Difference between revisions of "Non-equilibrium cluster states in colloids with competing interactions"

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== Summary ==
 
== Summary ==
As lab-on-a-chip technology becomes more advanced, there is an ever-growing need for improved control of cells. Also, the link between electronic and microfluidic systems has not yet been fully exploited. This paper describes a system using microelectrodes and dielectrophoresis (DEP) in conjunction with microfluidics to trap cells in prescribed patterns. Finite-element models were used in the design of the device. The authors also show the range over which the electric fields used do not harm the cells. Increasing the control of cells within such setups has the potential to greatly expand the possibly designs of biosensors and actuators.
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The authors examined a colloidal system in which there were competing short-range and long-range interactions. It has recently been found that such systems exhibit a stable cluster phase if low volume fractions of the particles are present. In this paper, the effects of varying attractive and repulsive strength were observed. In addition, at high volume fractions of particles, gelation is observed, which, like the clustering, is also dependent on attraction strength. By changing the strengths of attraction, the nucleation and rearrangement process can be controlled, leading to controlled gel formation.
  
 
== Methods/Results ==
 
== Methods/Results ==
  
The image below shows the setup and construction for the microelectrodes. One can see that it is a standard electrode design on a silicon dioxide substrate. It should also be noted that the cell-side surface is coated with fibronectin to enhance cell adhesion in islands (standard microcontact printing) and that the Pluronic solution is a blocking agent to contain the "islands" of cells. On the right, one can see how the device works. When cells flow through the channel, many are attracted to the electrodes via DEP. This occurs because the cells are more polarizable than the surrounding media, meaning that there is a net electric force towards the electrode. The cells that migrate to the vicinity of the electrode can bind to the fibronectin, which stabilizes them in the flow. The unbound cells are then flushed out, leaving the user with attached cells until the voltage ceases and new flow can rinse away the cells. The authors used finite element models to optimize the parameters of this design.
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The authors used fluorescently-labeled PMMA spheres in a mixture of cyclohexyl bromide (CHB) and cis-decalin as the colloid mixture. Here, the spheres, become slightly positively charged, which results in a long-range repulsion. Then, by adding polystyrene, the authors are able to introduce an attractive force as well, now resulting in competing interactions. Microscopy was performed on the colloid, as different volume fractions of the spheres were introduced. The figure below shows increasing volume fractions as one moves down the figure. It can be seen that at low-volume fractions, the spheres form clusters, but at higher volume fractions, the structures are more linear. The right half of the figure shows radius of gyration versus cluster size. One can see that indeed, as the volume fraction increases, the growth is more linear and even leads to branching of the structures.
  
 
[[image:Darnell8_1.jpg|thumb|500px|center]]
 
[[image:Darnell8_1.jpg|thumb|500px|center]]
  
The second figure shows the effects of DEP on cell health and morphology. In the first figure, it can be seen that above a threshold, the cells are not able to withstand the voltage, resulting in most of the cells dying. In the second plot, one can see that even after 48h, the cells with DEP proliferate a similar amount to those without. Also, one can see that with and without DEP, the morphology of the cells is similar, showing no large negative effect of the DEP on the cell viability.  
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The authors then used a model to predict the relationship between energy required for nucleation and cluster size. The first part of the figure below shows that there exists an ideal size at which these energies are minimized and that this energy increases with increasing strength of attraction. The second plot shows that the nucleation energy barrier decreases with increasing cluster size.
  
 
[[image:Darnell8_2.jpg|thumb|500px|center]]
 
[[image:Darnell8_2.jpg|thumb|500px|center]]
  
Finally, the authors showed the improvement in making cells adhere to micropatterned islands when these electrodes are used. The figure below shows that when electrodes are used the cells find the islands with 70% accuracy but that this number drops to 17% accuracy without the electrodes.
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Finally, the authors showed that these colloids can form gel structures by tuning their attractions. Similar to the first plot, one can see that at low volume fractions(on the left), the aggregates are clustered, while at high volume fractions, the aggregates are more linear.  
  
