Category Archives: mechanics

Tears in the heart

Figure 7 from Chew et al 1999A couple of weeks ago I wrote about speaking to a workshop on the aorta and reminisced about research on cardiac dynamics from about 15 years ago.  It triggered another memory of research we did more than 20 years ago on the tearing of the leaflets of artificial heart valves made from biological tissue.  We developed a computational model of the stresses associated with a tear developing in a porcine bioprosthetic heart valve.  The black and white images show snapshots of the predicted motion during the cardiac cycle of a damaged valve with a tear at about 11.30 along the edge of the top right leaflet.  The valve was simulated as being implanted to replace the aortic valve and the view is from the aorta, i.e., looking in the opposite direction to the blood flow out of the heart.  The tear causes part of the leaflet to flap outwards as can be seen in the middle snapshots.  The colour image shows the distribution of stress in the leaflet corresponding to the last snapshot of the motion and the concentration of stress around the tip of the tear can be seen which will tend to cause the leaflet to tear further leading to a bigger flap, more regurgitation of blood.  We were really excited about this research when we published it in 1999 but it has attracted relatively little attention in the last 23 years.  I would like to think that we were far ahead of our times but that’s unlikely and probably it was not as exciting as we thought, maybe because it lacked clinical relevance, our model lacked credibility or not many people have found our paper.

Source: Chew GG, Howard IC & Patterson EA, Simulation of damage in a porcine prosthetic heart valve, J. Medical Engineering & Technology, 23(5):178-189, 1999.

 

Aorta: structure to rupture

Decorative image from a video showing predicted flow through aortic valve and resultant stress in leaflets of valveRegular readers have probably already realised that I have very broad interests in engineering from aircraft and power stations [see ‘Conversations about engineering over dinner and haircut‘ on February 16th, 2022] to nanoparticles interacting with cells [see ‘Fancy a pint of science‘ on April 27th, 2022].  So, it will come as no surprise to hear that I gave a welcome address to a workshop on ‘Aorta: Structure to Rupture‘ last week.  The workshop was organised in Liverpool by one of my colleagues, with sponsorship from the British Heart Foundation, and I was invited to welcome delegates in my capacity as Dean of the School of Engineering.  It was exciting on two levels: speaking, for the first time in more than two years, to an audience who had travelled from around the world to discuss research. And because the topic was closely associated with cardiac dynamics, which is a field that I worked in for nearly twenty years until around 2006.  I was part of an interdisciplinary team modelling the fluid-structure interaction in the aortic valve as it opens when blood is pumped through it by the heart and then closes to prevent back flow into the heart.  The team dispersed after I moved to the USA in 2004.  So speaking to the workshop last week was something of a trip down memory lane for me and led me to look up our last publication in the field.  I was surprised to find it was cited seven times last year.

The image in the thumbnail is a snapshot from a video showing the predicted time-varying distribution of blood flow through the aortic valve and the resultant distribution of stress in the leaflets of the valve during a heart beat.  The simultation is described in our last publication in cardiac dynamics: Carmody, C. J., Burriesci, G., Howard, I. C., & Patterson, E. A.,  An approach to the simulation of fluid–structure interaction in the aortic valve. J. Biomechanics, 39(1), 158-169, 2006.

On flatness and roughness

Photograph of aircraft carrier in heavy seas for decorative purposes onlyFlatness is a tricky term to define.  Technically, it is the deviation, or lack of deviation, from a plane. However, something that appears flat to human eye often turns out not to be at all flat when looked at closely and measured with a high resolution instrument.  It’s a bit like how the ocean might appear flat and smooth to a passenger sitting comfortably in a window seat of an aeroplane and looking down at the surface of the water below but feels like a roller-coaster to a sailor in a small yacht.  Of course, if the passenger looks at the horizon instead of down at the yacht below then they will realise the surface of the ocean is curved but this is unlikely to be apparent to the sailor who can only see the next line of waves advancing towards them.  Of course, the Earth is not flat and the waves are better described as surface roughness.  Some months ago I wrote about our struggles to build a thin flat metallic plate using additive manufacturing [see ‘If you don’t succeed, try and try again…’ on September 29th, 2021].  At the time, we were building our rectangular plates in landscape orientation and using buttresses to support them during the manufacturing process; however, when we removed the plates from the machine and detached the buttresses they deformed into a dome-shape.  I am pleased to say that our perseverance has paid off and recently we have been much more successful by building our plates orientated in portrait mode, i.e., with the short side of the rectangle horizontal, and using a more sophisticated design of buttresses.  Viewed from the right perspective our recent plates could be considered flat though in reality they deviate from a plane by less than 3% of their in-plane dimensions and also have a surface roughness of several tens of micrometres (that’s the average deviation from the surface).  The funding organisations for our research expect us to publish our results in a peer-reviewed journal that will only accept novel unpublished results so I am not going to say anything more about our flat plates.  Instead let me return to the ocean analogy and try to make you seasick by recalling an earlier career in which I was on duty on the bridge of an aircraft carrier ploughing through seas so rough, or not flat, that waves were breaking over the flight deck and the ship felt like it was still rolling and pitching when we sailed serenely into port some days later.

The current research is funded jointly by the National Science Foundation (NSF) in the USA and the Engineering and Physical Sciences Research Council (EPSRC) in the UK (see Grants on the Web).

Image from https://laststandonzombieisland.com/2015/07/22/warship-wednesday-july-22-2015-the-giant-messenger-god/1977-hms-hermes-r-12-with-her-bows-nearly-out-of-the-water/

Ice bores and what they can tell us

Map of Greenland sheet showing depth of iceAbout forty years ago, I was lucky enough to be involved in organising a scientific expedition to North-East Greenland.  Our basecamp was on the Bersaerkerbrae Glacier in Scoresby Land, which at 72 degrees North is well within the Arctic Circle and forty years ago was only accessible in summer when the snow receded.  We measured ablation rates on the glacier [1], counted muskoxen in the surrounding landscape [2] [see ‘Reasons for publishing scientific papers‘ on April 21st 2021] and drilled boreholes in the ice of the glacier.  We performed mechanical tests on the ice cores obtained from different depths in the glacier and in various locations in order to assess the spatial distribution of the material properties of the ice in the glacier. This is important information for producing accurate simulations of the flow of the glacier, although our research did not extend to modelling the glacier.  We could also have used our ice cores to investigate the climatic history of the region.  The Greenland ice sheet contains an archive record of the climate on Earth for about the last half million years, stored in the snow and trapped air bubbles accumulated over that time period.  If the ice sheet melts then that unique record will be lost forever.

The thumbnail image is a map of the depth of ice in the Greenland ice sheet.  The map is about five years old and has a wide green fringe along the east coast.  Scoresby Land is the penisula to the north of the large fiord in the middle of the east coast.  In 1982, the edge of the ice sheet was about 80 miles from the Bersaerkerbrae Glacier, whereas today it is at least twice that distance because the ice sheet is receding.

References:

[1] Patterson EA, 1984, A mathematical model for perched block formation. Journal of Glaciology. 30(106):296-301.

[2] Patterson EA, 1984, ‘Sightings of Muskoxen in Northern Scoresby Land, Greenland’, Arctic, 37(1): 61-63.

Image: https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Greenland_ice_sheet_AMSL_thickness_map-en.svg/2000px-Greenland_ice_sheet_AMSL_thickness_map-en.svg.png