# Cell stimulation with optically manipulated microsources

Entry by Angelo Mao, AP 225, Fall 2010

Title: Cell stimulation with optically manipulated microsources

Authors: Holger Kress, Jin-Gyu Park, Cecile O Mejean, Jason D Forster, Jason Park, Spencer S Walse, Yong Zhang, Dianqing Wu, Orion D Weiner, Tarek M Fahmy & Eric R Dufresne

Journal: Nature Methods

Volume: 6(12)

Pages: 905-909

## Summary

The researchers developed an apparatus for creating gradients of small molecules using optically manipulated microsources. Briefly, the researchers loaded polylactic-co-glycolic acid (PLGA) particles on the micrometer scale with a chemical called formyl-methionine-leucine-phenylalanine (fMLP). The fMLP steadily diffused out of the PLGA beads, and the cells used in this study, neutrophils, would migrate up the concentration gradient of fMLP. Researchers demonstrated that it was possible to move microsources in real time and to capture the movement of neutrophils.

Soft Matter keywords: in vitro, diffusion, gradient

## Overview

The researchers formulated a steady state equation for the concentration gradient around microsources.

$c(\rho, z) = c_{b} + c_{0}\times a(\frac{1}{(\rho^{2}+(z+h)^{2})^{1/2}} + \frac{1}{(\rho^{2}+(z-h)^{2})^{1/2}})$

This equation is not very important, but there are several noteworthy aspects. First is the willingness of the researchers to assume steady state, even though the microsources should eventually run out of the chemoattractant agent. Second is the fact that it is strictly geometric around a point source, so the gradient quite simple.

The researchers tested their apparatus in various way, including by having a cell be drawn to a single microsource (figure 1), by having cell be simultaneously attracted by two microsources (figure 2), by loading microsources with a cell motility inhibitor called cytochalasin D (figure 3), and by moving microsources by optical tweezers and affecting cell response (figure 4).

Figure 1. Neutrophil migration in response to a microsource loaded with fMLP. In all figures, the time progression is by rows from left to right. The time scale is on the order of ~3 minutes..

Figure 1. Neutrophil migration in response to two microsources loaded with fMLP.

Figure 1. Neutrophil migration in response to a microsource loaded with cytochalasin D.

Figure 1. Neutrophil migration in response to a migrating microsource.

## Discussion

The researchers indeed demonstrate an apparatus that is capable of creating chemical gradients that can be manipulated using a technique called optical tweezers. However (presumably this was in an earlier paper), the researchers do not specify how to use their optical tweezers technique, or, more importantly, in what situations it can be used. The cells seem to be moving on top of a 2D glass surface. The problem with this surface is that it is not a "soft matter," and is not physiologically relevant. Materials in the body are often soft, three-dimensional, and best mimicked with polymer-based hydrogels. Another limitation with this apparatus may be, as mentioned earlier, the fact that steady state is assumed with microsources. Since the chemoattractant needs to be loaded into microsources, the steady state necessarily can only apply for a finite time, before the gradient deteriorates. This means that it is difficult to maintain the gradient for long term studies (although moving microsources around is a potential solution).