Strain mediated arterial degradation is critically influenced by matrix content and collagen crimp
03 April 2018
Gaul, RT (1,2); Nolan, DR (1,2); Ristori, T (3,4); Bouten, CVC (3,4); Loerakker, S (3,4); Lally, C (1,2)
1 Trinity Centre for Bioengineering, Trinity College Dublin
2 School of Engineering, Trinity College Dublin
3 Department of Biomedical Engineering, Eindhoven University of Technology, the Netherlands
4 Institute for Complex Molecular Systems, Eindhoven University of Technology, the Netherlands
The Engineers Ireland Biomedical Research Medal 2018, awarded to a PhD student who has been deemed to be making an outstanding contribution to the field of biomedical engineering research in Ireland, has been won by Robert Gaul. The criteria for the process involve an oral presentation, and the research paper will be judged on the following: importance/relevance of research topic; technical depth of the paper; and scientific novelty of the paper. There were 23 applicants in 2017, and this rose by 40 per cent to reach the figure of 32 this year. The prize is an Engineers Ireland Biomedical Medal and a €1,000 honorarium, kindly sponsored by DePuy Synthes.
Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide with more than 17 million deaths each year . Many of these diseases affect arteries in the body, which are the blood vessels that carry nutrient and oxygen-rich blood away from the heart to the surrounding tissue.
Blockages of arteries are often treated through the deployment of a stent: a metallic scaffold which expands a blocked blood vessel to restore blood flow. Every minute, six stents are deployed worldwide, with this number increasing year on year . A little known fact, outside of the medical devices industry, is that Ireland is responsible for producing 80 per cent of all stents manufactured globally . Unlike conventional engineering materials, arteries adapt and remodel to changes in their environment.
In many respects, they are the original smart material. Unfortunately, this ability to adapt can lead to a reoccurrence of the blockage in the vessel after stent deployment; this is known as in-stent restenosis. In order to better identify patients at risk of cardiovascular disease and improve the design of these crucial devices, it is important to understand how blood vessels respond and adapt to their mechanical environment.
Arteries are a class of fibre-reinforced material, where collagen fibres provide structural reinforcement of the vessel wall much in the same way that reinforced concrete is strengthened by steel rebar. In normal physiological conditions, arteries experience a dynamic, pulsatile flow; expanding and contracting with each heartbeat.
Arterial disease and stent deployment can dramatically alter the mechanical environment experienced by these vessels. Understanding the adaptation response of the vessel to these different loading environments will allow the development of clinical tools that are capable of identifying early disease development as well as the development and improvement of existing medical devices, such as stents.
To achieve this aim, our work investigates the influence of strain on the orientation of the collagen fibre network in arterial tissue, as well as the degradation response of this network, which may lead to the weakening and eventual rupture of tissue.
Fibre architecture using SALS
To investigate changes in the underlying arterial structure due to different mechanical environments, such as arterial disease and stenting, we have developed a small angle light scattering (SALS) system (Figure 1) . In SALS, laser light is passed through a sample to identify fibre orientation and alignment information.
The system was designed and developed in-house and optimised for use with arterial tissue for the very first time. The system incorporates a helium-neon (HeNe) laser which passes light through focusing lenses before passing through the sample which is held in an automated positioning stage controlled using labVIEW.
The resulting light which is scattered by the constituent fibres is recorded by a camera and analysed using a custom Matlab code to determine fibre orientation and alignment information across the tissue.
Once validated against current gold standards for assessing tissue microstructure, the SALS system was used to determine fibre orientation and reorientation within arterial tissue, which may occur in situations such as stenting (Figure 2).
SALS is a highly versatile technique for looking at structure and it has also enabled us to look at the fibre architecture in corneal tissue, the dura mater which covers the brain and the pericardial membrane which surrounds the heart but which is also used to manufacture leaflets in prosthetic heart valves.
Studies on these tissues can also aid in explaining other mechanically driven pathologies including the development of keratoconus in corneal tissue, the side effects of head trauma in the dura mater, and the performance and life span of prosthetic heart valves.
