Impact of synovial fluid flow on temperature regulation in knee cartilage

Several studies have reported an increase of temperature in cartilage submitted to cyclic sinusoidal loading. The temperature increase is in part due to the viscous behavior of this tissue, which partially dissipates the input mechanical energy into heat. While the synovial fluid flow within the intra-articular gap and inside the porous cartilage is supposed to play an important role in the regulation of the cartilage temperature, no specific study has evaluated this aspect. In the present numerical study, a poroelastic model of the knee cartilage is developed to evaluate first the temperature increase in the cartilage due to dissipation and second the impact of the synovial fluid flow in the cartilage heat transfer phenomenon. Our results showed that, the local temperature is effectively increased in knee cartilage due to its viscous behavior. The synovial fluid flow cannot significantly preventing this phenomenon. We explain this result by the low permeability of cartilage and the moderate fluid exchange at the surface of cartilage under deformation.

Cartilage self-heating contributes to chondrogenic expression

Philippe Abdel-Sayed, Mohamadreza Nassajian Moghadam, Dominique Pioletti, Arne Christian Vogel

Articular cartilage is a soft tissue showing inelastic properties. Under cyclic loading, inelastic materials may dissipate mechanical energy into heat. In knee cartilage, due to an intrinsic reduced heat convection related to its avascularity, a local temperature increase could be observed. This phenomenon is referred to as self-heating. As cells are sensitive to temperature variation, the energy dissipation could influence their metabolism. The goal of this study was to evaluate the effect of cartilage self-heating on chondrogenic expression. In the first part of this study, using a custom-made deformation calorimeter, we quantified the heat generated in cartilage submitted to cyclic loading at different frequencies. We calculated the corresponding local increase of cartilage temperature. At the cellular level, we then assessed the effect of these temperature variations on chondro-progenitor cell metabolism by measuring the gene expression of transcription factors involved in chondrogenesis. An up-regulation in mRNA expression levels of Sox9 and its co-activator PGC-1α was observed with an increase of temperature. Taken together, the results of this study suggest a dissipation contribution to chondrogenic gene expression. Dissipation phenomena might then be considered as a new variable in mechanobiology.

European Cells & Materials Ltd2013

Replicated microcellular aluminium structures for thermal management

Alexandros Athanasiou-Ioannou

Open-pore aluminium foams are of interest in thermal management of power electronics due to their large specific surface area, low weight and relatively high thermal conductivity. The replication process is a low-cost and simple foam manufacturing method that offers many opportunities for the investigation of the influence exerted by the microstructure (porosity, pore size) and geometry or their combination in the performance of metal foams in heat-exchange applications. In this study, we explored replicated aluminium foams as a part of an integrated heat sink manufactured in one step that seamlessly combines metal and ceramic phases. Replicated aluminium foams were manufactured and tested first under natural and then under forced convection. A dedicated test apparatus was built to test these samples under both natural and forced convection, using the guarding ring technique. In the case of natural convection, the aluminium foams had different pore sizes (125-180 µm, 400-450 µm and 5 mm), porosities (78 to 86%) and geometry (disk, cylinder and finned structures). In all cases, replicated aluminium foams were found to dissipate the same amount of heat as their bulk aluminium (alloy) equivalents. The thermal heat exchange performance of replicated microcellular aluminium was also measured under forced convection. The test rig was modified to measure the pressure drop and volumetric air flow in a cylindrically symmetric jet impingement configuration. Foams of different pore size (125-180, 400-450 and 900-1300 µm), porosity (between 75 and 85 %), height (1 and 2.8 cm) and infiltrated at various pressures (between 2.7 and 70 bars) were manufactured for thermal testing, all having a hollow central channel, through which air is injected at a range of flow rates. The results show that within the covered parameter range foams of higher porosities, larger pore sizes and lower infiltration pressures dissipate more heat at lower cost in terms of pumping power. Shorter heights also dissipate more heat for a given volumetric flow but do so at the cost of an increased pressure drop and hence greater pumping power expenditure. A numerical model, developed and written by Dr. D. Ingram, using the finite-volumes technique, was employed to elucidate how the various parameters influence the heat transfer. The model solves the Darcy-Forchheimer formulation of fluid flow in porous media, which is then coupled to a convective/conductive heat transfer model of the same structure. These simulations were used to perform parametric studies on the influence of height and pore size with regard to the thermal behaviour of replicated foams. Furthermore, the model was benchmarked against the experimental results; overall, despite some discrepancies with measured temperature profiles, the model captures data trends well, and provides insight on the physical phenomena that underlie the experimental data and observations. In particular, the presence and influence of recirculating air flow along the inner portion of the heat exchanger structures was identified. Finally, we successfully fabricated the proposed integrated multimaterial structure and tested it under forced convection, using the same conditions as the replicated aluminium foams. Results demonstrate that this type of structures can be effectively used to cool power electronics.

EPFL2017

Composite Double-Network Hydrogels To Improve Adhesion on Biological Surfaces

Pierre-Etienne Bourban, Martin Broome, Peyman Karami, Azadeh Khoushabi, Christophe Moser, Dominique Pioletti, Andreas Schmocker, Céline Samira Wyss

Despite the development of hydrogels with high mechanical properties, insufficient adhesion between these materials and biological surfaces significantly limits their use in the biomedical field. By controlling toughening processes, we designed a composite double-network hydrogel with ∼90% water content, which creates a dissipative interface and robustly adheres to soft tissues such as cartilage and meniscus. A double-network matrix composed of covalently cross-linked poly(ethylene glycol) dimethacrylate and ionically cross-linked alginate was reinforced with nanofibrillated cellulose. No tissue surface modification was needed to obtain high adhesion properties of the developed hydrogel. Instead, mechanistic principles were used to control interfacial crack propagation. Comparing to commercial tissue adhesives, the integration of the dissipative polymeric network on the soft tissue surfaces allowed a significant increase in the adhesion strength, such as ∼130 kPa for articular cartilage. Our findings highlight the significant role of controlling hydrogel structure and dissipation processes for toughening the interface. This research provides a promising path to the development of highly adhesive hydrogels for tissues repair.

2018

Replicated microcellular aluminium structures for thermal management

Alexandros Athanasiou-Ioannou

EPFL2017