Flexible polymer adhesive can be a life saver

A novel polymer developed by a high speed aerospace research programme is not only being used as an insulator on leads inserted to save the lives of heart patients, but also as an adhesive in high performance actuators and sensors, as a means of making multilayer circuits without need for conventional adhesives and as a resin in advanced composite and unfilled plastic components. LaRC-SI was invented at NASA, with the initials standing for Langley Research Centre Soluble Imide. It is made by casting or spraying a solution consisting of xylene, N-methyl-pyrrolidinone (NMP) and LaRS-SI powder. Its first patent was issued in 1997. It is, at the same time, biologically and chemically inert, can withstand high and low temperatures, is flexible and can easily be processed. Its inventor Dr Robert Bryant says: "The unique feature of this aromatic polyimide is that it remains soluble after solution imidisation in high boiling aprotic solvents, even at solids contents of 50% by weight. However, once isolated and heated above its Tg of 240°C, it becomes insoluble and exhibits high performance thermoplastic melt flow behaviour." This means that, at different drying temperatures, different amounts of solvent may be removed to make it insoluble, but its retains its melt processability. Recyclable, the polymer can be laminated by heat and pressure without need for adhesives, making it suitable for manufacturing lightweight flexible circuits that require no adhesive bonding between the layers and which can easily be made multilayer. By not requiring adhesives, potential problems arising from bubbling, wrinkling and delamination are avoided. Despite the material costing around $350 per pound, this has not stopped it being used as the adhesive in THUNDER – THin Layer UNimorph Ferroelectric DrivER – piezoelectric actuators made by Face International. These dome shaped devices, available commercially at prices starting at around $100 each, have been studied for a range of aerospace, defence and biomedical applications. In these devices, LaRC-SI is used to bond the PZT piezoelectric ceramic thermally to the stainless steel substrate. Intended applications, some of which may have already been implemented, include use in smart, deformable UAV wings and underwater propulsors. One recent commercial use of LaRC-SI is as a very thin varnish to provide electrical insulation on leads running into the human heart from a small device implanted in the chest. This is a crucial part of a technology called Cardiac Resynchronisation Therapy, or CRT, in which electrical impulses are delivered to the heart muscle in order to resynchronise heart beats and improve blood flow. NASA licensed the technology to Medtronic in July 2004, but the US Food and Drug Administration has only just approved its use in a left heart lead, one of the thinnest available for heart failure patients. The lead is delivered by an inner catheter that allows physicians to place the lead directly in areas of the heart that are difficult to access. Clinical studies show the lead was placed successfully 96.4% of the time. Other applications suggested by NASA include: low pressure bonding for sandwich panel construction; flexible flame retardant foam; dielectric coatings; and films, optical cladding and mounting electronic components on substrates with different coefficients of thermal expansion including metals. Pointers * Material can be applied in the form of a spray, spin, dip coating, paint or spread with a doctor blade. * It can be solution or melt processed and films can be made to bond to themselves by application of heat and pressure. * It is chemically and biologically inert and can be recycled