Jennifer Z. Paxton

Engineering the Bone–Ligament Interface Using Polyethylene Glycol Diacrylate Incorporated with Hydroxyapatite

Ligaments and tendons have previously been tissue engineered. However, without the bone attachment, implantation of a tissue-engineered ligament would require it to be sutured to the remnant of the injured native tissue. Due to slow repair and remodeling, this would result in a chronically weak tissue that may never return to preinjury function. In contrast, orthopaedic autograft reconstruction of the ligament often uses a bone-to-bone technique for optimal repair. Since bone-to-bone repairs heal better than other methods, implantation of an artificial ligament should also occur from bone-to-bone. The aim of this study was to investigate the use of a poly(ethylene glycol) diacrylate (PEGDA) hydrogel incorporated with hydroxyapatite (HA) and the cell-adhesion peptide RGD (Arg-Gly-Asp) as a material for creating an in vitro tissue interface to engineer intact ligaments (i.e., bone-ligament-bone). Incorporation of HA into PEG hydrogels reduced the swelling ratio but increased mechanical strength and stiffness of the hydrogels. Further, HA addition increased the capacity for cell growth and interface formation. RGD incorporation increased the swelling ratio but decreased mechanical strength and stiffness of the material. Optimum levels of cell attachment were met using a combination of both HA and RGD, but this material had no better mechanical properties than PEG alone. Although adherence of the hydrogels containing HA was achieved, failure occurs at about 4 days with 5% HA. Increasing the proportion of HA improved interface formation; however, with high levels of HA, the PEG HA composite became brittle. This data suggests that HA, by itself or with other materials, might be well suited for engineering the ligament-bone interface.

Construction of 3D biological matrices using rapid prototyping technology

Purpose – Hydrogels with low viscosities tend to be difficult to use in constructing tissue engineering (TE) scaffolds used to replace or restore damaged tissue, due to the length of time it takes for final gelation to take place resulting in the scaffolds collapsing due to their mechanical instability. However, recent advances in rapid prototyping have allowed for a new technology called bioplotting to be developed, which aims to circumvent these inherent problems. This paper aims to present details of the process.Design/methodology/approach – The paper demonstrates how by using the bioplotting technique complex 3D geometrical scaffolds with accurate feature sizes and good pore definition can be fabriated for use as biological matrices. PEG gels containing the cell‐adhesive RGD peptide sequence were patterned using this method to produce layers of directional microchannels which have a functionalised bioactive surface. Seeding these gels with C2C12 myoblasts showed that the cells responded to the topogra...

Engineered Muscle: A Tool for Studying Muscle Physiology and Function

Recent advances in skeletal muscle tissue engineering have resulted in an in vitro tissue model that can be used for studying the effects of genetic alterations, pharmacological interventions, and exercise on muscle physiology and function. Here, we present applications for this technology to further our understanding of the molecular mechanisms underlying skeletal muscle adaptation in response to exercise.

Degradation of polysaccharide hydrogels seeded with bone marrow stromal cells

In order to produce hydrogel cell culture substrates that are fit for the purpose, it is important that the mechanical properties are well understood not only at the point of cell seeding but throughout the culture period. In this study the change in the mechanical properties of three biopolymer hydrogels alginate, low methoxy pectin and gellan gum have been assessed in cell culture conditions. Samples of the gels were prepared encapsulating rat bone marrow stromal cells which were then cultured in osteogenic media. Acellular samples were also prepared and incubated in standard cell culture media. The rheological properties of the gels were measured over a culture period of 28 days and it was found that the gels degraded at very different rates. The degradation occurred most rapidly in the order alginate > Low methoxy pectin > gellan gum. The ability of each hydrogel to support differentiation of bone marrow stromal cells to osteoblasts was also verified by evidence of mineral deposits in all three of the materials. These results highlight that the mechanical properties of biopolymer hydrogels can vary greatly during in vitro culture, and provide the potential of selecting hydrogel cell culture substrates with mechanical properties that are tissue specific.

Formed 3D Bio-Scaffolds via Rapid Prototyping Technology

The construction of biomaterial scaffolds for cell seeding is now seen as the most common approach for producing artificial tissue as compared with cell self-assembly and Acellular matrix techniques. This paper describes the use of synthetic and natural polymeric material shaped into 3D biological matrices by using Rapid Prototyping (RP) technology. Recent advances in RP technology have greatly enhanced the range of biomaterials that can now be constructed into scaffolds, also allowing for maximized control of the pore size and architecture. Bioplotting is one such method which allows the dispensing of various biomaterials into a media bath which has similar rheological properties and acts as mechanical support and in most cases a cross-linking agent to produce high quality scaffolds. This method was used to construct scaffolds using agarose and gelatin with tight interconnecting pores which aim to enhance cell growth. Bioplotting was also used to pattern microchanneled layers in one direction with a PEG gel containing cell adhesive RGD peptide sequence, when seeded with C2C12 myoblasts demonstrated that cells responded to their topographical environment and aligned along the direction of the layered microchannels. This result indicates that this technique can be used to produce 3D scaffolds which aid tissue regeneration for physiologically functional tissue.

