Dr Timea Feller

Introduction

Since starting my scientific work as an undergraduate student in 2012, I have been interested in the unique mechanical behaviour of fibrin as a biopolymer. I obtained my PhD in 2019 in investigating morphological and mechanical aspects of fibrin network formation and breakdown using Atomic Force Microscopy (AFM). In 2020 I started on a postdoc position at University of Leeds on Prof. Ariëns’s BHF Programme grant investigating the role of fibrin clot architecture on thrombus stability. There I had the opportunity to work with and further develop two methodologies under the lead of Dr Simon Connell: the AFM based lateral fibre pulling and a magnetic microrheometer. While investigating the mechanical behaviour of fibrin fibres made from truncated variants with/without the flexible side chain called αC-region, I found a mechanical behaviour that could not be explained with any current models of the internal structure of fibrin fibres. This led me to a different concept about the possible organisation within fibrin fibres, where high extensibility and strain stiffening behaviour is explained with random, sparse but dynamic connections between the sub-units of fibres called protofibrils. To further investigate this structural model, I was awarded a Wellcome Fellowship (Early Career Award) that started in 2024.

Current major projects include:

• Investigating the internal structure of fibrin fibres to explain its mechanical behaviour
• Using magnetic microrheology and passive rheology to investigate local clot mechanics

Detailed Research Programme

The structure and mechanical properties of fibrin are of key importance in pathologic conditions such as venous thromboembolism, heart attack or ischaemic stroke, as well as in the dissolution of pathologic thrombi. Structure determines the mechanical properties of fibrin, yet the exact structure, especially on sub-fibre level is still not known. Targeting structures playing a key role in fibrin’s mechanical behaviour offers a promising route to the treatment of pathologic conditions and reveal details of diagnostic importance.

Fibrin fibres are highly deformable due to their high extensibility (fibres can extend up to 4-5 times of their original length before rupture) and the relatively low stress required for their extension (1-100 MPa fibre stiffness). This unique behaviour could be explained by entropic elasticity, where the elasticity arises from the force induced orientation of random, unstructured molecular chains. Fibrin does have such an unstructured chain called the αC-region. However, my very recent data showed that the absence of even the complete αC-region does not change fibrin fibre stiffness, especially at low strains. My results emphasize the role of the protofibril backbone: the half staggered, double stranded assemblies of fibrin monomer. Yet, protofibrils are highly organised structures that would suggest an elastic modulus ~3 orders of magnitude higher than shown by fibrin fibres. This led me to a concept of a novel model where frequent branching of protofibrils opens the possibility of internal branching of assembled protofibrils within the fibrin fibres. This would allow selective loading of the protofibrils, meaning that only a few protofibrils per fibre cross-section bear the mechanical load, while other protofibrils are initially unloaded and serve as a rescue mechanism. Such a model provides explanation for both the high extensibility and strain stiffening behaviour of fibrin fibres. In my model, highly branching protofibrils are the key load-bearing elements and the αC-region is important for structural integrity. This new model will provide a fresh perspective that could explain the unique mechanical properties of fibrin. I’m working on a broad investigation of both structure and mechanical properties, from molecule to network level. I plan to bring together all the results to provide a comprehensive model of the structure of fibrin, provide evidence how structure determines mechanical properties of fibrin and investigate how this new understanding can be used to target treatments for pathological conditions.

The viscoelastic behaviour of biological networks at the microscale is an important intermediate step between individual fibre and bulk level, already defined by the complex interplay of mechanical behaviour at multiple levels of molecular and structural organisation. We have developed a system capable of both active and passive microrheology and characterised the mechanical behaviour of blood clots at this microscale. For active microrheology, frequency-dependent viscoelastic properties (loss and storage moduli) of the clot are calculated from the resistance of the embedded superparamagnetic microbeads (4.5µm) against the applied magnetic force. This can be used when the mechanical behaviour is mainly elastic. In the meantime, passive rheology extends mechanical characterisation to earlier timepoints of gelation and later stages of breakdown, more dominated by viscous behaviour. For these experiments, polystyrene microparticles (1µm) were embedded, and the thermal motion tracked using high framerates (6000 Hz), with spatial precision of 10nm. We can quantify the increasing viscosity and the early development of elastic modulus upon gelation and clot breakdown, and the high-speed tracking extends the frequency range of active rheology by two orders of magnitude. Together with the extremely low sample volume (~10-20µl), our system provides a complex viscoelastic characterisation of not only fibrin but other soft hydrogels and biological polymers, where the relatively large forces and low frequency of active rheology are complemented by the sensitive (but weaker) passive rheology at high frequency, increasing the measurement range of the system.