Having established the potential of SALS to measure fibre orientation and reorientation, the system was used to identify degradation of this fibre architecture to gain insights into the degeneration of arterial tissue. The degradation response of collagen is of particular interest as it is also responsible for stimulating the production of new collagen, maintaining a mechanical equilibrium.
Other biological tissues have shown strain dependent degradation responses whereby the application of a strain increases or decreases the rate of degradation. These processes were assessed in arteries by developing a custom temperature controlled water bath incorporating a micrometer which allowed the application of a controlled and measurable uniaxial strain on the arterial sample.
A solution, known as collagenase, which breaks down the structurally important collagen fibres, was then added to the water bath under different loading environments to mimic in vivo degradation. Collagen fibre alignment was assessed before and after degradation to identify if preferential degradation or load induced protection of collagen fibres occurred.
Experimental findings indicated a ‘V-shaped’ degradation response took place at the fibre level (Figure 4A) whereby applying an increased strain initially decreased degradation, before increasing degradation once more when strain was above a critical threshold. These findings bear similarities with responses seen in other biological tissues such as bone which looks to maintain homeostasis or stable equilibrium through the laying down of new bone or removal of old unused bone.
Mechanically driven degradation
Having established the degradation response of the collagen fibres using SALS, we next wanted to investigate the degradation response occurring at the tissue level. To achieve this, a series of stress relaxation experiments were carried out using a Zwick uniaxial mechanical testing machine with a 20 N load cell.
These tests were also carried out in a temperature controlled bath in the presence of collagenase to mimic in vivo degradation which was calculated based on the rate of decay, 1⁄τ, of the measured force. This force decay is predominantly due to degradation of the load bearing collagen fibres (Figure 3).
Interestingly, a vastly different degradation response was seen at the whole tissue level, with an initial increase in degradation observed as opposed to a decrease (Figure 4B). These findings suggest that stiff diseased arterial tissue may undergo accelerated degradation before strain induced protection initiates. Increasing compliance, however, could lead to excessive tissue weakening and potentially tissue rupture.
To help explain the different degradation responses seen at the collagen and tissue scale, we have developed a novel numerical model explicitly incorporating a matrix material and collagen fibres with varying degrees of collagen fibre crimp as seen experimentally.
Despite modelling a ‘V-shaped’ degradation response for collagen as observed in the SALS results, the model was capable of predicting the more complex mechanical degradation response seen experimentally at the tissue scale. This was only possible after incorporating load bearing tissue matrix and variations in collagen crimp, highlighting the crucial role these play in the remodelling of arterial tissue.
Two seemingly conflicting experimentally observed results were explained through the development of a novel numerical model, but only once collagen fibre crimp and the influence of the load sharing arterial matrix were represented in the model. This model is capable of explaining the previously unknown degradation response of arterial tissue, which may have implications in the development and progression of arterial disease and the design and development of new medical devices.
In the longer term, these findings may aid in the development of clinical screening tools to detect and treat degenerative arterial diseases. In the case of an aneurysm, where an artery wall weakens and begins to balloon outward, the increased strain may lead to accelerated degradation, weakening the wall further.
If interventional measures are not taken, a runaway process of degradation and weakening may ensue, leading to eventual rupture, which is nearly always fatal. Our model may aid in detecting such early arterial degeneration and therefore help identify the need for clinical intervention before it is too late. By fully understanding what constitutes a healthy strain environment, we can optimise the design of medical devices, such as stents, to ensure healthy strain conditions are maintained, thereby allowing arteries to regain a homeostatic environment.
Robbie Gaul joined Prof Lally’s lab in September 2014 having been awarded an Irish Research Council postgraduate scholarship to carry out his PhD in the area of arterial remodelling. Prior to this, he studied for his BEng in biomedical engineering and received his degree from DCU in 2014, finishing top of his class. He is one of the developers of the recently launched, Engineers Ireland-funded Irish Biomedical Innovation Forum (www.ibif.ie), which aims to connect healthcare professionals and engineers to identify and provide creative, effective, and affordable solutions to current unmet clinical needs. Gaul recently won the 2018 Engineers Ireland annual Biomedical Research Medal and €1,000 honorarium, sponsored by DePuy Synthes.
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