Difference between revisions of "All-aqueous core-shell droplets produced in a microfluidic device"

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(New page: ''Entry by Max Darnell, AP 225, Fall 2011 '' Reference: '''Title:''' All-aqueous core-shell droplets produced in a microfluidic device '''Authors:''' Iwona Ziemecka, Volkert van Steijn,...)
 
 
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'''Journal:''' Soft Matter, 2011, Advance Article DOI: 10.1039/C1SM06517C  
 
'''Journal:''' Soft Matter, 2011, Advance Article DOI: 10.1039/C1SM06517C  
  
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'''Keywords:''' [[liposome]], [[aqueous two-phase system]], [[microfluidics]], [[emulsion]]
  
 
== Summary ==
 
== Summary ==
  
In biology, the cell utilizes a number of structures to encapsulate aqueous solutions. For example, liposomes and vesicles can encapsulate proteins and other cargo, as well isolate the surrounding environment from deleterious internal conditions such as low pH. Such methods are effective in biology in allowing enzymes to function at varied conditions without impacting the rest of the cell, as well as providing a means to transport cargoes within and outside the cell.  
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A current trend in bioengineering is the delivery and encapsulation of different cargoes for biological use. Relevant examples would include delivering drugs, or synthetic intracellular compartments. Although there are many technologies that address this issue, such as polymersomes or liposomes, the aqueous two-phase system (ATPS) is a simple, biocompatible model. In an ATPS, there exist two regions of water, one inside the other. Each layer contains a different solute, and the solute properties dictate the separation between these two phases. One key advantage of an ATPS from a bio-application perspective is that the structure allows for small molecules to freely diffuse between the phases, distinguishing the ATPS from membranous compartments such as liposomes.
  
The artificial leveraging of such structures for bioengineering uses holds great promise in a number of areas. For instance, polymersomes could be used for drug delivery, intra-cell bioreactors for enzymatic reactions, and could be used in artificial cells. one of the main issues, however, with developing artificial vesicles is that there is very little control over vesicle formation, as the process is mainly mediated by self-assembly, which is poorly understood. One alternative method has involved a stream of polymer containing suspension directed at the cargo of choice, but such a method has showed poor efficacy due to poor control over all of the polymer in the stream.
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Microfluidics is a valuable tool that can be leveraged to produce such structures, but up to this point, an oil phase or organic solvents were always used. This paper examines a microfluidic method to create ATPSs using only water and the solute.
 
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In the past, water-in-oil emulsions have been used to aggregate emulsions, but an approach involving water-in-oil emulsions formed via capillary microfluidics is a promising technique to create polymersomes with a high degree of control. The key to this technique is choosing different solvents, such as the disparity between solvation drives the attraction of the two phases of the double-emulsion.  
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== Methods/Results ==
 
== Methods/Results ==
  
Poly(ethylene glycol)-b-poly(lactic acid) [PEG(5000)-b-PLA(5000)] was chosen as the copolymer that would ultimately form the polymersome membrane. It was dissolved into a mixture of chloroform and hexane, and a double-emulsion was created via a previous method in a glass capillary microfluidic device. In this setup, the PEG is in the middle-phase while the PLA is in the solvent-phase. A relation yielding a negative spreading coefficient is given as follows:
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PEG and dextran were chosen as the solutes for a proof-of-concept. A microfluidic device was fabricated using standard methods, and is given in Figure 1. To create the double emulsions, a stream of PEG solution was combined with a stream of dextran solution. If this were to occur simply by merging to streams, the streams would flow in parallel because of the low Reynold's number. A key feature of the device architecture is a piezoelectric disc, which perturbs one of the flows in a pulsed fashion, allowing the to streams to form droplets. As seen in the figure, the first junction results in mixing, the second results in a sheathing by dextran, and the third acts to sever the fluid connection between successive droplets.
[[image:Darnell2_1.jpg|250px|center]]
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A negative spreading coefficient implies that there is an attractive force between the outer-middle and middle-inner phases, thus yielding the micelle. Due to the difference in solubility involving hexane and chloroform and the negative spreading coefficient, it becomes energetically favorable for dewetting to take place as shown in Figure 1:
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[[image:Darnell3_1.jpg|thumb|500px|center]]
 
[[image:Darnell3_1.jpg|thumb|500px|center]]
  
In other words, the dewetting and subsequent attraction of the two phases is partially driven by the diffusion of chloroform out of the polymersome. The resulting degree of attraction between the two phases was quantified by measuring the contact angle between the two, given in Figure 2:
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The authors then quantified the volume and diameter of the ATPSs as to better understand and control their morphology. The volume is given by <math>V=q/f</math>, where ''q'' is the sum of all input flows, and ''f'' is the frequency of perturbation. This relation yields that the diameter of the ATPS is given by <math>d=(6q/f\pi)^(1/3)</math>. To verify this relation, the diameter of the droplets were measured for different flow rates.
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Figure 2a) shows the device without the perturbation to cause droplet formation, while Figure 2b) shows the relationship between the model for droplet volume and experimental results.
 