Nanoscale crystallinity modulates cell proliferation on plasma sprayed surfaces

Calcium phosphate coatings have been applied to the surface of metallic prostheses to mediate hard and soft tissue attachment for more than 40years. Most coatings are formed of high purity hydroxyapatite, and coating methods are often designed to produce highly crystalline surfaces. It is likely however, that coatings of lower crystallinity can facilitate more rapid tissue attachment since the surface will exhibit a higher specific surface area and will be considerably more reactive than a comparable highly crystalline surface. Here we test this hypothesis by growing a population of MC3T3 osteoblast-like cells on the surface of two types of hip prosthesis with similar composition, but with differing crystallinity. The surfaces with lower crystallinity facilitated more rapid cell attachment and increased proliferation rate, despite having a less heterogeneous surface topography. This work highlights that the influence of the crystallinity of HA at the nano-scale is dominant over macro-scale topography for cell adhesion and growth. Furthermore, crystallinity could be easily adjusted by without compromising coating purity. These findings could facilitate designing novel coated calcium phosphate surfaces that more rapidly bond tissue following implantation.

Synthesis and in vitro degradation of a novel magnesium oxychloride cement

Magnesium oxychloride cement (MOC) has been used in civil engineering as it exhibits a relatively high early strength and a low coefficient of thermal expansion. Its poor water resistance, although, has prevented its widespread use. Steady degradation when immersed in an aqueous environment, however, could be a beneficial property for a resorbable bone replacement. In this study, we have evaluated how different concentrations of phosphoric acid may be used to enhance water resistance providing some control over the rate of degradation. The phase compositions, microstructures, mechanical properties, and the degradation of MOC have been evaluated. As a preliminary assessment of biological suitability, the response of a population of bone marrow stromal cells to the surface was evaluated. X-ray diffraction data demonstrate that 5Mg(OH)₂ ·MgCl₂·8H₂O (phase 5) was formed in all MOC samples. The MOC modified with H₃PO₄ exhibits good water resistance and can sustain strength in aqueous medium and by adjusting H₃ PO₄ concentration; degradation speed may be controlled. Cells cultured on the surface of the MOC attached and retained viability over the duration of the study.

Comparing physicochemical properties of printed and hand cast biocements designed for ligament replacement

Abstract Abstract In order to combat the low regenerative capabilities of ligaments, full ‘bone to bone’ replacements are required, which will integrate with bone while providing a smooth transition to the replacement soft tissue (tissues surrounding organs in the body, not being bone). This study investigated the use of three-dimensional powder printing technology to form calcium phosphate brackets, previously used for forming bespoke scaffold geometries, to 95±0·1% accuracy of their original computer aided design. The surface and internal structures of the printed samples were characterised both chemically and morphologically and compared with hand moulded cements in the dry state and after 3 days of immersion in phosphate buffered saline. X-ray diffraction, Raman spectroscopy and SEM all showed the presence of brushite in the hand moulded samples and brushite and monetite within the printed samples. Furthermore, the printed structures have a higher level of porosity in the dry state in comparison to the hand moulded samples (36±2·2% compared to 24±0·7%) despite exhibiting a compressive strength of almost double the hand cast material. Although the compressive strength of the printed cements decreases after the 3 day immersion, there was no significant difference between the printed and hand moulded cements under the same conditions. Three-dimensional powder printing technology has enabled the manufacture of bespoke calcium phosphate brackets with properties similar to those reported for hand moulded cements.

A novel method for monitoring mineralisation in hydrogels at the engineered hard–soft tissue interface

The capacity to study the deposition of mineral within a hydrogel structure is of significant interest to a range of therapies that seek to replace the hard tissues and the hard-soft tissue interface. Here, a method is presented that utilises Confocal Raman microscopy as a tool for monitoring mineralisation within hydrogels. Synthetic hard-soft material interfaces were fabricated by apposing brushite (a sparingly soluble calcium phosphate) and biopolymer gel monoliths. The resulting structures were matured over a period of 28 days in phosphate buffered saline. Confocal Raman microscopy of the interfacial region showed the appearance of calcium phosphate salt deposits away from the original interface within the biopolymeric structures. Furthermore, the appearance of octacalcium phosphate and carbonated hydroxyapatite was observed in the region of the brushite cement opposing the biopolymer gel. This study describes not only a method for analysing these composite structures, but also suggests a method for recapitulating the graduated tissue structures that are often found in vivo.

Current Progress in Enthesis Repair: Strategies for Interfacial Tissue Engineering

Complex interfaces have evolved at the junction between tissues of the musculoskeletal system to overcome the problems of impedance mismatch. In this review, we have revisited the anatomy, development, injury and repair of the interface between bone and tendon/ligament, also known as the osteotendinous/osteoligamentous junction or enthesis. Specifically, the existing options for repair have also been discussed along with the current progress of interfacial tissue engineers aiming to manufacture multiphase tissue constructs for repair of these complex interfaces in vitro.