Tag Archives: experimental mechanics

Nanoparticle motion through heterogeneous hydrogels

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Over the last couple of years, we have been transitioning a technique, which we developed for tracking the motion of nanoparticles using caustics [see ‘Slow moving nanoparticles‘ on December 13th 2017], from its initial use in exploring mechanics at the nanoscale to applications in nanobiology [See ‘Label-free real-time tracking of individual bacterium‘ on January 25th, 2023] where it has the advantages of functioning in real-time and being label-free (chemical labels can impact motion, protein interactions and cell behaviour).  In the summer, we had couple of articles published in consecutive issues of the Nature journal, Scientific Reports which describe our recent work.  In the first, we have explored the diffusion of nanoparticles through a synthetic analogue of the vitreous humour in order to support the design of novel therapeutics for retinal diseases.  Retinal diseases, such as macular degeneration and diabetic retinopathy, affects millions of people globally and treatment often involves frequent intravitreal injections of anti-vascular endothelium growth factor agents and corticoids.  Delivery of the appropriate dose to the retinal cell layer is challenging due to the complex nature of the vitreous and functionalised nanoparticles offer a potential solution.  In vivo animal testing is inappropriate because of the ethical concerns and poor representation of human eyes and ex vivo testing of cadaveric eyes is unreliable due to the instability of biomechanical and biochemical properties of the vitreous humour.  Hence, we used agar-hyaluronic acid hydrogels as an in vitro model of the vitreous and employed the caustic technique to track the motion of nanoparticles through the hydrogels.  The hydrogels had been validated as a representative model of the vitreous humour by other research groups.  Our tracking technique revealed that the electric charge on the nanoparticles did not affect their diffusion through the hydrogel; however, both the diameter of the particles and the heterogeneous nature of the gel influenced the diffusion.  Nanoparticles with diameters of 200, 100 and 50 nm moved progressively more quickly and over a larger area.  The diffusion rates in hydrogels with a high viscosity (about 450  Pa.s) were consistent throughout the gel implying that the gel was homogeneous, while gels with medium (about 40 Pa.s) to low (about 3 Pa.s) viscosity generated diffusion rates that were distributed bi-modally suggesting a heterogeneous gel with zones of low and high density in which the particles moved more or less freely.  The heterogeneity of a gel renders a global value for viscosity somewhat meaningless and makes comparisons difficult with the vitreous humour because it is also heterogeneous; however, global values of viscosity for porcine vitreous humour are typically 1 Pa.s.  We are continuing this research; however, our published work has demonstrated that the use of caustics in an optical microscope is a reproducible and inexpensive technique for exploring the design of novel nanoscale drug delivery systems for the eye.

Source: Lorenzo Lopez M, Kearns VR, Curran JM, Patterson EA. Diffusion of nanoparticles in heterogeneous hydrogels as vitreous humour in vitro substitutes. Scientific reports. 2024 Jul 29;14(1):1744.

Image: Random track of a nanoparticle superimposed on its image generated in the microscope using a pin-hole and narrowband filter.

More on fairy lights and volume decomposition (with ice cream included)

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Explanation in textLast June, I wrote about representing five-dimensional data using a three-dimensional stack of transparent cubes containing fairy lights whose brightness varied with time and also using feature vectors in which the data are compressed into a relatively short string of numbers [see ‘Fairy lights and decomposing multi-dimensional datasets’ on June 14th, 2023].  After many iterations, we have finally had an article published describing our method of orthogonally decomposing multi-dimensional data arrays using Chebyshev polynomials.  In this context, orthogonal means that components of the resultant feature vector are statistically independent of one another.  The decomposition process consists of fitting a particular form of polynomials, or equations, to the data by varying the coefficients in the polynomials.  The values of the coefficients become the components of the feature vector.  This is what we do when we fit a straight line of the form y=mx+c to set of values of x and y and the coefficients are m and c which can be used to compare data from different sources, instead of the datasets themselves.  For example, x and y might be the daily sales of ice cream and the daily average temperature with different datasets relating to different locations.  Of course, it is much harder for data that is non-linear and varying with w, x, y and z, such as the intensity of light in the stack of transparent cubes with fairy lights inside.  In our article, we did not use fairy lights or icecream sales, instead we compared the measurements and predictions in two case studies: the internal stresses in a simple composite specimen and the time-varying surface displacements of a vibrating panel.

