First principles of biomedical engineering – they’re closer than you think

It’s important to realise that beyond the realm of classical engineering principles lays the softer, squishier world of biology and medicine. By being intimately familiar with human anatomy, biomedical engineers are able to more rapidly question whether their concepts are truly viable prior to embarking on further development, using an often-neglected set of first principles – the fundamentals of human body design.

Using a scalpel, I slowly cut the failed tricuspid valve out of the heart of the cadaver. We had been told that our donor body had died of heart failure but nothing prepared me for the sensory impact of the hard, black necrotic tissue surrounding the three leaflet structure – it appropriately reflected what “dead” should look like physically. That was about half a decade ago and not a day goes by where I don’t reflect on my time spent wrist deep in a cadaver, and how it shaped how I approach my work as a biomedical engineer.

When you start designing medical devices, it’s tempting to seek out simplifying assumptions in order to get 80% of the solution in 20% of the time. We can model the circulatory system with pipes, we can model the lungs with foam, and we can model the bones with metal. Reasoning by analogy is often the first step to taking a first principles approach, utilising equations that govern the fundamental processes of our world. And there’s nothing inherently wrong with this approach until you discover that by simplifying the problem definition you’ve missed out on a wealth of potentially more appropriate and lucrative solutions as well.

Let’s take, for example, the case of my former cadaveric colleague’s heart – he had had an aortic valve replacement at one point in his life. Simplifying the complex procedure of heart valve replacement, we could think of the problem like fixing a broken piece of plumbing: restrict the flow of fluid, remove the defective valve, put the new valve in, and let the fluid flow resume hoping that everything worked the first time. And for valve replacement, that’s been more or less the gold standard since advancements made to cardiopulmonary bypass (artificial heart and lung) machines enabled the heart (blood flow) to be stopped during surgery. Using the plumbing mentality, it’s difficult to imagine how you could improve on this procedure which is why it’s important to question and occasionally reject the simplifying models we use for medical device design. Finding the optimal level of detail at which to examine the problem is critical to the success of the endeavour.

In reality, heart valve replacement has started heading towards the plumbing equivalent of replacing leaky valves from your neighbours couch by snaking tools through their plumbing, into the main pipes, back into your flat to the leaky valve and there, instead of taking the old valve out, you just put the new valve in on top of it. This procedure, known as Transcatheter Aortic Valve Replacement (TAVR), in which a new valve is implanted into the heart by navigating through an artery in your leg, is expected to worth $5.5B USD by 2020. If we remained focused solely on the heart valve in absence of the larger circulatory system and body-scale system as a whole, the ability to utilise a less-invasive approach would have been lost before the first ideation session even began.

Ultrasound Elastography is another example where conventional analogies fall short of an optimal solution. Frequently surgeons will probe or palpate tissue with their fingers to try and differentiate boundaries of different structures, like a tumour amongst healthy tissue. This physical sensation can aid with the detection of diseases in certain organs, particularly when the tumour doesn’t show on certain imaging modalities but relies on subjective tactile feedback and cannot be used in regions that can’t be palpated. Obviously then we need to improve the ability of existing imaging systems to “see” the tissue but there’s something significantly cleverer available.

Researchers realised that “seeing” a tumour depended on the type of information being made available. Traditional x-ray imaging relies on the absorption of ionizing radiation by anatomical structures and maps this absorption onto a display, whereas palpation creates a tactile sensation map in the physician’s head. So instead of thinking, “we need a more sensitive imaging system to detect the differences in radiation absorption in the tissue”, they realised that they could blast the target tissue with ultrasound waves, mimicking a palpation then measuring the resulting mechanical properties by observing the response to this “push”. What was created is a novel imaging system that provides a tactile map of tissue stiffness, enabling doctors to more clearly see the tissue they’re interested in without requiring physical access to it.

As efficiency and cost reductions pressure healthcare systems globally, the demand from next generation medical devices requires successful inventions to not only improve outcomes but reduce costs compared to the existing gold standard. By recognising that your medical device needs to be designed with respect to not only the classical mechanical and electrical first principles but the anatomical first principles as well, you can get that much closer to a safer and more effective device.

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