Monday, 4 April 2016

Paper published in Scientific Reports

Our work on assessing the chage in deformability of red blood cells from patients with the condition diabetic retinopathy has been published as R. Agrawal et al.  'Assessment of red blood cell deformability in type 2 diabetes mellitus and diabetic retinopathy by dual optical tweezers stretching technique', Scientific Reports 6 15873, doi:10.1038/srep15873 (2016).

From the abstract: A pilot cross sectional study was conducted to investigate the role of red blood cells (RBC) deformability in type 2 diabetes mellitus (T2DM) without and with diabetic retinopathy (DR) using a dual optical tweezers stretching technique. A dual optical tweezers was made by splitting and recombining a single Nd:YAG laser beam. RBCs were trapped directly (i.e., without microbead handles) in the dual optical tweezers where they were observed to adopt a “side-on” orientation. RBC initial and final lengths after stretching were measured by digital video microscopy, and a Deformability index (DI) calculated. Blood from 8 healthy controls, 5 T2DM and 7 DR patients with respective mean age of 52.4yrs, 51.6 yrs and 52 yrs was analysed. Initial average length of RBCs for control group was 8.45 ± 0.25 μm, 8.68 ± 0.49 μm for DM RBCs and 8.82 ± 0.32 μm for DR RBCs (p < 0.001). The DI for control group was 0.0698 ± 0.0224, and that for DM RBCs was 0.0645 ± 0.03 and 0.0635 ± 0.028 (p < 0.001) for DR group. DI was inversely related to basal length of RBCs (p = 0.02). DI of RBC from DM and DR patients was significantly lower in comparison with normal healthy controls. A dual optical tweezers method can hence be reliably used to assess RBC deformability.

The movie below shows a red blood cell being stretched using our optical tweezers. 


Tuesday, 15 March 2016

Optical Tweezers: Principles & Applications

The textbook Optical Tweezers: Principles and Applications, by P H Jones, O M Marago and G Volpe was published in Europe in 2015, and in the Americas in February 2016.  

Combining state-of-the-art research with a strong pedagogic approach, this text provides a detailed and complete guide to the theory, practice and applications of optical tweezers. In-depth derivation of the theory of optical trapping and numerical modelling of optical forces are supported by a complete step-by-step design and construction guide for building optical tweezers, with detailed tutorials on collecting and analysing data. Also included are comprehensive reviews of optical tweezers research in fields ranging from cell biology to quantum physics. Featuring numerous exercises and problems throughout, this is an ideal self-contained learning package for advanced lecture and laboratory courses, and an invaluable guide to practitioners wanting to enter the field of optical manipulation. The text is supplemented by, a forum for discussion and a source of additional material including free-to-download, customisable research-grade software (OTS) for calculation of optical forces, digital video microscopy, optical tweezers calibration and holographic optical tweezers.

" is quite simply excellent..." (M Padgett)
" amazing Optical Tweezers Software toolbox..." (J J Saenz)
"...a must for all Ph.D. students, researchers, and engineers [...] dealing with optical tweezers..." (M I Mishchenko)

Dr Zhirong Liu joins the group

Dr Zhirong Liu has joined the UCL Optical Tweezers Group as a China Scholarship Council finded visiting researcher.  Dr Liu is an Associate Professor at East China Jiaotong University where his research interests include optical trapping, wave propagation and scattering.  He will be staying with us for one year to work on both experiments and theory of optical tweezers.

Tuesday, 19 January 2016

LCN / Physics / School of Pharmacy "Meet & Greet"

The department of Physics and Astronomy (P&A), the London Centre for Nanotechnology (LCN), and the School of Pharmacy (SOP) have organised a "meet and greet" event to facilitate links between physics / nanotechnology research on one hand, and pharmaceutical research on the other. The main aim of this event is to highlight respective research activities, expertise and facilities, and identify potential for cross-departmental collaborations and funding applications. This will be achieved by a series of short presentations on research questions, expertise, facilities and industrial collaborations, and by ample opportunity to chat over coffee/tea and lunch.

The event is open for UCL academics, postdocs and PhD students working in relevant or potentially relevant areas.

Wednesday 20th January 2016, 10:00am - 3:30pm

LMCB seminar room, UCL Laboratory for Molecular Cell Biology (map)


10.00 – 10.15 Welcome and coffee

10.15 – 10.30 Introduction to the morning, with Simon Gaisford (SOP, morning chair), Jon Butterworth (
P&A, Head of Department), Andrew Fisher (LCN, Director)

10.30 – 12:00 Short presentations (15min each)
- Soma Somavarapu (SOP)
- Andela Saric (
- Sudax Murdan (SOP)
- Bart Hoogenboom (LCN / P&A)
- Thanh Nguyen (LCN/

12.00 – 1.00 Networking lunch

1.00 – 1.15 Introduction to the afternoon, with Bart Hoogenboom (LCN/P&A, afternoon chair) and Duncan Craig (SOP, Director)

1.15 – 2.45 Short presentations (15min each)
- Duncan Craig (SOP)
- Jochen Blumberger (LCN/P&A)
- Simon Gaisford (SOP)
- Phil Jones (
- Gareth Williams (SOP)
- David Bowler (LCN/

2.45 – 3:30 Wrap up followed by coffee/tea

Monday, 23 November 2015

COST Visit to IPCF-CNR, Messina

Tom and Onofrio in Messina
UCL Optical Tweezers Group PhD student Tom Smart recently undertook a Short-Term Scientific Mission (STSM) to our colleagues at the NanoSoft Lab, IPCF-CNR (Messina), supported by the COST Action MP1205 Optofluidics.  The aim of Tom's visit was to learn the techniques of Raman spectroscopy on optically trapped particles, which directly addresses the COST action goal of "detection, identification and manipulation of biomolecules and nanomaterials."

