Expanding structures reach out to save lives

Tom Shelley reports on mechanical developments in novel erectable structures with a wide range of potential uses

Tom Shelley reports on mechanical developments in novel erectable structures with a wide range of potential uses A mechanical linkage invented a hundred years ago forms a basis for novel expanding building frames while oriental paper folding has provided the inspiration for new devices to keep open human arteries and air passages. The linkages are additionally applicable to expanding cylinders, columns, apertures and arched roofs, many of striking artistic elegance while the paper folding mechanisms have been expanded to novel columns and masts. Despite their ease of construction, none of the developments have so far been used in anger, mainly, we think, because so few engineers know about them. This, however, could soon change now that they are being promoted by such as the Royal Society, as well as Eureka of course. Once they have been seen in demonstration, the mechanisms provide obvious solutions for roofing sports arenas and convertible cars as well as usage in expandable furniture, emergency buildings and camping tents. The base idea, according Dr Zhong You, a lecturer in the Department of Engineering Science at the University of Oxford, is a three dimensional four bar chain, originally published by one Dr Geoffrey Thomas Bennett in 1903. Dr Bennett was a fellow of Emmanuel College, Cambridge. Typical of Oxford and Cambridge academics of his time, he never patented or even applied his idea to anything useful, but was merely interested in its spatial mathematical properties, part of a lifetime study that led to his becoming a fellow of the Royal Society in 1914. Whereas the four bar chain in two dimensions is well known to all engineers and forms the basis of a plethora of useful devices, Bennett's chain is different in that the axes of the hinges are neither parallel, nor meet at a single point, but point in different directions. The result is that if one pulls two opposite joints away from each other, the arrangement starts to fold so that the other two joints not only move towards each other, but also move in a direction perpendicular to the original plane of the joints. What makes the arrangement really interesting and useful, however, is the effect of joining a large number of linkages together as a network of jointed rods. What starts out as a slightly curving bundle of rods, pulls out to form a rigid arch, or if pulled out even further, forms a cylinder. In its original form, it looks just perfect to stow on the floor of a small helicopter, around the side of the seats, and one of its many potential applications has to be immediately erectable tents and shelters for either or civilian or military use. Constructions are not limited to expandable arches, however. Dr You told Eureka at the Royal Society annual summer exhibition that it is possible to use the mechanism as the basis of any shape, whether it be a stadium roof, or a framework to support an antenna reflector on a space craft. As well as four bar linkages, Dr You is also an expert on three dimensional six bar arrangements called Bricard linkages which open up yet more possibilities. But of even greater possible commercial potential than the chain based structures, the team at Oxford has also been studying structures derived from folded sheets, with a particular view to developing stent grafts superior to those presently available which are made of wire mesh covered with a membrane. Possibly in deference to Dr Kaori Kuribayashi, who has just completed her thesis on the possible manufacture of stents using this technology, they are described as 'Origami', although to our eyes, they bear an even closer relationship to traditional unfolding Chinese paper lanterns. According to Dr Kuribayashi, folding of the stent is achieved by dividing the structure into a series of identical elements with hill and valley folds. The patterns allow the stent to be folded and subsequently expanded both radially and longitudinally. The elements have to deform when the structure is expanded and an important part of the study can been to optimise designs to as to minimise this deformation. As a result of these studies, a stent graft with helical folds has been devised which improves radial strength and ease of deployment. The folds are introduced by adjusting the joining position of the two edges of a sheet that had been symmetrically jointed in an earlier symmetric design. The locations of the helical folds have been optimised for easy folding by considering both geometric aspects of folding and the buckling patterns of a thin walled tube under torsion, which have been found analytically. Folliowing Finite Element Analysis, a stent graft has been made to verify the concept. A number of prototypes of the stent graft of the same size as standard oesophagal and aortal stent grafts have been made using stainless steel and shape memory alloys. The patterns of folds are produced by photochemical etching. It has been demonstrated that the SMA stent grafts self expand smoothly and gradually at a near body temperature. Whereas Bennett had no interest in patenting, the same is not true of his successors, and Oxford University has applied for patents on both the expanding arch and the Origami stents through Isis Innovation. Dr You and his colleagues are actively seeking industry partners to develop rapidly erectable large span shelters for live performance, storage and sports facilities. In addition, a company, Origami Instruments, has been set up to raise funds to take the stent to a production level where clinical trials can be undertaken. At the Summer Exhibition, the team also showed a number of demonstration self expanding masts and beams for possible deployment on space craft, based on the folding metal stent technology, as well as various novel expanding ring supports and irises based on combinations of four bar chains. University of Oxford Department of Engineering Science Dr Zhong's web pages Email Dr Zhong You Eureka says: These structures are wonderful to behold and should have a great future in a wide range of industries Pointers * By constructing assemblages of jointed rods using mathematical rules, it is possible to devise expandable arches, cylinders, stadium roofs and other structures * Based on traditional oriental paper folding technologies - Origami and Chinese paper patterns it has been possible to devise a totally new type of stent, as well as expandable pillars and beams for use on spacecraft