributed to efficient hair regrowth during skin tissue regeneration in both immunodeficient mice and androgenetic alopecia (AGA) mice models. Thus, this study proposes a novel 3D micropatterned multicellular system assembling a biomimetic micro-structure and modulating the cell-cell interaction for hair regeneration during skin reconstruction.

Polyhedron-Like Biomaterials for Innervated and Vascularized Bone Regeneration.

Neural-vascular networks are densely distributed through periosteum, cortical bone, and cancellous bone, which is of great significance for bone regeneration and remodeling. Although significant progress has been made in bone tissue engineering, ineffective bone regeneration, and delayed osteointegration still remains an issue due to the ignorance of intrabony nerves and blood vessels. Herein, inspired by space-filling polyhedra with open architectures, polyhedron-like scaffolds with spatial topologies are prepared via 3D-printing technology to mimic the meshwork structure of cancellous bone. Benefiting from its spatial topologies, polyhedron-like scaffolds greatly promoted the osteogenic differentiation of bone mesenchymal stem cells (BMSCs) via activating PI3K-Akt signals, and exhibiting satisfactory performance on angiogenesis and neurogenesis. Computational fluid dynamic (CFD) simulation elucidates that polyhedron-like scaffolds have a relatively lower area-weighted average static pressure, which is beneficial to osteogenesis. Furthermore, in vivo experiments further demonstrate that polyhedron-like scaffolds obviously promote bone formation and osteointegration, as well as inducing vascularization and ingrowth of nerves, leading to innervated and vascularized bone regeneration. Taken together, this work offers a promising approach for fabricating multifunctional scaffolds without additional exogenous seeding cells and growth factors, which holds great potential for functional tissue regeneration and further clinical translation.

Cell-Laden Scaffolds for Vascular-Innervated Bone Regeneration.

For regeneration of highly vascularized and innervated tissues, like bone, simultaneous ingrowth of blood vessels and nerves is essential but largely neglected. To address this issue, a "pre-angiogenic" cell-laden scaffold with durable angiogenic functions is prepared according to the bioactivities of silicate bioceramics and the instructive effects of vascular cells on neurogenesis and bone repair. Compared with traditional cell-free scaffolds, the prepared cell-laden scaffolds printed with active cells and bioactive inks can support long-term cell survival and growth for three weeks. The long-lived scaffolds exhibited durable angiogenic capability both in vitro and in vivo. The pre-angiogenic scaffolds can induce the neurogenetic differentiation of neural cells and the osteogenic differentiation of mesenchymal stem cells by the synergistic effects of released bioactive ions and the ability of vascular cells to attract neurons. The enhanced bone regeneration with both vascularization and innervation is attributed to these physiological functions of the pre-angiogenic cell-laden scaffolds, which is defined as "vascular-innervated" bone regeneration. It is suggested that the concept of "vascular-innervated scaffolds" may represent the future direction of biomaterials for complex tissue/organ regeneration.

3D Printing of Diatomite Incorporated Composite Scaffolds for Skin Repair of Deep Burn Wounds.

Deep burn injury always causes severe damage of vascular network and collagen matrix followed by delayed wound healing process. In this study, natural diatomite (DE) microparticles with porous nanostructure were separated based on the particles size through a dry sieving method and combined with gelatin methacryloyl (GelMA) hydrogel to form a bioactive composite ink. The DE-containing inorganic/organic composite scaffolds, which were successfully prepared through three-dimensional (3D) printing technology, were used as functional burn wound dressings. The scaffolds incorporated with DE are of great benefit to several cellular activities, including cell spreading, proliferation, and angiogenesis-related gene expression in vitro, which can mainly be attributed to the positive effect of bioactive silicon (Si) ions released from the embedded DE. Moreover, due to establishment of bioactive ionic environment, the deep burn wounds treated with 3D-printed DE incorporated scaffolds exhibited rapid wound healing rate, enhanced collagen deposition, and dense blood vessel formation in vivo. Therefore, the present study demonstrates that the cost-effective DE can be used as biocompatible Si source to significantly promote the bioactivities of wound dressings for effective tissue regeneration.

Bioactive inorganic particles-based biomaterials for skin tissue engineering.

The challenge for treatment of severe cutaneous wound poses an urgent clinical need for the development of biomaterials to promote skin regeneration. In the past few decades, introduction of inorganic components into material system has become a promising strategy for improving performances of biomaterials in the process of tissue repair. In this review, we provide a current overview of the development of bioactive inorganic particles-based biomaterials used for skin tissue engineering. We highlight the three stages in the evolution of the bioactive inorganic biomaterials applied to wound management, including single inorganic materials, inorganic/organic composite materials, and inorganic particles-based cell-encapsulated living systems. At every stage, the primary types of bioactive inorganic biomaterials are described, followed by citation of the related representative studies completed in recent years. Then we offer a brief exposition of typical approaches to construct the composite material systems with incorporation of inorganic components for wound healing. Finally, the conclusions and future directions are suggested for the development of novel bioactive inorganic particles-based biomaterials in the field of skin regeneration.

Dynamic crosslinked polymeric nano-prodrugs for highly selective synergistic chemotherapy.

