The properties of articular cartilage, which lines bones in joints, depend partly on repulsion between components of the material. A new synthetic gel that mimics this feature has rare, direction-dependent properties. See Letter p.68
The work of materials scientists usually focuses on attractive forces, which underpin the reinforcement of polymers by strong fibres or particles1, and the self-repairing abilities of rubbery materials through hydrogen bonding2. But on page 68 of this issue, Liu et al.3 report their use of repulsive forces in the design of a hydrogel — a water-swollen polymer network — that exhibits fascinating direction-dependent behaviour. The material might be useful in applications that require a reduction of vibrations.
The new hydrogel contains nanometre-scale sheets of a titanium oxide known as titanate(IV) nanosheets (TiNS), arranged cofacially (in planes with their faces aligned towards each other). TiNS consist of only surface atoms, and adopt an ultrathin (7.5-ångström) two-dimensional crystal-like structure — 150 stacked nanosheets would have the same thickness as a human hair. In water-based media, the surfaces have a high density of negative charges, which are balanced by an over-layer of positively charged ions, thus forming electric double layers that repel each other and ensure efficient dispersion of the nanosheets.
If TiNS were simply mixed into a hydrogel, they would adopt the thermodynamically most favourable orientation, in which the sheets are orthogonal to each other. But Liu and colleagues observed that TiNS align cofacially when placed in a magnetic field strong enough to overcome the energy barrier to the formation of this arrangement. They therefore magnetically aligned TiNS in a solution of a hydrogel precursor, and then polymerized the precursor, trapping the nanosheets in the resulting polymer network so that they did not orient back to the orthogonal position.
The hydrogel turned out to have some remarkable properties. First, a simple visual inspection revealed that it is almost transparent from one angle, but completely opaque from another. This is clear evidence of strong orientation (structural order) in the material, and confirms that the nanosheets are ideally aligned. Such perfect structural order is rarely seen.
Second, Liu and co-workers observed that the hydrogel exhibits impressive mechanical behaviour, even though the concentration of the nanosheets is a mere 0.8% by weight. When the authors compressed the material orthogonal to the nanosheet plane, the resistance from the hydrogel was several times higher than that from compression parallel to that plane. This is the opposite of what happens in conventional fibre-reinforced materials, which are more resistant to compression parallel to the axis of alignment of the fibres. This unusual behaviour of the new material arises from the repulsive forces between the nanosheets, which prevents the layers of sheets from getting closer when compressed. And when the researchers applied shear (a force coplanar with the cross-section of the material) parallel to the nanosheet plane, resistance was about four times less than when shear was applied orthogonal to that plane. This is because the layers of nanosheets can slide across each other with almost no friction when parallel shear is applied.
Liu et al. report that the material's unusual mechanical behaviour has excellent vibration-damping properties when a large, continuous oscillatory motion is applied vertically to cylindrical samples that carry a load on top. Conventional hydrogels would initially compress and then undergo cycles of expansion and compression — this was also what happened when the authors applied an oscillatory force parallel to the nanosheet planes of their material (Fig. 1a). But when they applied such a force orthogonally to the nanosheet planes, the hydrogel counterintuitively showed restricted, almost solely horizontal deformation (Fig. 1b).
The hydrogel can thus efficiently direct force from one direction to the plane orthogonal to the force — in Liu and colleagues' words it is an excellent vibration isolator. An explanation for this rare behaviour is that, when a vertical force is applied to the hydrogel, the easiest way for the material to dissipate the energy is by shearing in the plane orthogonal to the deformation. Such efficient vibration isolation is reminiscent of a trampolinist at the end of a competitive routine, when the gymnast needs to become perfectly still after landing on the elastic trampoline. Gymnasts achieve this through tremendous body control largely involving technique, but also assisted by articular cartilage that isolates vertical movement in the soft joints of the body.
Articular cartilage is a complex hydrogel consisting of proteoglycan gel — essentially a protein-based substance that underpins the extracellular matrix of connective tissue — within which chondroitin sulphate (which consists of chains of sugars) is dissolved. The load-bearing and damping properties required for joints are provided by systematic structuring of collagen fibres and chondrocytes (the cells that maintain cartilage), along with repulsive forces between the negatively charged proteoglycan molecules4. The load-bearing and damping properties of Liu and co-workers' hydrogel closely resemble those of articular cartilage, although its structure is much simpler than that of the natural material.
So could the new hydrogel, or another material that exploits repulsive forces, be used as artificial cartilage? It is estimated that a human knee or hip joint can experience one million loading cycles per year. These large, cyclic stresses and strains may cause tiny cracks on the surface of articular cartilage or within the bulk material. These cracks can be repaired by chondrocytes, but may grow and accumulate into microscopically observable damage4. It would therefore probably be too ambitious to replace articular cartilage with the hydrogel unless a self-healing mechanism for the new material can be developed. But self-healing mechanisms are based on attractive forces; exploiting such forces in a system based on repulsion may not be easy without destroying desirable properties. Even so, the new hydrogel will surely be developed into many interesting products — for example, it could be used in microelectronic applications, in which the hydrogel could act as a matrix between electronic elements to strongly reduce harmful vibrations.