What: Shell Lace Stent Where: Human trachea
Stephen Lawrence Prize winner Tonkin Liu’s smallest scaled project ever gives new meaning to the idea of a tight site. Since 2015, the practice has been developing a new strand of its long-running Shell Lace research into single surface structures with Arup. But this time, instead of applying its biomimicry concept to lightweight structures such as canopies and pavilions, the site is the human windpipe.
Designed to support transplants of the trachea and collapsed airways, the Shell Lace Stent invention has been approved as patent pending.
This unusual piece of technology transfer came about when a medical researcher heard Tonkin Liu give a talk about the Shell Lace concept. She saw potential for its use as a medical stent that performed better than conventional trachea stents. Made of tubular non-tailored mesh, these are prone to slippage, injuries and infection and need frequent replacement.
The practice took up the challenge and set about developing a brief in conjunction with Northwick Park Medical Institute with the aid of a grant from Innovate UK. The key requirements were that it should be easy to install, comfortable, and able to remain in place without migrating up or down. The stent also needed to facilitate air ventilation and be smooth in texture yet able to ‘hold’ a coating of medicine.
Tonkin Liu attended an operation to understand the process for installing a stent in the body and explored the form of the trachea that the stent would be internally bracing. For this, the architects worked with a trachea from a dead pig, which closely matches that of humans.
This led to the first Eureka moment – the trachea was C-shaped yet regular stents were tubular. Another revelation was the amount of tension when the windpipe was cut, and the need to tense against it. The practice researched helix options using 3D printed plastic before finding inspiration for the eventual solution from nature, specifically the way certain petals unfurl, with the Calla Lily a particular reference point.
Tonkin Liu’s design solution was a C-shaped furled stent rather than a closed tube to give more flexibility for different diameter windpipes. This is contoured with three ‘petals’ at top and bottom that overlap against the healthy trachea that the donor trachea is stitched to. The vertical sides of the stent are contoured to ease the fit. Throughout prototyping, the architect worked with Arup to structurally map the design. In particular, this maximised the number of perforations to make the structure as light as possible and facilitate both air flow and the delivery of medicine held within the perforated surface – without compromising its strength. Testing different thicknesses for performance showed 0.7mm was the optimum size, and many iterations of the petals were prototyped.
The next step will be fundraising to enable further research towards bringing the stent to market. This will require work with material experts to explore producing the stent in medical grade silicone, as well as various stages of extensive testing and approvals for the use of it for humans. There is also the potential to explore further locations within the body where the concept could be used.
Although the prototype stent is by far the smallest application of Shell Lace, it is perhaps the biggest in terms of ambition.
‘It’s great for us to be able to go on this journey,’ says Tonkin Liu co-founder Anna Liu.
Unfurling mechanism
The stent is designed to be preloaded into the healthy donor tissue that is transplanted into the body. At this point, the stent is furled, with the two sets of petals each inverted at their tops and tied down together in their loading position via pin holes in the top of each petal. Once the donor stent is in place, these ties are removed to allow the previously tucked in petals to unfurl from their inverted position and settle against the healthy trachea. As it is deployed and relaxes, the sides of the stents brace in an outward ‘hoop’ to push against the inside of the trachea to hold it in place.