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Development of a highly compliant superelastic compression hosiery for the over 18 millions European citizens suffering from Chronic Venous Insufficiency

Final Report Summary - LOOSEANDTIGHT (Development of a superelastic material that enables its cost-effective application in highly compliant compression hosiery for the over 18 millions Europeans ...)

The main objective of the project was to develop a novel hybrid fabric based on superelastic fibres to manufacture a new concept of compression hosiery that overcomes current limitations.

In order to design the innovative compression hosiery, a preliminary scientific analysis has been carried out to analyse the properties of several superelastic alloys (nickel titanium-based binary alloys and copper-based ternary alloys) and complementary textile fibres to be used in order to manufacture the hybrid textile structure. Furthermore, also an investigation on methodologies and technologies to cover the superelastic wires were performed as well. The investigation on shape memory alloys has led to the conclusion that the nickel-titanium alloy (NiTinol) is the best alloy for the envisaged application. It is interesting to notice that the performed scientific analysis allowed to the selection of practically only a type of NiTi wire, considered as the best choice, but provided in two different state: as drawn or straight annealed, depending on when the final heat treatment to the hybrid textile structure has to be applied.

The project developed a novel manufacturing process to produce virtually infinite length superelastic wires properly shape set (in the frame of the project the chosen shape is the snake shape, but the methodology can be extended to different shapes according to the envisaged application), and the associated processes to wind and un-wind the produced superelastic wires on proper spools by controlling the tensioning during the winding process. A novel hybrid superelastic yarn was also developed. It was composed of Nitinol wires (both straight or in the snake shape set from) covered with different complementary textile fibres including elastane, cotton) with the desired superelastic properties and improved aesthetic and comfort properties for wearing purposes.

Furthermore, a novel hybrid textile structure was developed. It was represented by a semi-integrated approach consisting of knitting an interlay NiTi yarn in a form of shape set wire structure (snake form), properly covered with complementary textile fibres, into the common elastane knitted fabric. This structure is able to exhibit a double effect very effective in compression hosiery application: a geometric elongation effect, thanks to which it is possible to consistently reduce the wearing force and a superelastic effect that guarantees the constant compression level during wearing.

A mathematical model (design tool) for the prediction of the compression level exerted by the compression hosiery was also created. Within the proposed concept of the design tool, the behaviour of the knitted elastan fabric is taken as an input (determined experimentally). The superelastic force-elongation response of the NiTi snaked yarn is simulated and both mechanical responses are deemed to be connected in parallel (the snaked NiTi yarn is integrated as interlayed yarn into the elastan fabric). The model is able to predict with good results the behaviour of the compression hosiery in terms of compression profile, and, from the following figure, it is possible to see the great difference between standard compression hosiery (red curve, high force required during wearing) and the novel developed compression hosiery (blue curve), in which is evident the advantage in terms of very low force required to obtain the necessary elongation for wearing, but at the same time the right compression force exerted during usage.

During the development of such tasks, a novel heat treatment method was conceived and developed. This novel heat treatment method is really very interesting for a number of applications (from compression hosiery and medical stents, to technical and geo-technical textiles, hybrid composites structures, etc.) because it would greatly simplify the production process, only adding this final heat treatment at the end of the cycle. In this way, the superelastic wire, once covered with complementary textile fibres, can be inserted into the usual knitting machines without any changes to the cycle, and in the desired architecture, with no stresses and alteration of its behaviour.