eprintid: 235
rev_number: 15
eprint_status: archive
userid: 7
dir: disk0/00/00/02/35
datestamp: 2009-08-26 16:22:00
lastmod: 2015-05-29 19:50:38
status_changed: 2009-08-26 16:22:00
type: report
metadata_visibility: show
item_issues_count: 0
creators_name: Anthony, Adam
creators_name: Breward, Chris
creators_name: Edwards, David A.
creators_name: Gratton, Michael
creators_name: Haider, Mansoor
creators_name: Joshi, Yogesh
creators_name: Milgrom, Timur
creators_name: Pelesko, John A.
creators_name: Schleiniger, Gilberto
creators_name: Xiao, Zunlei
corp_creators: Melinda K. Duncan
corp_creators: Brian P. Danysh
corp_creators: Kirk J. Czymmek
corp_creators: Tapan P. Patel
title: Characterizing Molecular Diffusion in the Lens Capsule
ispublished: pub
subjects: medicine
studygroups: mpi23
companyname: University of Delaware
full_text_status: public
abstract: We tackle the following questions:

1. Why does the double exponential fit better than the single exponential?
2. Why does the time at which you terminate the data alter the predictions?
3. Is there a more robust way to fit the data than is currently being used?
4. Does the effect of transfer between the mobile molecules and the bound ones explain the difference between the two types of graphs?
problem_statement: We are interested in studying diffusion through the lens capsule because diffusion is important for lens development and growth, nutrient and waste release, drug delivery, ocular inflammation and for cataract formation and treatment. To determine the diffusivity of various molecules through the lens capsule, experiments have been carried out using a technique known as fluorescence recovery after photo-bleaching (FRAP). In this process, the whole lens is first immersed in a bath of fluorescing molecules and left to soak for more than an hour, which allows the molecules to diffuse into the lens capsule and to ensure that the whole system is in chemical and diffusional equilibrium. Some of the molecules within the scaffold remain free to diffuse around in the medium and some become bound to the scaffold. The proportion in each of these "compartments", and the affinity of the molecules for the scaffold, depends on their chemistry (e.g., size, charge, etc.).

Currently, the post-experiment data processing involves the following recipe. First, the normalized data is plotted and fitted with either a "single exponential" or a "double exponential" where they treat all the parameters in as independent. Unsurprisingly, the "double exponential fit" with 5 independent fitting parameters fits the data better than the "single exponential" with 3 parameters. After the "half life" parameter" has been determined, the diffusivity is calculated from it using a formula taken from a paper by Axelrod, et al.

There are two types of behavior exhibited by the kind of molecules used in the experiments:

1. The intensity curve tends to a steady state over the time scale of the experiment. This graph is associated with molecules that are tightly bound to the scaffold (and cannot leave).
2. After the initial exponential transient, the intensity slowly increases back to its original value. This graph is associated with experiments where the bound molecules are exchanging with unbound ones.

The experimentalists are concerned with the procedures they use to interpret the data.
Firstly, they are worried about the fact that they neglect all the influence of the third dimension, assuming that diffusion occurs only in a plane with no z-axis influence.
Secondly, they are worried about whether the bleaching creates an aurora at the edge of the ROI which would alter their results.
Thirdly, they are worried about when to truncate their time series and to fit the data: they find that truncating their data 10s later can alter the fit parameters by 10%.

There are also concerns about how the data is normalized. During the actual experimental run, two regions are imaged: the bleached region and a control region. In an ideal situation, these two regions would have the same properties before the experiment, and only after bleaching would the readings diverge. Unfortunately, due to the random nature of the porous matrix, as well as varying optical properties of the device, the bleaching is not the same in both areas.
date: 2007
citation:   Anthony, Adam and Breward, Chris and Edwards, David A. and Gratton, Michael and Haider, Mansoor and Joshi, Yogesh and Milgrom, Timur and Pelesko, John A. and Schleiniger, Gilberto and Xiao, Zunlei  (2007) Characterizing Molecular Diffusion in the Lens Capsule.  [Study Group Report]     
document_url: http://miis.maths.ox.ac.uk/miis/235/1/debio.pdf