Tunable biomimetic materials elaborated by ice templating and self-assembly of collagen for tubular tissue engineering

Synthetic tubular grafts currently used in clinical context fail frequently, and the expectations that biomimetic materials could tackle these limitations are high. However, developing tubular materials presenting structural, compositional and functional properties close to those of native tissues remains an unmet challenge. Here we describe a combination of ice templating and topotactic fibrillogenesis of type I collagen, the main component of tissues' extracellular matrix, yielding highly concentrated yet porous tubular collagen materials with controlled hierarchical architecture at multiple length scales, the hallmark of native tissues' organization. By modulating the thermal conductivity of the cylindrical molds, we tune the macroscopic porosity defined by ice. Coupling the aforementioned porosity patterns with two different fibrillogenesis routes results in a new family of tubular materials whose textural features and the supramolecular arrangement of type I collagen are achieved. The resulting materials present hierarchical elastic properties and are successfully colonized by human endothelial cells and alveolar epithelial cells on the luminal side, and by human mesenchymal stem cells on the external side. The proposed straightforward protocol is likely to be adapted for larger graft sizes that address ever-growing clinical needs, such as peripheral arterial disease or tracheal and bronchial reconstructions.

Magnetically anisotropic hydrogels for tissue engineering

Many soft tissues of the human body possess hierarchically anisotropic structures, exhibiting orientation-specific mechanical properties and biological functionality. Hydrogels have been proposed as promising scaffold materials for tissue engineering applications due to their water-rich composition, excellent biocompatibility, and tunable physico-chemical properties. However, conventional hydrogels with homogeneous structures often exhibit isotropic properties that differ from those of biological tissues, limiting their further application. Recently, magnetically anisotropic hydrogels containing long-range ordered magneto-structures have attracted widespread interest owing to their highly controllable assembly strategy, rapid magnetic responsiveness and remote spatiotemporal regulation. In this review, we summarize the latest progress of magnetically anisotropic hydrogels for tissue engineering. The fabrication strategy of magnetically anisotropic hydrogels from magnetic nanofillers with different dimensions is systemically introduced. Then, the effects of magnetically anisotropic cues on the physicochemical properties of hydrogels and the cellular biological behavior are discussed. And the applications of magnetically anisotropic hydrogels in the engineering of different tissues are emphasized. Finally, the current challenges and the future perspectives for magnetically anisotropic hydrogels are presented.

Low-temperature deposition manufacturing technology: a novel 3D printing method for bone scaffolds

The application of three-dimensional printing technology in the medical field has great potential for bone defect repair, especially personalized and biological repair. As a green manufacturing process that does not involve liquefication through heating, low-temperature deposition manufacturing (LDM) is a promising type of rapid prototyping manufacturing and has been widely used to fabricate scaffolds in bone tissue engineering. The scaffolds fabricated by LDM have a multi-scale controllable pore structure and interconnected micropores, which are beneficial for the repair of bone defects. At the same time, different types of cells or bioactive factor can be integrated into three-dimensional structural scaffolds through LDM. Herein, we introduced LDM technology and summarize its applications in bone tissue engineering. We divide the scaffolds into four categories according to the skeleton materials and discuss the performance and limitations of the scaffolds. The ideas presented in this review have prospects in the development and application of LDM scaffolds.

Circulating Tumor Cells in Cancer Diagnostics and Prognostics by Single-Molecule and Single-Cell Characterization

Circulating tumor cells (CTCs) represent an interesting source of biomarkers for diagnosis, prognosis, and the prediction of cancer recurrence, yet while they are extensively studied in oncobiology research, their diagnostic utility has not yet been demonstrated and validated. Their scarcity in human biological fluids impedes the identification of dangerous CTC subpopulations that may promote metastatic dissemination. In this Perspective, we discuss promising techniques that could be used for the identification of these metastatic cells. We first describe methods for isolating patient-derived CTCs and then the use of 3D biomimetic matrixes in their amplification and analysis, followed by methods for further CTC analyses at the single-cell and single-molecule levels. Finally, we discuss how the elucidation of mechanical and morphological properties using techniques such as atomic force microscopy and molecular biomarker identification using nanopore-based detection could be combined in the future to provide patients and their healthcare providers with a more accurate diagnosis.