To achieve highly selective synergistic chemotherapy attractive for clinical translation, the precise polymeric nano-prodrugs (PPD-NPs) were successfully constructed via the facile crosslinking reaction between pH-sensitive poly(ortho ester)s and reduction-sensitive small molecule synergistic prodrug (Pt(IV)-1). PPD-NPs endowed the defined structure and high drug loading of cisplatin and demethylcantharidin (DMC). Moreover, PPD-NPs exhibited steady long-term storage and circulation via the crosslinked structure, suitable negative potentials and low critical micelle concentration (CMC), improved selective tumour accumulation and cellular internalization via dynamic size transition and surficial amino protonation at tumoural extracellular pH, promoted efficient disintegration and drug release at tumoural intracellular pH/glutathione, and enhanced cytotoxicity via the synergistic effect between cisplatin and DMC with the feed ratio of 1:2, achieving significant tumour suppression while decreasing the side effects. Thus, the dynamic crosslinked polymeric nano-prodrugs exhibit tremendous potential for clinically targeted synergistic cancer therapy.

3D Printing of Black Bioceramic Scaffolds with Micro/Nanostructure for Bone Tumor-Induced Tissue Therapy.

It is common to improve the relevant performance in the field of energy storage materials or catalytic materials by regulating the number of defects. However, there are few studies on the biomaterials containing defects for tissue engineering. Herein, a new type of defect-rich scaffolds, black akermanite (B-AKT) bioceramic scaffolds with micro/nanostructure, the thickness of which is from 0.14 to 1.94 µm, is fabricated through introducing defects on the surface of bioceramic scaffolds. The B-AKT scaffolds have advantages on the degradation rate and the osteogenic capacity over the AKT (Ca2 MgSi2 O7 ) scaffolds due to the surface defects which stimulate the osteogenic differentiation of rabbit bone mesenchymal stem cells via activating bone morphogenetic protein 2 （BMP2） signaling pathway and further promote bone formation in vivo. In addition, the prepared B-AKT scaffolds, the temperature of which can be over 100 °C under the near infrared （NIR） irradiation (0.66 W cm-2 ), possess excellent performance on photothermal and antitumor effects. The work develops an introducing-defect strategy for regulating the biological performance of bioceramic scaffolds, which is expected to be applied in the next generation of bioceramic scaffolds for regenerative medicine.

3D-printed bioceramic scaffolds with Fe3S4microflowers for magnetothermal and chemodynamic therapy of bone tumor and regeneration of bone defects.

Elimination of residual osteosarcoma cells and repair of bone defects remain major challenges for osteosarcoma in clinic. To address this problem, it is required that multifunctional therapeutic platform possess high tumor-killing efficiency and simultaneous bone regeneration capabilities. In this work, an intelligent therapeutic platform was developed to achieve highly-efficient tumor therapy and simultaneous significantly improved bone defect repairing ability, which was realized byin situgrowing ferromagnetic Fe3S4layers with tuned microstructures on the surface of 3D-printed akermanite bioceramic scaffolds via hydrothermal method. The Fe3S4layers exploited magnetic thermal energy to enhance chemodynamic treatment, thus achieving a synergistic effect between magnetothermal and chemodynamic therapy on the elimination of residual tumor cells. Moreover, the micro-structured surface of the 3D-printed bioceramic scaffolds further enhanced the osteogenic activityin vitroand accelerated the bone regenerationin vivo. The scaffolds with multi-mode tumor-killing and bone repairing capabilities indicated that such a therapeutic platform is applicable for a stepwise treatment strategy of osteosarcoma and provides inspiration for the design of multifunctional biomaterials.

3D Printing of Strontium Silicate Microcylinder-Containing Multicellular Biomaterial Inks for Vascularized Skin Regeneration.

The reconstruction of dermal blood vessels is essential for skin regeneration process. However, the lack of vascular structure, insufficient angiogenesis induction, and ineffective graft-host anastomosis of the existing skin substitutes are major bottle-necks for permanent skin replacement in tissue engineering. In this study, the uniform strontium silicate (SS) microcylinders are successfully synthesized and integrated into the biomaterial ink to serve as stable cell-induced factors for angiogenesis, and then a functional skin substitute based on a vascularization-induced biomimetic multicellular system is prepared via a "cell-writing" bioprinting technology. With an unprecedented combination of vascularized skin-mimicking structure and vascularization-induced function, the SS-containing multicellular system exhibits outstanding angiogenic activity both in vitro and in vivo. As a result, the bioprinted skin substitutes significantly accelerate the healing of both acute and chronic wounds by promoting the graft-host integration and vascularized skin regeneration in three animal models. Therefore, the study provides a referable strategy to fabricate biomimetic multicellular constructs with angiogenesis-induced function for regeneration of vascularized complex and hierarchical tissues.

3D printing of Haversian bone-mimicking scaffolds for multicellular delivery in bone regeneration.

The integration of structure and function for tissue engineering scaffolds is of great importance in mimicking native bone tissue. However, the complexity of hierarchical structures, the requirement for mechanical properties, and the diversity of bone resident cells are the major challenges in constructing biomimetic bone tissue engineering scaffolds. Herein, a Haversian bone-mimicking scaffold with integrated hierarchical Haversian bone structure was successfully prepared via digital laser processing (DLP)-based 3D printing. The compressive strength and porosity of scaffolds could be well controlled by altering the parameters of the Haversian bone-mimicking structure. The Haversian bone-mimicking scaffolds showed great potential for multicellular delivery by inducing osteogenic, angiogenic, and neurogenic differentiation in vitro and accelerated the ingrowth of blood vessels and new bone formation in vivo. The work offers a new strategy for designing structured and functionalized biomaterials through mimicking native complex bone tissue for tissue regeneration.