The image shows the normalised out-of-plane displacements as the colour as a function of time in the z-direction for the surface of a panel represented by the xy-plane.

Source:

Amjad KH, Christian WJ, Dvurecenska KS, Mollenhauer D, Przybyla CP, Patterson EA. Quantitative Comparisons of Volumetric Datasets from Experiments and Computational Models. IEEE Access. 11: 123401-123417, 2023.

Enduring, authentic, ancient and modern

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Decorative photograph across SevernTwo weeks ago, over a period of forty-eight hours, I visited four churches. An unusual event for me.  We travelled from Liverpool to Bristol one afternoon to attend a Thanksgiving Service the following morning for an extraordinary engineer and a lovely man, Eddie O’Brien.  The evening before the service, we stayed in a village pub in Oldbury-on-Severn and after dinner walked up the hill to the 13th century church dedicated to St Arilda.  It was locked so we strolled around the overgrown churchyard along a narrow mowed path and enjoyed the view across the Severn to Wales.  The following morning we drove into Bristol city centre to attend the Thanksgiving Service which was held in the Zetland Evangelical church.  The church was plain, unpretentious and packed.  The service was led by a retired pastor who preached with a gentle, thoughtful passion about Eddie’s life and its meaning.  I knew only one, possibly two, facets of his life: his professional life as an engineer and leading exponent of experimental mechanics; and his life as a student.  Eddie was twenty years my senior and thirty years ago I supervised his MPhil and PhD in experimental mechanics.  He was in his fifties and I was in my thirties – it was a challenge for both of us and we learnt from each other.  When he graduated he presented me with a copy of his PhD thesis that he had hand-bound in leather himself.  We left Bristol after the service and drove north across the Severn Bridge to Tintern Abbey where we stopped for lunch looking out over an empty cricket pitch across a green enclosed valley before exploring the ruins of the Cistercian abbey.  The abbey was founded in 1131 and in 1536 it was surrendered to Henry VIII during the dissolution of the monasteries.  The lead from the roof was removed and five hundred years of decay started creating the ruins you can wander around today.  Back in Liverpool, the following evening we went, with our neighbours, to a ‘Music at the Met’ concert at the Liverpool Metropolitan Cathedral called ‘Music for a King’ and featuring uplifting pieces, including ‘Zadok the Priest’ and ‘Crown Imperial’.  The bold grandeur of the concrete structure, richly coloured stained glass, thunderous organ and combined choirs of the anglican and catholic cathedrals contrasted starkly with the simple service of Thanksgiving for Eddie O’Brien we had attended the previous day when we sang hymns recalled from childhood, including ‘The Lord’s My Shepherd’.

Image: view across River Severn to Wales from St Arilda’s churchyard.

Reasons I became an engineer: #2

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Decorative photograph showning the entrance to the Engineering Faculty at the University of SheffieldThis is the second in a series of posts reflecting on my route to becoming an engineer.  In the first one I described how I chose a degree in mechanical engineering so that I would have appreciation of the technical difficulties that engineers might cite when requesting operational changes for a ship that I hoped one day to command [see ‘Reasons I became an engineer: #1’ on April 19th, 2023].  I think I selected mechanical engineering because it provided a broader engineering education than other engineering degrees and I did not know enough to choose any other branch of engineering.  I went to the University of Sheffield and during vacations returned to the Royal Navy serving onboard HMS Active and flew out to join her wherever she was in the world, except when I went to the Royal Navy Engineering College at Manadon outside Plymouth to undertake engineering applications training.  I cast a brass nameplate, which I still have in my office, and made a toolbox that I also still have at home.  After graduation, I returned full-time to the Royal Navy as a sub-lieutenant and started my career as a naval officer in the executive or seaman branch.  However, I did not settle and missed engineering so I asked for and was refused a transfer to the Royal Corps of Naval Constructors who work on the design and development of warships.  As a result, I resigned my commission in the Royal Navy and got a job as a research assistant in the Department of Mechanical Engineering at the University of Sheffield where I registered for a PhD in engineering.  I had taken a positive step towards becoming an engineer but perhaps on the premise of what I did not want to do rather than what I did want to do.