Micro-CT based quantification of non-mineralized tissue on cultured hydroxyapatite scaffolds

Evaluation of the osteoconductivity and the degradation of novel hydroxyapatite/polyurethane combined with mesoporous silica microspheres in a rabbit osteomyelitis model

Bone defects caused by osteomyelitis can lead to severe disability. Surgeons still face significant challenges in treating bone defects. Nano-hydroxyapatite (n-HA) plays an important role in bone tissue engineering due to its excellent biocompatibility and osteoconductivity. Levofloxacin (Levo) was encapsulated in mesoporous silica nanoparticles (MSNs) via electrostatic attraction to serve as a drug delivery system. MSNs were incorporated with n-HA and polyurethane (PU). The degradation and osteoconductivity properties of these novel composite scaffolds and their effectiveness in treating chronic osteomyelitis in a rabbit model were assessed. Gross pathology, radiographic imaging, micro-computed tomography, Van Gieson staining, and hematoxylin and eosin staining were conducted at 6 and 12 weeks. The group of composite scaffolds combining n-HA/PU with MSNs containing 5 mg Levo (n-HA/PU + Nano +5 mg Levo) composite scaffolds showed superior antibacterial properties compared to the other groups. At 12 weeks, the n-HA/PU + Nano +5 mg Levo composite scaffolds group exhibited significantly greater volume of new trabecular bone formation compared to the other three groups. The surface of the novel composite scaffolds exhibited degradation after 6 weeks implantation. The internal structure of the scaffolds collapsed noticeably after 12 weeks of implantation. The rate of material degradation corresponded to the rate of new bone ingrowth. This novel composite scaffold, which is biodegradable and osteoconductive, has potential as a drug delivery system for treating chronic osteomyelitis accompanied by bone defects.

Characterization of Biocompatibility of Functional Bioinks for 3D Bioprinting

Three-dimensional (3D) bioprinting with suitable bioinks has become a critical tool for fabricating 3D biomimetic complex structures mimicking physiological functions. While enormous efforts have been devoted to developing functional bioinks for 3D bioprinting, widely accepted bioinks have not yet been developed because they have to fulfill stringent requirements such as biocompatibility and printability simultaneously. To further advance our knowledge of the biocompatibility of bioinks, this review presents the evolving concept of the biocompatibility of bioinks and standardization efforts for biocompatibility characterization. This work also briefly reviews recent methodological advances in image analyses to characterize the biocompatibility of bioinks with regard to cell viability and cell-material interactions within 3D constructs. Finally, this review highlights a number of updated contemporary characterization technologies and future perspectives to further advance our understanding of the biocompatibility of functional bioinks for successful 3D bioprinting.

Tracheal reconstruction with pedicled tandem grafts engineered by a radial stretch bioreactor

The engineering of tracheal substitutes is pivotal in improving tracheal reconstruction. In this study, we aimed to investigate the effects of biomechanical stimulation on tissue engineering tracheal cartilage by mimicking the trachea motion through a novel radial stretching bioreactor, which enables to dynamically change the diameter of the hollow cylindrical implants. Applying our bioreactor, we demonstrated that chondrocytes seeded on the surface of Poly (ε-caprolactone) scaffold respond to mechanical stimulation by improvement of infiltration into implants and upregulation of cartilage-specific genes. Further, the mechanical stimulation enhanced the accumulation of cartilage neo-tissues and cartilage-specific extracellular macromolecules in the muscle flap-remodeled implants and reconstructed trachea. Nevertheless, the invasion of fibrous tissues in the reconstructed trachea was suppressed upon mechanical loading.

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.