[[image:Darnell3_2.jpg|thumb|500px|center]]
 
[[image:Darnell3_2.jpg|thumb|500px|center]]
  
 
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Key to this work is the notion that the ATPSs will form two phases a period of time after mixing. Figure 3 shows the formation of distinct phases over a period of time, as the ATPS travels down the microfluidic channel.
Finally, the authors tested the practical application of such an approach, in this case encapsulating a fluorescent dye in one of the polymersomes created using such a method. Figure 3 shows the dyes encapsulated by the polymersomes.
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[[image:Darnell3_3.jpg|thumb|center|350px]]
 
[[image:Darnell3_3.jpg|thumb|center|350px]]
 
== Connection to Soft Matter ==
 
== Connection to Soft Matter ==
  
Wetting is one of the most important phenomena in soft matter physics. This paper is a great example of how wetting can be coupled to diffusion to achieve a desired result. It is also significant in that it shows how the phenomena of wetting is not confined only to hard surfaces and liquids, but also applies at smaller length scales. It would be interesting to see such coupling in other areas, between wetting and elasticity and other mechanical properties, for instance.
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The problem of excluding oil from a double emulsion is an interesting one. Over the course of phase formation, the authors noticed that the phases grow faster than in the case of Ostwald ripening, which is a case in which small solutes dissolve and recrystalize on larger crystals. Ostwald ripening, however, is more commonly used to describe emulsions involving an oil phase. The homogeneity of the solvent (water) in this case may provide an interesting case of double emulsion physics in which to study the effects of the solvent.

Latest revision as of 15:04, 30 November 2011

Entry by Max Darnell, AP 225, Fall 2011

Reference:

Title: All-aqueous core-shell droplets produced in a microfluidic device

Authors: Iwona Ziemecka, Volkert van Steijn,* Ger J. M. Koper, Michiel T. Kreutzer and Jan H. van Esch

Journal: Soft Matter, 2011, Advance Article DOI: 10.1039/C1SM06517C

Keywords: liposome, aqueous two-phase system, microfluidics, emulsion

Summary

A current trend in bioengineering is the delivery and encapsulation of different cargoes for biological use. Relevant examples would include delivering drugs, or synthetic intracellular compartments. Although there are many technologies that address this issue, such as polymersomes or liposomes, the aqueous two-phase system (ATPS) is a simple, biocompatible model. In an ATPS, there exist two regions of water, one inside the other. Each layer contains a different solute, and the solute properties dictate the separation between these two phases. One key advantage of an ATPS from a bio-application perspective is that the structure allows for small molecules to freely diffuse between the phases, distinguishing the ATPS from membranous compartments such as liposomes.

Microfluidics is a valuable tool that can be leveraged to produce such structures, but up to this point, an oil phase or organic solvents were always used. This paper examines a microfluidic method to create ATPSs using only water and the solute.

Methods/Results

PEG and dextran were chosen as the solutes for a proof-of-concept. A microfluidic device was fabricated using standard methods, and is given in Figure 1. To create the double emulsions, a stream of PEG solution was combined with a stream of dextran solution. If this were to occur simply by merging to streams, the streams would flow in parallel because of the low Reynold's number. A key feature of the device architecture is a piezoelectric disc, which perturbs one of the flows in a pulsed fashion, allowing the to streams to form droplets. As seen in the figure, the first junction results in mixing, the second results in a sheathing by dextran, and the third acts to sever the fluid connection between successive droplets.

Darnell3 1.jpg

The authors then quantified the volume and diameter of the ATPSs as to better understand and control their morphology. The volume is given by <math>V=q/f</math>, where q is the sum of all input flows, and f is the frequency of perturbation. This relation yields that the diameter of the ATPS is given by <math>d=(6q/f\pi)^(1/3)</math>. To verify this relation, the diameter of the droplets were measured for different flow rates.

Figure 2a) shows the device without the perturbation to cause droplet formation, while Figure 2b) shows the relationship between the model for droplet volume and experimental results.

Darnell3 2.jpg

Key to this work is the notion that the ATPSs will form two phases a period of time after mixing. Figure 3 shows the formation of distinct phases over a period of time, as the ATPS travels down the microfluidic channel.

Darnell3 3.jpg

Connection to Soft Matter

The problem of excluding oil from a double emulsion is an interesting one. Over the course of phase formation, the authors noticed that the phases grow faster than in the case of Ostwald ripening, which is a case in which small solutes dissolve and recrystalize on larger crystals. Ostwald ripening, however, is more commonly used to describe emulsions involving an oil phase. The homogeneity of the solvent (water) in this case may provide an interesting case of double emulsion physics in which to study the effects of the solvent.