This EPSRC funded project aims to explore the potential for additively manufacturing replacement heart valve frames using laser metal fusion, LMF.

There are three main areas of focus:

• Optimal process settings for 3D printed thin strut heart valve frames
• Frame design for redo-TAVI
• Humanitarian availability of heart valve replacement in the Global South

A new concept in replacement heart valve design

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Degenerative heart valve disease

Degenerative heart valve disease is a growing problem in the ageing populations of Europe and North America. Further, tens of millions of both young and old people in low- and middle-income countries experience valve failure following bacterial throat infections.

3D printed phantoms of diseased valves are manufactured from
computer models constructed from patient scans

The current state of the art for the treatment of heart valve disease is rapidly moving towards transcatheter approaches, seeking to deploy replacement valves comprising frames that are laser cut from metallic tubes. However, to develop new valve concepts that can better address existing and emerging challenges globally, alternative manufacturing methods will likely be necessary. In response to this opportunity, we successfully conducted a pilot study that additively manufactured valve frames using laser metal fusion (LMF).

Now, we seek to fundamentally optimise and assess the precision, accuracy and quality of the LMF manufacturing process for valve frames. It will draw upon world-leading expertise in device design, precision manufacturing and high-resolution imaging.

At the end of the project, we will have (i) a detailed understanding of optimal settings for maximum precision, accuracy and structural integrity of metal sintered valve frames and (ii) developed two novel device concepts made possible by additive manufacturing, one of which will be specifically designed for treating young patients all over the world.

Transcatheter aortic valve implantation, TAVI

TAVI offers a minimally invasive option for implanting a replacement heart valve, RHV, in a failing degenerated aortic valve inside the heart.

TAVI valves typically comprise three components: the valve leaflets, a stent-like frame and a skirt. The skirt serves two purposes. First, the leaflets are stitched onto the skirt such that the skirt is attached to the frame, thus avoiding directly stitching the leaflets to the frame. Second, the skirt provides a seal between the frame and the aorta.

A SAPIEN XT valve being prepared before a TAVI procedure

Redo-TAVI

Redo-TAVI addresses the full life pathway requirement for patients when an implanted valve fails and needs to be replaced. With redo-TAVI, a failing prosthetic valve is removed and replaced by a new valve, thus avoiding the reduction in flow area produced when implanting a new valve inside the failing valve. The redo procedure can be repeated as many times as necessary during a patient's lifetime. The concept is depicted in the Youtube video, playable in the right hand banner.

A humanitarian focus

Heart valve disease in the Gloabl South, including South Asia, South America and Africa is largely due to degeneration of valve leaflets following streptococcal throat infection. This is known as rheumatic heart valve disease (RHVD) and it affects tens of millions of patients in low- and middle-income countries, LMIC. In contrast to the developed parts of the world

• both young and old people are vulnerable to RHVD in LMIC;
• there is low availability of treatment and cardiology expertise and
• treatment, if available, is unaffordable for most patients.

Our aim is to develop replacement heart valves that offer a viable treatment pathway for patients in the Global South.

Methods

(a) Laser metal fusion

RHV frames are being manufactured by Croft AM Ltd and by the University of Southampton using a Concept Laser M2 3D metal printer.

University of Southampton 3D metal printer and

high resolution computer tomography facilities

A key advantage exists in the ability to manufacture multiple frames in a single print cycle. Each frame needs to be carefully removed from the build plate.

Multiple frames are manufactured on a single build plate. Frame close-up (right).

(b) Experimental analysis

Manufacturing settings such as laser power and scan speed are varied leading to quality maps of manufacturing accuracy and print quality.

Variation of strut thickness with laser power. Surface roughness map (right).

Wide ranging tests and assessments are being performed on the printed frames in order to determine optimal manufacturing settings. Typically, the frames are sand-blasted before measuring the porosity within the frame struts and the surface roughness.

(c) Benchtop testing

There is a significant challenge with respect to the strength of very thin struts, less than one third of a mm. Extensive testing starts with tensile tests aiming to measure the ultimate tensile strength and elongation to break for different strut thicknesses.

Tensile testing of thin structures.

Tensile test results demonstrate the reduced strength of very thin structures.

(d) Computer modelling

Computer modelling is being used to assess replacement heart valve performance and to develop new design concepts.

Redo-TAVI demonstration - see the Youtube video.

The aim is to support the design process by comparing computer simulations and benchtop experiments of the deployment of replacement heart valves into models of real diseased aortic valves. In this process, patient heart scans are processed to recreate computer models of the aortic valve which are then used in the computer simulations and from which phantom models are 3D printed for use in benchtop experiments. Multiple materials are used in the phantoms to replicate, as closely as possibe, the different material propoerties of relatively soft, elastic aortic tissue, valve leaflets and the hard calcified plaques that tend to impregnate the native valve leaflets.

The deployed shapes of the frames - following deployment in the 3D printed phantoms - are determined experimentally by scanning them in the high resolution CT scanners available in the MuVis Centre at the University of Southampton.