Porous yet dense matrices: using ice to shape collagen 3D cell culture systems with increased physiological relevance

Standard in vitro cell cultures are one of the pillars of biomedical sciences. However, there is increasing evidence that 2D systems provide biological responses that are often in disagreement with in vivo observations, partially due to limitations in reproducing the native cellular microenvironment. 3D materials that are able to mimic the native cellular microenvironment to a greater extent tackle these limitations. Here, we report Porous yet Dense (PyD) type I collagen materials obtained by ice-templating followed by topotactic fibrillogenesis. These materials combine extensive macroporosity, favouring the cell migration and nutrient exchange, as well as dense collagen walls, which mimic locally the extracellular matrix. When seeded with Normal Human Dermal Fibroblasts (NHDFs), PyD matrices allow for faster and more extensive colonisation when compared with equivalent non-porous matrices. The textural properties of the PyD materials also impact cytoskeletal and nuclear 3D morphometric parameters. Due to the effectiveness in creating a biomimetic 3D environment for NHDFs and the ability to promote cell culture for more than 28 days without subculture, we anticipate that PyD materials could configure an important step towards in vitro systems applicable to other cell types and with higher physiological relevance.

Camphene as a Mild, Bio-Derived Porogen for Near-Ambient Processing and 3D Printing of Porous Thermoplastics

Porous structures are ubiquitous in nature due to their advantageous mechanical and transport properties. These structures have inspired various synthetic porous polymer technologies, including lightweight structural materials and membranes. While many manufacturing processes have been developed to generate porous thermoplastics, these usually include hazardous processes, such as high pressures and temperatures, or chemical components. Furthermore, few are compatible with additive manufacturing methods, such as 3D printing. Herein, we introduce bio-derived terpene camphene as a solvent and porogen for the freeze-casting of thermoplastic parts under mild conditions. Enabled by a low melting point (50 °C), camphene is used as a solvent for melt processing camphene-polymer solutions at moderate temperatures that later undergo room-temperature crystallization to template polymer-rich domains. Due to its high vapor pressure, camphene can be sublimed directly from these biphasic structures, resulting in an interconnected microporous polymer structure. Various polymers are demonstrated to be soluble in camphene, including polystyrene, an olefinic elastomer, a polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene elastomer, a cyclic olefin copolymer, and poly(ethyl methacrylate). Porous samples of each polymer were generated from camphene mixtures via compression molding, cooling, and subsequent vacuum annealing at room temperature to remove camphene. The porosity and pore structures were dependent on solution composition, including both the polymer type and polymer loading. Across the compositions investigated, porosity decreased monotonically from 93 to 65% with increasing polymer content. In the case of polystyrene, samples with pore diameters varying from ∼20 to <5 μm were generated. Rheological measurements were conducted on a series of polystyrene-camphene solutions to understand and optimize the formulation and conditions for direct ink write 3D printing. Porous parts with complex structures were successfully printed under mild conditions. These results underscore the advantages of camphene as a sustainable, nontoxic porogen and will inform future development of porous polymer systems derived from these methods.

Complex Architectural Control of Ice-Templated Collagen Scaffolds Using a Predictive Model

The architectural and physiomechanical properties of regenerative scaffolds have been shown to improve engineered tissue function at both a cellular and tissue level. The fabrication of regenerative three-dimensional scaffolds that precisely replicate the complex hierarchical structure of native tissue, however, remains a challenge. The aim of this work is therefore two-fold: i) demonstrate an innovative multidirectional freeze-casting system to afford precise architectural control of ice-templated collagen scaffolds; and ii) present a predictive simulation as an experimental design tool for bespoke scaffold architecture. We used embedded heat sources within the freeze-casting mold to manipulate the local thermal environment during solidification of ice-templated collagen scaffolds. The resultant scaffolds comprised complex and spatially varied lamellar orientations that correlated with the imposed thermal environment and could be readily controlled by varying the geometry and power of the heat sources. The complex macro-architecture did not interrupt the hierarchical features characteristic of ice-templated scaffolds, but pore orientation had a significant impact on the stiffness of resultant structures under compression. Furthermore, our finite element model (FEM) accurately predicted the thermal environment and illustrated the freezing front topography within the mold during solidification. The lamellar orientation of freeze-cast scaffolds was also predicted using thermal gradient vector direction immediately prior to phase change. In combination our FEM and bespoke freeze-casting system present an exciting opportunity for tailored architectural design of ice-templated regenerative scaffolds that mimic the complex hierarchical environment of the native extracellular matrix. STATEMENT OF SIGNIFICANCE: Biomimetic scaffold structure improves engineered tissue function, but the fabrication of three-dimensional scaffolds that precisely replicate the complex hierarchical structure of native tissue remains a challenge. Here, we leverage the robust relationship between thermal gradients and lamellar orientation of ice-templated collagen scaffolds to develop a multidirectional freeze-casting system with precise control of the thermal environment and consequently the complex lamellar structure of resultant scaffolds. Demonstrating the diversity of our approach, we identify heat source geometry and power as control parameters for complex lamellar orientations. We simultaneously present a finite element model (FEM) that describes the three-dimensional thermal environment during solidification and accurately predicts lamellar structure of resultant scaffolds. The model serves as a design tool for bespoke regenerative scaffolds.

