Hydrogels are attractive biomaterials for replicating cellular microenvironments, but attention needs to be given to hydrogels diffusion properties. A large body of literature shows the promise of hydrogels as 3D culture models, cell expansion systems, cell delivery vehicles, and tissue constructs. Surprisingly, literature seems to have overlooked the important effects of nutrient diffusion on the viability of hydrogel-encapsulated cells. In this paper, we present the methods and results of an investigation into glucose and oxygen diffusion into a silated-hydroxypropylmethylcellulose (Si-HPMC) hydrogel. Using both an implantable glucose sensor and implantable oxygen sensor, we continuously monitored core glucose concentration and oxygen concentration at the centre of hydrogels. We demonstrated that we could tune molecular transport in Si-HPMC hydrogel by changing the polymer concentration. Specifically, the oxygen diffusion coefficient was found to significantly decrease from 3.4 × 10 to 2.4 × 10 m s as the polymer concentration increased from 1% to 4% (w/v). Moreover, it was revealed during in vitro culture of cellularized hydrogels that oxygen depletion occurred before glucose depletion, suggesting oxygen diffusion is the major limiting factor for cell survival. Insight was also gained into the mechanism of action by which oxygen and glucose diffuse. Indeed, a direct correlation was found between the average polymer crosslinking node size and glucose parameters, and this correlation was not observed for oxygen. Overall, these experiments provide useful insights for the analysis of nutrient transport and gas exchange in hydrogels and for the development of future cellular microenvironments based on Si-HPMC or similar polysaccharide hydrogels.

Membranes for guided bone regeneration (GBR) were prepared from the synthetic biodegradable polymer poly-D,L-lactic/glycolic acid (PLGA). This GBR membrane has a bi-layered structure with a dense film to prevent gingival fibroblast ingrowth and ensure mechanical function, and a micro-fibrous layer to support colonization by osteogenic cells and promote bone regeneration. Hydrolysis and biodegradation were both studied in vitro through soaking in phosphate buffered saline (PBS) and in vivo by implantation in the subcutis of rats for 4, 8, 16, 26, 48 and 52 weeks. Histology revealed an excellent colonization of the micro-fibrous layer by cells with a minimal inflammatory reaction during resorption. GBR using the synthetic PLGA membrane was evaluated on critical-size calvaria defects in rats for 4 and 8 weeks. Radiographs, micro-computed tomography and histology showed bone regeneration with the PLGA membrane, while the defects covered with a collagen membrane showed a limited amount of mineralized bone, similar to that of the defect left empty. The biofunctionality of the PLGA membranes was also compared to collagen membranes in mandible defects in rabbits, associated or not with beta-tricalcium phosphate granules. This study revealed that the bi-layered synthetic membrane made of PLGA was safer, more biocompatible, and had a greater controlled resorption rate and bone regeneration capacity than collagen membranes. This new PLGA membrane could be used in pre-implantology and peri-odontology surgery.

A major limitation of the 2D culture systems is that they fail to recapitulate the in vivo 3D cellular microenvironment whereby cell-cell and cell-extracellular matrix (ECM) interactions occur. In this paper, a biomaterial scaffold that mimics the structure of collagen fibers was produced by jet-spraying. This micro-fiber polycaprolactone (PCL) scaffold was evaluated for 3D culture of human bone marrow mesenchymal stromal cells (MSCs) in comparison with a commercially available electrospun scaffold. The jet-sprayed scaffolds had larger pore diameters, greater porosity, smaller diameter fibers, and more heterogeneous fiber diameter size distribution compared to the electrospun scaffolds. Cells on jet-sprayed constructs exhibited spread morphology with abundant cytoskeleton staining, whereas MSCs on electrospun scaffolds appeared less extended with fewer actin filaments. MSC proliferation and cell infiltration occurred at a faster rate on jet-sprayed compared to electrospun scaffolds. Osteogenic differentiation of MSCs and ECM production as measured by ALP, collagen and calcium deposition was superior on jet-sprayed compared to electrospun scaffolds. The jet-sprayed scaffold which mimics the native ECM and permits homogeneous cell infiltration is important for 3D in vitro applications such as bone cellular interaction studies or drug testing, as well as bone tissue engineering strategies.

In situ forming hydrogels that can be injected into tissues in a minimally-invasive fashion are appealing as delivery vehicles for tissue engineering applications. Ideally, these hydrogels should have mechanical properties matching those of the host tissue, and a rate of degradation adapted for neo-tissue formation. Here, the development of in situ forming hyaluronic acid hydrogels based on the pH-triggered condensation of silicon alkoxide precursors into siloxanes is reported. Upon solubilization and pH adjustment, the low-viscosity precursor solutions are easily injectable through fine-gauge needles prior to in situ gelation. Tunable mechanical properties (stiffness from 1 to 40 kPa) and associated tunable degradability (from 4 days to more than 3 weeks in vivo) are obtained by varying the degree of silanization (from 4.3% to 57.7%) and molecular weight (120 and 267 kDa) of the hyaluronic acid component. Following cell encapsulation, high cell viability (> 80%) is obtained for at least 7 days. Finally, the in vivo biocompatibility of silanized hyaluronic acid gels is verified in a subcutaneous mouse model and a relationship between the inflammatory response and the crosslink density is observed. Silanized hyaluronic acid hydrogels constitute a tunable hydrogel platform for material-assisted cell therapies and tissue engineering applications.