Nanofibrous hydrogel composites as mechanically robust tissue engineering scaffolds.

Hydrogels closely resemble the extracellular matrix (ECM) and can support cell proliferation while new tissue is formed, making them materials of choice as tissue engineering scaffolds. However, their sometimes-poor mechanical properties can hinder their application. The addition of meshes of nanofibers embedded in their matrix forms a composite that draws from the advantages of both components. Given that these materials are still in the early stages of development, there is a lack of uniformity across methods for characterizing their mechanical properties. Here, we propose a simple metric to enable comparisons between materials. The fibrous constituent improves the mechanical properties of the hydrogel, while the biocompatibility and functionality of the gels are maintained or even improved.The authors acknowledge the support of the EPSRC through a doctoral training award (ALB) and via the Nano Science and Technology Doctoral Training Centre (NanoDTC), EP/G037221/1 (GSO).This is the accepted manuscript. The final version is available from http://www.cell.com/trends/biotechnology/references/S0167-7799%2814%2900181-4

Multi-scale mechanical response of freeze-dried collagen scaffolds for tissue engineering applications.

Tissue engineering has grown in the past two decades as a promising solution to unresolved clinical problems such as osteoarthritis. The mechanical response of tissue engineering scaffolds is one of the factors determining their use in applications such as cartilage and bone repair. The relationship between the structural and intrinsic mechanical properties of the scaffolds was the object of this study, with the ultimate aim of understanding the stiffness of the substrate that adhered cells experience, and its link to the bulk mechanical properties. Freeze-dried type I collagen porous scaffolds made with varying slurry concentrations and pore sizes were tested in a viscoelastic framework by macroindentation. Membranes made up of stacks of pore walls were indented using colloidal probe atomic force microscopy. It was found that the bulk scaffold mechanical response varied with collagen concentration in the slurry consistent with previous studies on these materials. Hydration of the scaffolds resulted in a more compliant response, yet lesser viscoelastic relaxation. Indentation of the membranes suggested that the material making up the pore walls remains unchanged between conditions, so that the stiffness of the scaffolds at the scale of seeded cells is unchanged; rather, it is suggested that thicker pore walls or more of these result in the increased moduli for the greater slurry concentration conditions.The authors are grateful to the Nano Doctoral Training Centre (NanoDTC), University of Cambridge, and the EPSRC who supported this work through the EP/G037221/1 grant.This is the final published version. It originally appeared at http://www.sciencedirect.com/science/article/pii/S1751616114003397#

Scale and structure dependent solute diffusivity within microporous tissue engineering scaffolds

Abstract: Diffusion of nutrients to cells cultured within three-dimensional scaffolds is fundamental for cell survival during development of the tissue construct, when no vasculature is present to aid transport. Significant efforts have been made to characterize the effect of structure on solute diffusivity in nanoporous hydrogels, yet a similar thorough characterization has not been attempted for microporous scaffolds. Here, we make use of freeze-dried collagen scaffolds, possessing pore sizes in the range 150–250 μm and isotropic or aligned morphology, to study the diffusivity of fluorescent dextran molecules. Fluorescence recovery after photobleaching is used to measure the self diffusivity of the solutes within single pores, while Fickian diffusion over scales larger than the pore size is studied by assessing the solute concentration profile within the materials over time. We show that, not only do the morphological parameters of the scaffolds significantly affect the diffusivity of the solutes, but also that the assessment of such diffusivity depends on the length scale of diffusion of the molecules under investigation, with the resulting diffusion coefficients being differently affected by the scaffold structure. The results provided can guide the design of scaffolds with tailored diffusivity and nutrient concentration profiles

A Multiscale Model for Solute Diffusion in Hydrogels.

The number of biomedical applications of hydrogels is increasing rapidly on account of their unique physical, structural, and mechanical properties. The utility of hydrogels as drug delivery systems or tissue engineering scaffolds critically depends on the control of diffusion of solutes through the hydrogel matrix. Predicting or even modeling this diffusion is challenging due to the complex structure of hydrogels. Currently, the diffusivity of solutes in hydrogels is typically modeled by one of three main theories proceeding from distinct diffusion mechanisms: (i) hydrodynamic, (ii) free volume, and (iii) obstruction theory. Yet, a comprehensive predictive model is lacking. Thus, time and capital-intensive trial-and-error procedures are used to test the viability of hydrogel applications. In this work, we have developed a model for the diffusivity of solutes in hydrogels combining the three main theoretical frameworks, which we call the multiscale diffusion model (MSDM). We verified the MSDM by analyzing the diffusivity of dextran of different sizes in a series of poly(ethylene glycol) (PEG) hydrogels with distinct mesh sizes. We measured the subnanoscopic free volume by positron annihilation lifetime spectroscopy (PALS) to characterize the physical hierarchy of these materials. In addition, we performed a meta-analysis of literature data from previous studies on the diffusion of solutes in hydrogels. The model presented outperforms traditional models in predicting solute diffusivity in hydrogels and provides a practical approach to predicting the transport properties of solutes such as drugs through hydrogels used in many biomedical applications

Scale and structure dependent solute diffusivity within microporous tissue engineering scaffolds