Biomimetic Silk Macroporous Materials for Drug Delivery Obtained via Ice-Templating

Silk from Bombyx mori is one of the most exciting materials in nature. The apparently simple arrangement of its two major components─two parallel filaments of silk fibroin (SF) coated by a common sericin (SS) sheath─provides a combination of mechanical and surface properties that can protect the moth during its most vulnerable phase, the pupal stage. Here, we recapitulate the topology of native silk fibers but shape them into three-dimensional porous constructs using an unprecedented design strategy. We demonstrate, for the first time, the potential of these macroporous silk foams as dermal patches for wound protection and for the controlled delivery of Rifamycin (Rif), a model antibiotic. The method implies (i) removing SS from silk fibers; (ii) shaping SF solutions into macroporous foams via ice-templating; (iii) stabilizing the SF macroporous foam in a methanolic solution of Rif; and (iv) coating Rif-loaded SF foams with a SS sheath. The resulting SS@SF foams exhibit water wicking capacity and accommodate up to ∼20% deformation without detaching from a skin model. The antibacterial behavior of Rif-loaded SS@SF foams against Staphylococcus aureus on agar plates outperforms that of SF foams (>1 week and 4 days, respectively). The reassembly of natural materials as macroporous foams─illustrated here for the reconstruction of silk-based materials─can be extended to other multicomponent natural materials and may play an important role in applications where controlled release of molecules and fluid transport are pivotal.

(Bio)manufactured Solutions for Treatment of Bone Defects with Emphasis on US-FDA Regulatory Science Perspective

Bone defects, with second highest demand for surgeries around the globe, may lead to serious health issues and negatively influence patient lives. The advances in biomedical engineering and sciences have led to the development of several creative solutions for bone defect treatment. This review provides a brief summary of bone graft materials, an organized overview of top-down and bottom-up (bio)manufacturing approaches, plus a critical comparison between advantages and limitations of each method. We specifically discuss additive manufacturing techniques and their operation mechanisms in detail. Next, we review the hybrid methods and promising future directions for bone grafting, while giving a comprehensive US-FDA regulatory science perspective, biocompatibility concepts and assessments, and clinical considerations to translate a technology from a research laboratory to the market. The topics covered in this review could potentially fuel future research efforts in bone tissue engineering, and perhaps could also provide novel insights for other tissue engineering applications.

Three-dimensional Culture and Chondrogenic Differentiation of Mesenchymal Stem Cells in Interconnected Collagen Scaffolds

Interconnected scaffolds are useful for promoting the chondrogenic differentiation of stem cells. Collagen scaffolds with interconnected pore structures were fabricated with poly(lactic acid-co-glycolic acid) (PLGA) sponge templates. The PLGA-templated collagen scaffolds were used to culture human bone marrow-derived mesenchymal stem cells (hMSCs) to investigate their promotive effect on the chondrogenic differentiation of hMSCs. The cells adhered to the scaffolds with a homogeneous distribution and proliferated with culture time. The expression of chondrogenesis-related genes was upregulated, and abundant cartilaginous matrices were detected. After subcutaneous implantation, the PLGA-templated collagen scaffolds further enhanced the production of cartilaginous matrices and the mechanical properties of the implants. The good interconnectivity of the PLGA-templated collagen scaffolds promoted chondrogenic differentiation. In particular, the collagen scaffolds prepared with large pore-bearing PLGA sponge templates showed the highest promotive effect.

Near-infrared analysis of nanofibrillated cellulose aerogel manufacturing

Biomaterial aerogel fabrication by freeze-drying must be further improved to reduce the costs of lengthy freeze-drying cycles and to avoid the formation of spongy cryogels and collapse of the aerogel structures. Residual water content is a critical quality attribute of the freeze-dried product, which can be monitored in-line with near-infrared (NIR) spectroscopy. Predictive models of NIR have not been previously applied for biomaterials and the models were mostly focused on the prediction of only one formulation at a time. We recorded NIR spectra of different nanofibrillated cellulose (NFC) hydrogel formulations during the secondary drying and set up a partial least square regression model to predict their residual water contents. The model can be generalized to measure residual water of formulations with different NFC concentrations and the excipients, and the NFC fiber concentrations and excipients can be separated with the principal component analysis. Our results provide valuable information about the freeze-drying of biomaterials and aerogel fabrication, and how NIR spectroscopy can be utilized in the optimization of residual water content.

