Characterization of Mechanical Properties of Tissue Scaffolds by Phase Contrast Imaging and Finite Element Modeling

Comprehensive Review on Design and Manufacturing of Bio-scaffolds for Bone Reconstruction

Bio-scaffolds are synthetic entities widely employed in bone and soft-tissue regeneration applications. These bio-scaffolds are applied to the defect site to provide support and favor cell attachment and growth, thereby enhancing the regeneration of the defective site. The progressive research in bio-scaffold fabrication has led to identification of biocompatible and mechanically stable materials. The difficulties in obtaining grafts and expenditure incurred in the transplantation procedures have also been overcome by the implantation of bio-scaffolds. Drugs, cells, growth factors, and biomolecules can be embedded with bio-scaffolds to provide localized treatments. The right choice of materials and fabrication approaches can help in developing bio-scaffolds with required properties. This review mostly focuses on the available materials and bio-scaffold techniques for bone and soft-tissue regeneration application. The first part of this review gives insight into the various classes of biomaterials involved in bio-scaffold fabrication followed by design and simulation techniques. The latter discusses the various additive, subtractive, hybrid, and other improved techniques involved in the development of bio-scaffolds for bone regeneration applications. Techniques involving multimaterial printing and multidimensional printing have also been briefly discussed.

Quality control methods in musculoskeletal tissue engineering: from imaging to biosensors

Tissue engineering is rapidly progressing toward clinical application. In the musculoskeletal field, there has been an increasing necessity for bone and cartilage replacement. Despite the promising translational potential of tissue engineering approaches, careful attention should be given to the quality of developed constructs to increase the real applicability to patients. After a general introduction to musculoskeletal tissue engineering, this narrative review aims to offer an overview of methods, starting from classical techniques, such as gene expression analysis and histology, to less common methods, such as Raman spectroscopy, microcomputed tomography, and biosensors, that can be employed to assess the quality of constructs in terms of viability, morphology, or matrix deposition. A particular emphasis is given to standards and good practices (GXP), which can be applicable in different sectors. Moreover, a classification of the methods into destructive, noninvasive, or conservative based on the possible further development of a preimplant quality monitoring system is proposed. Biosensors in musculoskeletal tissue engineering have not yet been used but have been proposed as a novel technology that can be exploited with numerous advantages, including minimal invasiveness, making them suitable for the development of preimplant quality control systems.

Noninvasive Three-Dimensional In Situ and In Vivo Characterization of Bioprinted Hydrogel Scaffolds Using the X-ray Propagation-Based Imaging Technique

Hydrogel-based three-dimensional (3D) bioprinting has been illustrated as promising to fabricate tissue scaffolds for regenerative medicine. Notably, bioprinting of hydrated and soft 3D hydrogel scaffolds with desired structural properties has not been fully achieved so far. Moreover, due to the limitations of current imaging techniques, assessment of bioprinted hydrogel scaffolds is still challenging, yet still essential for scaffold design, fabrication, and longitudinal studies. This paper presents our study on the bioprinting of hydrogel scaffolds and on the development of a novel noninvasive imaging method, based on synchrotron propagation-based imaging with computed tomography (SR-PBI-CT), to study the structural properties of hydrogel scaffolds and their responses to environmental stimuli both in situ and in vivo. Hydrogel scaffolds designed with varying structural patterns were successfully bioprinted through rigorous printing process regulations and then imaged by SR-PBI-CT within physiological environments. Subjective to controllable compressive loadings, the structural responses of scaffolds were visualized and characterized in terms of the structural deformation caused by the compressive loadings. Hydrogel scaffolds were later implanted in rats as nerve conduits for SR-PBI-CT imaging, and the obtained images illustrated their high phase contrast and were further processed for the 3D structure reconstruction and quantitative characterization. Our results show that the scaffold design and printing conditions play important roles in the printed scaffold structure and mechanical properties. More importantly, our obtained images from SR-PBI-CT allow us to visualize the details of hydrogel 3D structures with high imaging resolution. It demonstrates unique capability of this imaging technique for noninvasive, in situ characterization of 3D hydrogel structures pre- and post-implantation in diverse physiological milieus. The established imaging platform can therefore be utilized as a robust, high-precision tool for the design and longitudinal studies of hydrogel scaffold in tissue engineering.

Personalized Design of Functional Gradient Bone Tissue Engineering Scaffold

The porous structure of the natural bone not only has the characteristics of lightweight and high strength, but also is conducive to the growth of cells and tissues due to interconnected pores. In this paper, a novel gradient-controlled parametric modeling technology is presented to design bone tissue engineering (BTE) scaffold. First of all, the method functionalizes the pore distribution in the bone tissue, and reconstructs the pore distribution of the bone tissue in combination with the pathological analysis of the bone defect area of the individual patient. Then, based on the reconstructed pore distribution, the Voronoi segmentation algorithm and the contour interface optimization method are used to reconstruct the whole model of the bone tissue. Finally, the mechanical properties of the scaffold are studied by the finite element analysis (FEA) of different density gradient scaffolds. The results show that the method is highly feasible. BTE scaffold can be designed by irregular design methods and adjustment of pore distribution parameters, which is similar with natural bone in structural characteristics and biomechanical properties.