The 3D imaging of mesenchymal stem cells on porous scaffolds using high-contrasted x-ray computed nanotomography

This study presents an X-ray computed nanotomography (nano-CT) based, high-resolution imaging technique. Thanks to a voxel resolution of 540 nm, this novel technique is suitable for observing the 3D morphology of soft biopolymeric scaffolds seeded with stem cells. A sample of highly porous collagen scaffold seeded with contrasted mesenchymal stem cells (MSC) was investigated by using lab-based nano-CT. The whole volume of the sample was analysed without its destruction. To evaluate the potential of nano-CT, a comparison measurement was done using a standard microscopy technique. Scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX) established an extension and local accumulation of the contrasting agent - heavy metallic osmium tetroxide. The presented imaging technique is novel as it will help to understand better the behaviour of cells while interacting with three-dimensional biomaterials. This is crucial for both experimental and clinical tissue engineering applications in order to limit the risk of uncontrolled cell growth, and potentially tumour formation. LAY DESCRIPTION: Biomaterials play a crucial role in tissue engineering by serving as 3D scaffolds for cellular attachment, proliferation, and in growth ultimately leading to new tissue formation. Cell morphology and proliferation inside the 3D scaffold are necessary to know for assessing cell viability. However, these studies are usually negatively affected by the limitations of imaging techniques. We demonstrate that X-ray computed nanotomography (nano-CT), based on high-resolution imaging technique providing voxel resolution of 540 nm, is a suitable method for observing the 3D morphology of soft biopolymeric scaffolds seeded with stem cells. A sample of highly porous collagen scaffold seeded with contrasted mesenchymal stem cells (MSC) was investigated by using a lab-based nano-CT. The whole volume of the sample was analysed without its destruction. To evaluate the potential of nano-CT, a comparison measurement was done using a standard microscopy technique. Scanning electron microscopy in a combination with energy dispersive X-ray analysis established an extension and local accumulation of the contrasting agent - heavy metallic osmium tetroxide. The presented imaging technique is novel as it will help to understand better the behaviour of cells while interacting with three-dimensional biomaterials. This is crucial for both experimental and clinical tissue engineering applications in order to limit the risk of uncontrolled cell growth, and potentially tumour formation.

Three-dimensional characterization of tissue-engineered constructs by contrast-enhanced nanofocus computed tomography

To successfully implement tissue-engineered (TE) constructs as part of a clinical therapy, it is necessary to develop quality control tools that will ensure accurate and consistent TE construct release specifications. Hence, advanced methods to monitor TE construct properties need to be further developed. In this study, we showed proof of concept for contrast-enhanced nanofocus computed tomography (CE-nano-CT) as a whole-construct imaging technique with a noninvasive potential that enables three-dimensional (3D) visualization and quantification of in vitro engineered extracellular matrix (ECM) in TE constructs. In particular, we performed a 3D qualitative and quantitative structural and spatial assessment of the in vitro engineered ECM, formed during static and perfusion bioreactor cell culture in 3D TE scaffolds, using two contrast agents, namely, Hexabrix® and phosphotungstic acid (PTA). To evaluate the potential of CE-nano-CT, a comparison was made to standardly used techniques such as Live/Dead viability/cytotoxicity, Picrosirius Red staining, and to net dry weight measurements of the TE constructs. When using Hexabrix as the contrast agent, the ECM volume fitted linearly with the net dry ECM weight independent from the flow rate used, thus suggesting that it stains most of the ECM. When using PTA as the contrast agent, comparing to net weight measurements showed that PTA only stains a part of the ECM. This was attributed to the binding specificity of this contrast agent. In addition, the PTA-stained CE-nano-CT data showed pronounced distinction between flow conditions when compared to Hexabrix, indicating culture-specific structural ECM differences. This novel type of information can contribute to optimize bioreactor culture conditions and potentially critical quality characteristics of TE constructs such as ECM quantity and homogeneity, facilitating the gradual transformation of TE constructs in well-characterized TE products.

Mathematical modelling of tissue-engineered angiogenesis

We present a mathematical model for the vascularisation of a porous scaffold following implantation in vivo. The model is given as a set of coupled non-linear ordinary differential equations (ODEs) which describe the evolution in time of the amounts of the different tissue constituents inside the scaffold. Bifurcation analyses reveal how the extent of scaffold vascularisation changes as a function of the parameter values. For example, it is shown how the loss of seeded cells arising from slow infiltration of vascular tissue can be overcome using a prevascularisation strategy consisting of seeding the scaffold with vascular cells. Using certain assumptions it is shown how the system can be simplified to one which is partially tractable and for which some analysis is given. Limited comparison is also given of the model solutions with experimental data from the chick chorioallantoic membrane (CAM) assay.