Recent advances of natural biopolymeric culture scaffold: synthesis and modification

Traditionally existing 2D culture scaffold has been inappropriately validated due to the failure in generating the precise therapeutic response. Therefore, this leads to the fabrication of 3D culture scaffold resolving the limitations in the in vivo environment. In recent years, tissue engineering played an important role in the field of bio-medical engineering. Biopolymer material, a novel natural material with excellent properties of nontoxic and biodegradable merits can be served as culture scaffold. This review summarizes the modifications of natural biopolymeric culture scaffold with different crosslinkers and their application. In addition, this review provides the recent progress of natural biopolymeric culture scaffold mainly focusing on their properties, synthesizing and modification and application.

Enhanced BMP-2-Mediated Bone Repair Using an Anisotropic Silk Fibroin Scaffold Coated with Bone-like Apatite

The repair of large bone defects remains challenging and often requires graft material due to limited availability of autologous bone. In clinical settings, collagen sponges loaded with excessive amounts of bone morphogenetic protein 2 (rhBMP-2) are occasionally used for the treatment of bone non-unions, increasing the risk of adverse events. Therefore, strategies to reduce rhBMP-2 dosage are desirable. Silk scaffolds show great promise due to their favorable biocompatibility and their utility for various biofabrication methods. For this study, we generated silk scaffolds with axially aligned pores, which were subsequently treated with 10× simulated body fluid (SBF) to generate an apatitic calcium phosphate coating. Using a rat femoral critical sized defect model (CSD) we evaluated if the resulting scaffold allows the reduction of BMP-2 dosage to promote efficient bone repair by providing appropriate guidance cues. Highly porous, anisotropic silk scaffolds were produced, demonstrating good cytocompatibility in vitro and treatment with 10× SBF resulted in efficient surface coating. In vivo, the coated silk scaffolds loaded with a low dose of rhBMP-2 demonstrated significantly improved bone regeneration when compared to the unmineralized scaffold. Overall, our findings show that this simple and cost-efficient technique yields scaffolds that enhance rhBMP-2 mediated bone healing.

Interconnected collagen porous scaffolds prepared with sacrificial PLGA sponge templates for cartilage tissue engineering

Interconnected pore structures of scaffolds are important to control the cell functions for cartilage tissue engineering. In this study, collagen scaffolds with interconnected pore structures were prepared using poly(D,L-lactide-co-glycolide) (PLGA) sponges as sacrificial templates. Six types of PLGA sponges of different pore sizes and porosities were prepared by the solvent casting/particulate leaching method and used to regulate the interconnectivity of the collagen scaffolds. The integral and continuous templating structure of PLGA sponges generated well-interconnected pore structures in the collagen scaffolds. Bovine articular chondrocytes cultured in collagen scaffolds showed homogenous distribution, fast proliferation, high expression of cartilaginous genes and high secretion of cartilaginous extracellular matrix. In particular, the collagen scaffold templated by the PLGA sacrificial sponge that was prepared with a high weight ratio of PLGA and large salt particulates showed the most promotive effect on cartilage tissue formation. The interconnected pore structure facilitated cell distribution, cell-cell interaction and cartilage tissue regeneration.

Colonization versus encapsulation in cell-laden materials design: porosity and process biocompatibility determine cellularization pathways

Seeding materials with living cells has been-and still is-one of the most promising approaches to reproduce the complexity and the functionality of living matter. The strategies to associate living cells with materials are limited to cell encapsulation and colonization, however, the requirements for these two approaches have been seldom discussed systematically. Here we propose a simple two-dimensional map based on materials' pore size and the cytocompatibility of their fabrication process to draw, for the first time, a guide to building cellularized materials. We believe this approach may serve as a straightforward guideline to design new, more relevant materials, able to seize the complexity and the function of biological materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Preservation of biomaterials and cells by freeze-drying: Change of paradigm

Freeze-drying is the most widespread method to preserve protein drugs and vaccines in a dry form facilitating their storage and transportation without the laborious and expensive cold chain. Extending this method for the preservation of natural biomaterials and cells in a dry form would provide similar benefits, but most results in the domain are still below expectations. In this review, rather than consider freeze-drying as a traditional black box we "break it" through a detailed process thinking approach. We discuss freeze-drying from process thinking aspects, introduce the chemical, physical, and mechanical environments important in this process, and present advanced biophotonic process analytical technology. In the end, we review the state of the art in the freeze-drying of the biomaterials, extracellular vesicles, and cells. We suggest that the rational design of the experiment and implementation of advanced biophotonic tools are required to successfully preserve the natural biomaterials and cells by freeze-drying. We discuss this change of paradigm with existing literature and elaborate on our perspective based on our new unpublished results.