Wednesday, 23 September 2015

SPIE OTOM XII Conference Proceedings

The proceedings of the SPIE Optical Trapping and Optical Manipulation Conference XII, held in San Diego in August 2015 have been published, including three contributions from the UCL Optical Tweezers Group:

T. J. Smart, C. J. Richards, R. Bhatnagar, C. Pavesio, R. Agrawal and P. H. Jones.  'A study of red blood cell deformability in diabetic retinopathy using optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945820, doi 10.1117/12.2191281 (2015)
From the abstract: Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus (DM) in which high blood sugar levels cause swelling, leaking and occlusions in the blood vessels of the retina, often resulting in a loss of sight. The microvascular system requires red blood cells (RBCs) to undergo significant cellular deformation in order to pass through vessels whose diameters are significantly smaller than their own. There is evidence to suggest that DM impairs the deformability of RBCs, and this loss of deformability has been associated with diabetic kidney disease (or nephropathy) - another microvascular complication of DM. However, it remains unclear whether reduced deformability of RBCs correlates with the presence of DR.

Here we present an investigation into the deformability of RBCs in patients with diabetic retinopathy using optical tweezers. To extract a value for the deformability of RBCs we use a dual-trap optical tweezers set-up to stretch individual RBCs. RBCs are trapped directly (i.e. without micro-bead handles), so rotate to assume a `side-on' orientation. Video microscopy is used to record the deformation events, and shape analysis software is used to determine parameters such as initial and maximum RBC length, allowing us to calculate the deformability for each RBC.  A small decrease in deformability of diabetes cells subject to this stretching protocol is observed when compared to control cells.

T. J. Smart, C. J. Richards, Xiang Han, S. Siwiak-Jaszek and P. H. Jones.  'Correlated fluctuations of optically trapped particles',  Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945823, doi 10.1117/12.2190820 (2015)
From the abstract: We present a study of correlated Brownian fluctuations between optically confined particles in a number of different configurations.  First we study colloidal particles held in separate optical tweezers.  In this configuration the particles are known to interact through their hydrodynamic coupling, leading to a pronounced anti-correlation in their position fluctuations at short times.  We study this system and the behavior of the correlated motion when the trapped particles are subject to an external force such as viscous drag.

The second system considered is a chain of optically bound particles in an evanescent wave surface trap.  In this configuration the particles interact both through hydrodynamic and optical coupling.  Using digital video microscopy and subsequent particle tracking analysis we study the thermal motion of the chain and map the covariance of position fluctuations between pairs of particles in the chain.  The experiments are complemented by Brownian motion simulations. 

C. J. Richards, T. J. Smart, P. H. Jones and D. Cubero.  'Low frequency dynamical stabilisation in optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945825, doi 10.1117/12.2190822 (2015)
From the abstract: It is well known that a rigid pendulum with minimal friction will occupy a stable equilibrium position vertically upwards when its suspension point is oscillated at high frequency.  The phenomenon of the inverted pendulum was explained by Kapitza by invoking a separation of timescales between the high frequency modulation and the much lower frequency pendulum motion, resulting in an effective potential with a minimum in the inverted position.

We present here a study of a microscopic optical analogue of Kapitza's pendulum that operates in different regimes of both friction and driving frequency.  The pendulum is realized using a microscopic particle held in a scanning optical tweezers and subject to a viscous drag force.  The motion of the optical pendulum is recorded and analyzed by digital video microscopy and particle tracking to extract the trajectory and stable orientation of the particle.  In these experiments we enter the regime of low driving frequency, where the period of driving is comparable to the characteristic relaxation time of the radial motion of the pendulum with finite stiffness.

In this regime we find stabilization of the pendulum at angles other than the vertical (downwards) is possible for modulation amplitudes exceeding a threshold value where, unlike the truly high frequency case studied previously, both the threshold amplitude and equilibrium position are found to be functions of friction.  Experimental results are complemented by an analytical theory for induced stability in the low frequency driving regime with friction.

Friday, 21 August 2015

Paper published in Optics Letters

Our paper on measuring the optical binding interaction between microparticles in an evanescent wave surface trap has been published as Xiang Han and P. H. Jones, Evanescent wave optical binding forces on spherical microparticles Optics Letters 40 4042-4045 (2015).
From the abstract: In this Letter, we demonstrate stable optical binding of spherical microparticles in counter-propagating evanescent optical fields formed by total reflection at a dielectric interface. The microspheres are observed to form one-dimensional chains oriented parallel to the direction of propagation of the beams. We characterize the strength of the optical binding interaction by measuring the extent of Brownian position fluctuations of the optically bound microspheres and relating this to a binding spring constant acting between adjacent particles. A stronger binding interaction is observed for particles near the middle of the chain, and the dependence of the binding strength on incident laser power and number of particles in the chain is determined.