The curve integration method is comparable to manual segmentation for the analysis of bone/scaffold composites using micro-CT

Microcomputed tomography (micro-CT) is becoming a more common imaging technique in tissue engineering and has been used to characterize scaffold pore size, pore fraction, and bone ingrowth, among other characteristics. Despite the increasingly widespread use, no standards exist for segmenting images. Manual segmentation, a common segmentation method, is subjective, time consuming, and has been shown to be inaccurate and unreliable. The curve integration method was previously introduced as a method to accurately calculate the volume fraction of constituents in bone scaffolds from micro-CT data. In this article, the curve integration method is compared to manual image segmentation in order to validate the former method. Three cases are presented from two in vivo bone regeneration studies that include cross-sections from a rabbit calvarial defect used to study drug delivery, and cross-sections and small volumes of hydroxyapatite scaffold-bone composites from a porcine intramuscular study. The analysis shows that the curve integration method models the data accurately and can be used to calculate volume fractions of the materials in the sample. Furthermore, the curve integration method is faster and less labor intensive than manual image segmentation.

Impact of bone geometry on effective properties of bone scaffolds

The characterization of bone/scaffold composite mechanical properties is essential for translation to the clinic, but in vivo studies require resources and personnel not available to many investigators. Therefore, the ability to predict composite properties could facilitate scaffold evaluation and reduce the number of in vivo studies required. To date, there have been no studies that have used experimental data to formulate a model of bone morphology or that have examined morphology as a variable in composite properties. In this study, a simple model was developed to predict the effective elastic properties of hydroxyapatite (HA) scaffold/bone composites using representative volume elements (RVE) and finite element analysis. While the RVE for the scaffold is clear, the choice of RVE for bone is not. Two bone geometries were generated for the RVE based on data from an in vivo study: a uniform coating and bridges in pores. Three scaffolds were evaluated in order to consider the effects of scaffold material modulus and porosity. Results showed that the bone geometry had little influence on composite elastic properties when compared to experimental error from the in vivo study. The implication is that such properties can be estimated by measuring the volume fraction of bone using a non-destructive method like microcomputerized tomography and the simple RVE model.

Quantifying the 3D macrostructure of tissue scaffolds

The need to shift from tissue replacement to tissue regeneration has led to the development of tissue engineering and in situ tissue regeneration. Both of these strategies often employ the use of scaffolds--templates that allow cells to attach and then guide the new tissue growth. There are many design criteria for an ideal scaffold. These criteria vary depending on the tissue type and location in the body. In any application of a scaffold it is vital to be able to characterise the scaffold before it goes into in vitro testing. In vitro testing allows the cell response to be investigated before its in vivo performance is assessed. A full characterisation of events in vitro and in vivo, in three dimensions (3D), is necessary if a scaffold's performance and effectiveness is to be fully quantified. This paper focuses on porous scaffolds for bone regeneration, suggests appropriate design criteria for a bone regenerating scaffold and then reviews techniques for obtaining the vitally important quantification of its pore structure. The techniques discussed will include newly developed methods of quantifying X-ray microtomography (microCT) images in 3D and for predicting the scaffolds mechanical properties and the likely paths of fluid flow (and hence potential cell migration). The complications in investigating scaffold performance in vitro are then discussed. Finally, the use of microCT for imaging scaffolds for in vivo tests is reviewed.

3D imaging of tissue integration with porous biomaterials

Porous biomaterials designed to support cellular infiltration and tissue formation play a critical role in implant fixation and engineered tissue repair. The purpose of this Leading Opinion Paper is to advocate the use of high resolution 3D imaging techniques as a tool to quantify extracellular matrix formation and vascular ingrowth within porous biomaterials and objectively compare different strategies for functional tissue regeneration. An initial over-reliance on qualitative evaluation methods may have contributed to the false perception that developing effective tissue engineering technologies would be relatively straightforward. Moreover, the lack of comparative studies with quantitative metrics in challenging pre-clinical models has made it difficult to determine which of the many available strategies to invest in or use clinically for companies and clinicians, respectively. This paper will specifically illustrate the use of microcomputed tomography (micro-CT) imaging with and without contrast agents to nondestructively quantify the formation of bone, cartilage, and vasculature within porous biomaterials.