Hypoxia-mimicking Co doped TiO2 microporous coating on titanium with enhanced angiogenic and osteogenic activities

A microfluidic oxygen sink to create a targeted cellular hypoxic microenvironment under ambient atmospheric conditions

Physiological oxygen levels within the tissue microenvironment are usually lower than 14%, in stem cell niches these levels can be as low as 0-1%. In cell cultures, such low oxygen levels are usually mimicked by altering the global culture environment either by O₂ removal (vacuum or oxygen absorption) or by N₂ supplementation for O₂ replacement. To generate a targeted cellular hypoxic microenvironment under ambient atmospheric conditions, we characterised the ability of the dissolved oxygen-depleting sodium sulfite to generate an in-liquid oxygen sink. We utilised a microfluidic design to place the cultured cells in the vertical oxygen gradient and to physically separate the cells from the liquid. We demonstrate generation of a chemical in-liquid oxygen sink that modifies the surrounding O₂ concentrations. O₂ level control in the sink-generated hypoxia gradient is achievable by varying the thickness of the polydimethylsiloxane membrane. We show that intracellular hypoxia and hypoxia response element-dependent signalling is instigated in cells exposed to the microfluidic in-liquid O₂ sink-generated hypoxia gradient. Moreover, we show that microfluidic flow controls site-specific microenvironmental kinetics of the chemical O₂ sink reaction, which enables generation of intermittent hypoxia/re-oxygenation cycles. The microfluidic O₂ sink chip targets hypoxia to the cell culture microenvironment exposed to the microfluidic channel architecture solely by depleting O₂ while other sites in the same culture well remain unaffected. Thus, responses of both hypoxic and bystander cells can be characterised. Moreover, control of microfluidic flow enables generation of intermittent hypoxia or hypoxia/re-oxygenation cycles.

STATEMENT OF SIGNIFICANCE

Specific manipulation of oxygen concentrations in cultured cells' microenvironment is important when mimicking low-oxygen tissue conditions and pathologies such as tissue infarction or cancer. We utilised a sodium sulfite-based in-liquid chemical reaction to consume dissolved oxygen. When this liquid was pumped into a microfluidic channel, lowered oxygen levels could be measured outside the channel through a polydimethylsiloxane PDMS membrane allowing only for gaseous exchange. We then utilised this setup to deplete oxygen from the microenvironment of cultured cells, and showed that cells responded to hypoxia on molecular level. Our setup can be used for specifically removing oxygen from the cell culture microenvironment for experimental purposes and for generating a low oxygen environment that better mimics the cells' original tissue environments.

TiO2 nanorod arrays modified Ti substrates promote the adhesion, proliferation and osteogenic differentiation of human periodontal ligament stem cells

Nanostructure coating on titanium (Ti) implants is well known as a cue for directing osteoblast behavior and function. However, effects of nanostructure coatings on dental stem cells have been rarely explored. In this work, assembled TiO₂ nanorod arrays (TNRs) were fabricated on the polished Ti substrates using hydrothermal and sintering methods. The adhesion, morphology, proliferation and osteogenic differentiation of human periodontal ligament stem cells (PDLSCs) seeded onto TNRs substrates were evaluated. Ti substrates were used as control. Rougher TNRs showed better hydrophilicity and protein adsorption capacity compared with Ti control. When seeded on TNRs substrates, PDLSCs exhibited more stretched morphology and higher proliferation rate. Cytoskeletal F-actin expression was markedly promoted for PDLSCs cultured on TNRs substrates under osteogenic induction. Alkaline phosphatase (ALP) activity and mineral deposition were also enhanced by TNRs. Moreover, osteogenesis-related markers of ALP, runt related transcription factor 2 (Runx2) and osteopontin (OPN) of PDLSCs cultured on TNRs substrates were significantly up-regulated at both gene and protein levels when compared to Ti substrates. In conclusion, the unique structure of TNRs provided a biocompatible platform for modulating morphology and function of PDLSCs. The promotion of osteogenic differentiation indicated that the surface modification of implants with TNRs may improve the osteogenic activity of implants and the bone-implant integration in future clinical applications.

Mesenchymal stem cells in the aseptic loosening of total joint replacements

Peri-prosthetic osteolysis remains as the main long-term complication of total joint replacement surgery. Research over four decades has established implant wear as the main culprit for chronic inflammation in the peri-implant tissues and macrophages as the key cells mediating the host reaction to implant-derived wear particles. Wear debris activated macrophages secrete inflammatory mediators that stimulate bone resorbing osteoclasts; thus bone loss in the peri-implant tissues is increased. However, the balance of bone turnover is not only dictated by osteoclast-mediated bone resorption but also by the formation of new bone by osteoblasts; under physiological conditions these two processes are tightly coupled. Increasing interest has been placed on the effects of wear debris on the cells of the bone-forming lineage. These cells are derived primarily from multipotent mesenchymal stem cells (MSCs) residing in bone marrow and the walls of the microvasculature. Accumulating evidence indicates that wear debris significantly impairs MSC-to-osteoblast differentiation and subsequent bone formation. In this review, we summarize the current understanding of the effects of biomaterial implant wear debris on MSCs. Emerging treatment options to improve initial implant integration and treat developing osteolytic lesions by utilizing or targeting MSCs are also discussed. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1195-1207, 2017.