Computational Design and Optimization of Nerve Guidance Conduits for Improved Mechanical Properties and Permeability

Nerve guidance conduits (NGCs) are tubular tissue engineering scaffolds used for nerve regeneration. The poor mechanical properties and porosity have always compromised their performances for guiding and supporting axonal growth. Therefore, in order to improve the properties of NGCs, the computational design approach was adopted to investigate the effects of different NGC structural features on their various properties, and finally design an ideal NGC with mechanical properties matching human nerves and high porosity and permeability. Three common NGC designs, namely hollow luminal, multichannel, and microgrooved, were chosen in this study. Simulations were conducted to study the mechanical properties and permeability. The results show that pore size is the most influential structural feature for NGC tensile modulus. Multichannel NGCs have higher mechanical strength but lower permeability compared to other designs. Square pores lead to higher permeability but lower mechanical strength than circular pores. The study finally selected an optimized hollow luminal NGC with a porosity of 71% and tensile modulus of 8 MPa to achieve multiple design requirements. The use of computational design and optimization was shown to be promising in future NGC design and nerve tissue engineering research.

Modeling of the Mechanical Behavior of 3D Bioplotted Scaffolds Considering the Penetration in Interlocked Strands

Three-dimensional (3D) bioplotting has been widely used to print hydrogel scaffolds for tissue engineering applications. One issue involved in 3D bioplotting is to achieve the scaffold structure with the desired mechanical properties. To overcome this issue, various numerical methods have been developed to predict the mechanical properties of scaffolds, but limited by the imperfect representation of one key feature of scaffolds fabricated by 3D bioplotting, i.e., the penetration or fusion of strands in one layer into the previous layer. This paper presents our study on the development of a novel numerical model to predict the elastic modulus (one important index of mechanical properties) of 3D bioplotted scaffolds considering the aforementioned strand penetration. For this, the finite element method was used for the model development, while medium-viscosity alginate was selected for scaffold fabrication by the 3D bioplotting technique. The elastic modulus of the bioplotted scaffolds was characterized using mechanical testing and results were compared with those predicted from the developed model, demonstrating a strong congruity between them. Once validated, the developed model was also used to investigate the effect of other geometrical features on the mechanical behavior of bioplotted scaffolds. Our results show that the penetration, pore size, and number of printed layers have significant effects on the elastic modulus of bioplotted scaffolds; and also suggest that the developed model can be used as a powerful tool to modulate the mechanical behavior of bioplotted scaffolds.

Remote Determination of Time-Dependent Stiffness of Surface-Degrading-Polymer Scaffolds Via Synchrotron-Based Imaging

Surface-degrading polymers have been widely used to fabricate scaffolds with the mechanical properties appropriate for tissue regeneration/repair. During their surface degradation, the material properties of polymers remain approximately unchanged, but the scaffold geometry and thus mechanical properties vary with time. This paper presents a novel method to determine the time-dependent mechanical properties, particularly stiffness, of scaffolds from the geometric changes captured by synchrotron-based imaging, with the help of finite element analysis (FEA). Three-dimensional (3D) tissue scaffolds were fabricated from surface-degrading polymers, and during their degradation, the tissue scaffolds were imaged via the synchrotron-based imaging to characterize their changing geometry. On this basis, the stiffness behavior of scaffolds was estimated from the FEA, and the results obtained were compared to the direct measurements of scaffold stiffness from the load–displacement material testing. The comparison illustrates that the Young's moduli estimated from the FEA and characterized geometry are in agreement with the ones of direct measurements. The developed method of estimating the mechanical behavior was also demonstrated effective with a nondegrading scaffold that displays the nonlinear stress–strain behavior. The in vivo monitoring of Young's modulus by morphology characterization also suggests the feasibility of characterizing experimentally the difference between in vivo and in vitro surface degradation of tissue engineering constructs.

Effect of Nanoparticle Incorporation and Surface Coating on Mechanical Properties of Bone Scaffolds: A Brief Review

Mechanical properties of a scaffold play an important role in its in vivo performance in bone tissue engineering, due to the fact that implanted scaffolds are typically subjected to stress including compression, tension, torsion, and shearing. Unfortunately, not all the materials used to fabricate scaffolds are strong enough to mimic native bones. Extensive research has been conducted in order to increase scaffold strength and mechanical performance by incorporating nanoparticles and/or coatings. An incredible improvement has been achieved; and some outstanding examples are the usage of nanodiamond, hydroxyapatite, bioactive glass particles, SiO₂, MgO, and silver nanoparticles. This review paper aims to present the results, to summarize significant findings, and to give perspective for future work, which could be beneficial to future bone tissue engineering.

Synchrotron-Based in Situ Characterization of the Scaffold Mass Loss from Erosion Degradation

The mass loss behavior of degradable tissue scaffolds is critical to their lifespan and other degradation-related properties including mechanical strength and mass transport characteristics. This paper presents a novel method based on synchrotron imaging to characterize the scaffold mass loss from erosion degradation in situ, or without the need of extracting scaffolds once implanted. Specifically, the surface-eroding degradation of scaffolds in a degrading medium was monitored in situ by synchrotron-based imaging; and the time-dependent geometry of scaffolds captured by images was then employed to estimate their mass loss with time, based on the mathematical model that was adopted from the literature of surface erosion with the experimentally-identified model parameters. Acceptable agreement between experimental results and model predictions was observed for scaffolds in a cylindrical shape, made from poly(lactic-co-glycolic) acid (PLGA) and polycaprolactone (PCL). This study illustrates that geometry evaluation by synchrotron-based imaging is an effective means to in situ characterize the scaffold mass loss as well as possibly other degradation-related properties.