 
[[image:Darnell8_3.jpg|thumb|center|350px]]
 
[[image:Darnell8_3.jpg|thumb|center|350px]]
 
== Connection to Soft Matter ==
 
== Connection to Soft Matter ==
  
Electric fields and forces are extremely important in soft matter physics, especially as more biological systems are considered. This paper showed one application of electric fields in harnessing the properties of cells to control them, but there exist many more. For instance, considering cells such as cardiomyocytes or neurons, which use action potentials, the leveraging of the electrical considerations within soft matter physics could lead to novel interactions between artificial systems and these cell types. In addition, work such as that conducted in Bob Westervelt's lab shows the next steps in the work reported here, where the same phenomenon is leveraged, but where the location of the DEP field is also controllable.
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Colloids are an important aspect of soft matter physics, in part due to their ability to self-assemble into higher-order structures. This paper is relevant in that it examines different parameters that can be fine-tuned to achieve various self-assembled structures. By understanding these interactions, the prospect of building materials starting with particles and tweaking environmental conditions to control self-assembly becomes possible. Especially with the recent focus on gels for biomaterial applications, it also seems possible that biomolecules could be incorporated early in this gelation process, providing a new method of encapsulation for drug delivery.

Revision as of 14:19, 11 November 2011

Entry by Max Darnell, AP 225, Fall 2011

Reference:

Title: Non-equilibrium cluster states in colloids with competing interactions

Authors: Tian Hui Zhang, Jan Klok, R. Hans Tromp, Jan Groenewold and Willem K. Kegel

Journal: Soft Matter DOI: 10.1039/c1sm06570j (2011)


Summary

The authors examined a colloidal system in which there were competing short-range and long-range interactions. It has recently been found that such systems exhibit a stable cluster phase if low volume fractions of the particles are present. In this paper, the effects of varying attractive and repulsive strength were observed. In addition, at high volume fractions of particles, gelation is observed, which, like the clustering, is also dependent on attraction strength. By changing the strengths of attraction, the nucleation and rearrangement process can be controlled, leading to controlled gel formation.

Methods/Results

The authors used fluorescently-labeled PMMA spheres in a mixture of cyclohexyl bromide (CHB) and cis-decalin as the colloid mixture. Here, the spheres, become slightly positively charged, which results in a long-range repulsion. Then, by adding polystyrene, the authors are able to introduce an attractive force as well, now resulting in competing interactions. Microscopy was performed on the colloid, as different volume fractions of the spheres were introduced. The figure below shows increasing volume fractions as one moves down the figure. It can be seen that at low-volume fractions, the spheres form clusters, but at higher volume fractions, the structures are more linear. The right half of the figure shows radius of gyration versus cluster size. One can see that indeed, as the volume fraction increases, the growth is more linear and even leads to branching of the structures.

Darnell8 1.jpg

The authors then used a model to predict the relationship between energy required for nucleation and cluster size. The first part of the figure below shows that there exists an ideal size at which these energies are minimized and that this energy increases with increasing strength of attraction. The second plot shows that the nucleation energy barrier decreases with increasing cluster size.

Darnell8 2.jpg

Finally, the authors showed that these colloids can form gel structures by tuning their attractions. Similar to the first plot, one can see that at low volume fractions(on the left), the aggregates are clustered, while at high volume fractions, the aggregates are more linear.

Darnell8 3.jpg

Connection to Soft Matter

Colloids are an important aspect of soft matter physics, in part due to their ability to self-assemble into higher-order structures. This paper is relevant in that it examines different parameters that can be fine-tuned to achieve various self-assembled structures. By understanding these interactions, the prospect of building materials starting with particles and tweaking environmental conditions to control self-assembly becomes possible. Especially with the recent focus on gels for biomaterial applications, it also seems possible that biomolecules could be incorporated early in this gelation process, providing a new method of encapsulation for drug delivery.