Abstract: Diffusion of nutrients to cells cultured within three-dimensional scaffolds is fundamental for cell survival during development of the tissue construct, when no vasculature is present to aid transport. Significant efforts have been made to characterize the effect of structure on solute diffusivity in nanoporous hydrogels, yet a similar thorough characterization has not been attempted for microporous scaffolds. Here, we make use of freeze-dried collagen scaffolds, possessing pore sizes in the range 150–250 μm and isotropic or aligned morphology, to study the diffusivity of fluorescent dextran molecules. Fluorescence recovery after photobleaching is used to measure the self diffusivity of the solutes within single pores, while Fickian diffusion over scales larger than the pore size is studied by assessing the solute concentration profile within the materials over time. We show that, not only do the morphological parameters of the scaffolds significantly affect the diffusivity of the solutes, but also that the assessment of such diffusivity depends on the length scale of diffusion of the molecules under investigation, with the resulting diffusion coefficients being differently affected by the scaffold structure. The results provided can guide the design of scaffolds with tailored diffusivity and nutrient concentration profiles

Scale and structure dependent solute diffusivity within microporous tissue engineering scaffolds

Abstract: Diffusion of nutrients to cells cultured within three-dimensional scaffolds is fundamental for cell survival during development of the tissue construct, when no vasculature is present to aid transport. Significant efforts have been made to characterize the effect of structure on solute diffusivity in nanoporous hydrogels, yet a similar thorough characterization has not been attempted for microporous scaffolds. Here, we make use of freeze-dried collagen scaffolds, possessing pore sizes in the range 150–250 μm and isotropic or aligned morphology, to study the diffusivity of fluorescent dextran molecules. Fluorescence recovery after photobleaching is used to measure the self diffusivity of the solutes within single pores, while Fickian diffusion over scales larger than the pore size is studied by assessing the solute concentration profile within the materials over time. We show that, not only do the morphological parameters of the scaffolds significantly affect the diffusivity of the solutes, but also that the assessment of such diffusivity depends on the length scale of diffusion of the molecules under investigation, with the resulting diffusion coefficients being differently affected by the scaffold structure. The results provided can guide the design of scaffolds with tailored diffusivity and nutrient concentration profiles

A Multiscale Model for Solute Diffusion in Hydrogels.

The number of biomedical applications of hydrogels is increasing rapidly on account of their unique physical, structural, and mechanical properties. The utility of hydrogels as drug delivery systems or tissue engineering scaffolds critically depends on the control of diffusion of solutes through the hydrogel matrix. Predicting or even modeling this diffusion is challenging due to the complex structure of hydrogels. Currently, the diffusivity of solutes in hydrogels is typically modeled by one of three main theories proceeding from distinct diffusion mechanisms: (i) hydrodynamic, (ii) free volume, and (iii) obstruction theory. Yet, a comprehensive predictive model is lacking. Thus, time and capital-intensive trial-and-error procedures are used to test the viability of hydrogel applications. In this work, we have developed a model for the diffusivity of solutes in hydrogels combining the three main theoretical frameworks, which we call the multiscale diffusion model (MSDM). We verified the MSDM by analyzing the diffusivity of dextran of different sizes in a series of poly(ethylene glycol) (PEG) hydrogels with distinct mesh sizes. We measured the subnanoscopic free volume by positron annihilation lifetime spectroscopy (PALS) to characterize the physical hierarchy of these materials. In addition, we performed a meta-analysis of literature data from previous studies on the diffusion of solutes in hydrogels. The model presented outperforms traditional models in predicting solute diffusivity in hydrogels and provides a practical approach to predicting the transport properties of solutes such as drugs through hydrogels used in many biomedical applications

Research data supporting "Structural Determinants of Hydration, Mechanics and Fluid Flow in Collagen Scaffolds"

The following data files are provided: Summary of results: numerical values used to plot each figure in the manuscript, as well as single measurements for each parameter when applicable. Raw indentation data: the load vs. time curves obtained by spherical indentation for the dry and hydrated samples. CT Analysis: Ellipse fit data giving the pore size analysis for each condition, scaling analysis for calculation of percolation diameter, 3D analysis giving the specific surface area for each sample. Representative CT stacks: contains a 1 mm x 1 mm x 1 mm representative Micro-CT dataset, presented as a .tif file, for each control sample in the dry and hydrated/wet states. READ ME: A word document describing each file and its contents.This work was supported by the ERC [advanced grant number 320598 3D-E], EPSRC Doctoral Training Account [grant number EP/G037221/1], Nano Doctoral Training Centre (NanoDTC), Geistlich Pharma AG and Switzerland

Physical and Biological characterization of composite scaffolds for cartilage replacement

The following data files are provided: Physical characterization - Raw indentation curves for the composite materials and the two single components, as well as an Excel file containing the numerical results obtained from mechanical testing and swelling analysis. Biological characterization - Excel files containing raw data and numerical results for the number of cells over time in 2D and 3D, cell morphology in 2D, metabolic activity in 3D, and cell migration in 3D

Stiffening by Osmotic Swelling Constraint in Cartilage-Like Cell Culture Scaffolds.

Cartilage wounds result in chronic pain and degradation of the quality of life for millions of people. A synthetic cellular scaffold able to heal the damage by substituting the natural tissue is of great potential value. Here, it is shown for the first time that the unique interplay between the molecular components of cartilage can be reproduced in composite materials made of a polyelectrolyte hydrogel embedding a collagen scaffold. These composites possess a mechanical response determined by osmotic and electrostatic effects, comparable to articular cartilage in terms of elastic modulus, time-dependent response, and permeability to interstitial fluid flow. Made entirely from biocompatible materials, the cartilage-like composite materials developed permit 3D culture of chondrocyte-like cells through their microporosity. The biomimetic materials presented here constitute an entirely new class of osmotically stiffened composites, which may find use outside of biomedical applications.This work was supported by the European Research Council [ERC Advanced Grant 320598 3D-E], and the Engineering and Physical Sciences Research Council [grant EPSRC EP/G037221/1]