Effect of water on mechanical properties of mineralized tissue composites

In the current study, the effects of polar solvents on tissue volume and mechanical properties are considered. Area shrinkage measurements are conducted for mineralized bone tissue samples soaked in polar solvents. Area shrinkage is used to calculate approximate linear and volume shrinkage. Results are compared with viscoelastic mechanical parameters for bone in the same solvents (as measured previously) and with both shrinkage measurements and mechanical data for nonmineralized tissues, as taken from the existing literature. As expected, the shrinkage of mineralized tissues is minimal when compared with shrinkage of nonmineralized tissues immersed in the same polar solvents. The mechanical changes in bone are also substantially less than in nonmineralized tissues. The largest stiffness values are found in shrunken bone samples (immersed in acetone and ethanol). The mineral phase in bone thus resists tissue shrinkage that would otherwise occur in the pure soft tissue phase. © 2007 Materials Research Society.

Electrospun fiber - Hydrogel composites for nucleus pulposus tissue engineering

New materials are needed to replace degenerated intervertebral disc tissue and to provide longer-term solutions for chronic back-pain. Replacement tissue potentially could be engineered by seeding cells into a scaffold that mimics the architecture of natural tissue. Many natural tissues, including the nucleus pulposus (the central region of the intervertebral disc) consist of collagen nanofibers embedded in a gel-like matrix. Recently it was shown that electrospun micro- or nano-fiber structures of considerable thickness can be produced by collecting fibers in an ethanol bath. Here, randomly aligned polycaprolactone electrospun fiber structures up to 50 mm thick are backfilled with alginate hydrogels to form novel composite materials that mimic the fiber-reinforced structure of the nucleus pulposus. The composites are characterized using both indentation and tensile testing. The composites are mechanically robust, exhibiting substantial strain-to-failure. The method presented here provides a way to create large biomimetic scaffolds that more closely mimic the composite structure of natural tissue. © 2012 Materials Research Society.

Composite hydrogels for nucleus pulposus tissue engineering

Tissue engineering offers a paradigm shift in the treatment of back pain. Engineered intervertebral discs could replace degenerated tissue and overcome the limitations of current treatments, which substantially alter the biomechanical properties of the spine. The centre of the disc, the nucleus pulposus, is an amorphous gel with a large bound water content and it can resist substantial compressive loads. Due to similarities in their compositions, hydrogels have frequently been considered as substitutes for the nucleus pulposus. However, there has been limited work characterising the time-dependent mechanical behaviour of hydrogel scaffolds for nucleus pulposus tissue engineering. Poroelastic behaviour, which plays a key role in nutrient transport, is of particular importance. Here, we investigate the time-dependent mechanical properties of gelatin and agar hydrogels and of gelatin-agar composites. The time-dependent properties of these hydrogels are explored using viscoelastic and poroelastic frameworks. Several gel formulations demonstrate comparable equilibrium elastic behaviour to the nucleus pulposus under unconfined compression, but permeability values that are much greater than those of the native tissue. A range of time-dependent responses are observed in the composite gels examined, presenting the opportunity for targeted design of custom hydrogels with combinations of mechanical properties optimized for tissue engineering applications. © 2011 Elsevier Ltd.

Structural optimization of lattice materials

Lattice materials are characterized at the microscopic level by a regular pattern of voids confined by walls. Recent rapid prototyping techniques allow their manufacturing from a wide range of solid materials, ensuring high degrees of accuracy and limited costs. The microstructure of lattice material permits to obtain macroscopic properties and structural performance, such as very high stiffness to weight ratios, highly anisotropy, high specific energy dissipation capability and an extended elastic range, which cannot be attained by uniform materials. Among several applications, lattice materials are of special interest for the design of morphing structures, energy absorbing components and hard tissue scaffold for biomedical prostheses. Their macroscopic mechanical properties can be finely tuned by properly selecting the lattice topology and the material of the walls. Nevertheless, since the number of the design parameters involved is very high, and their correlation to the final macroscopic properties of the material is quite complex, reliable and robust multiscale mechanics analysis and design optimization tools are a necessary aid for their practical application. In this paper, the optimization of lattice materials parameters is illustrated with reference to the design of a bracket subjected to a point load. Given the geometric shape and the boundary conditions of the component, the parameters of four selected topologies have been optimized to concurrently maximize the component stiffness and minimize its mass. Copyright © 2011 by ASME.

Composite electrospun gelatin fiber-alginate gel scaffolds for mechanically robust tissue engineered cornea

A severe shortage of good quality donor cornea is now an international crisis in public health. Alternatives for donor tissue need to be urgently developed to meet the increasing demand for corneal transplantation. Hydrogels have been widely used as scaffolds for corneal tissue regeneration due to their large water content, similar to that of native tissue. However, these hydrogel scaffolds lack the fibrous structure that functions as a load-bearing component in the native tissue, resulting in poor mechanical performance. This work shows that mechanical properties of compliant hydrogels can be substantially enhanced with electrospun nanofiber reinforcement. Electrospun gelatin nanofibers were infiltrated with alginate hydrogels, yielding transparent fiber-reinforced hydrogels. Without prior crosslinking, electrospun gelatin nanofibers improved the tensile elastic modulus of the hydrogels from 78±19. kPa to 450±100. kPa. Stiffer hydrogels, with elastic modulus of 820±210. kPa, were obtained by crosslinking the gelatin fibers with carbodiimide hydrochloride in ethanol before the infiltration process, but at the expense of transparency. The developed fiber-reinforced hydrogels show great promise as mechanically robust scaffolds for corneal tissue engineering applications. © 2013 Elsevier Ltd.

Gelatin nanofiber-reinforced alginate gel scaffolds for corneal tissue engineering.

A severe shortage of donor cornea is now an international crisis in public health. Substitutes for donor tissue need to be developed to meet the increasing demand for corneal transplantation. Current attempts in designing scaffolds for corneal tissue regeneration involve utilization of expensive materials. Yet, these corneal scaffolds still lack the highly-organized fibrous structure that functions as a load-bearing component in the native tissue. This work shows that transparent nanofiber-reinforced hydrogels could be developed from cheap, non-immunogenic and readily available natural polymers to mimic the cornea's microstructure. Electrospinning was employed to produce gelatin nanofibers, which were then infiltrated with alginate hydrogels. Introducing electrospun nanofibers into hydrogels improved their mechanical properties by nearly one order of magnitude, yielding mechanically robust composites. Such nanofiber-reinforced hydrogels could serve as alternatives to donor tissue for corneal transplantation.

Mechanical characterisation of hydrogel materials

Interest in hydrogel materials is growing rapidly, due to the potential for hydrogel use in tissue engineering and drug delivery applications, and as coatings on medical devices. However, a key limitation with the use of hydrogel materials in many applications is their relatively poor mechanical properties compared with those of (less biocompatible) solid polymers. In this review, basic chemistry, microstructure and processing routes for common natural and synthetic hydrogel materials are explored first. Underlying structure-properties relationships for hydrogels are considered. A series of mechanical testing modalities suitable for hydrogel characterisation are next considered, including emerging test modalities, such as nanoindentation and atomic force microscopy (AFM) indentation. As the data analysis depends in part on the material's constitutive behaviour, a series of increasingly complex constitutive models will be examined, including elastic, viscoelastic and theories that explicitly treat the multiphasic poroelastic nature of hydrogel materials. Results from the existing literature on agar and polyacrylamide mechanical properties are compiled and compared, highlighting the challenges and uncertainties inherent in the process of gel mechanical characterisation. © 2014 Institute of Materials, Minerals and Mining and ASM International.