Polyelectrolyte Complexes for Microfabrication - Falcucci, Foster, and Kallin

Background

Table 1 ^[1]: Several naturally occurring and synthetic polymers commonly used for polyectrolectrolyte complex formation. For example, poly(L-lysine), a polycation, and poly(L-glutamic acid), a polyanion could be mixed to form a polyelectrolyte complex.

In traditional polymers, covalent crosslinkage is responsible for unique properties. However, in polyelectrolyte complexes(PECs), these crosslinkages are replaced by ionic interactions between oppositely charged polymer chains. As its name suggests, a polyelectrolyte is a polymer consisting of multiple charges due to the charges on the base monomers. Polyelectrolytes are typically soluble in water. For example, DNA is a polyanion. When a polyanion and polycation are mixed, they ionically interact, forming a polyelectrolyte complex. Table 1 demonstrates some of the most common polyanion and polycations used to form polyelectrolyte complexes. In polyelectrolyte complexes, charges are typically buried within the complex due to the electrostatic attractions between oppositely charged ions. This creates a surface that is not soluble in water. When oppositely charged polyelectrolytes in counter-ion salt solutions are mixed, a coacervate phase can be reached ^[2]. This ability to form coacervates, and other unique properties of polyelectrolytes give them a promising range of applications, which is continued to be explored.

Current Uses

Chemical resistant coatings, fluorescent indicators for certain bio markers. Some invertebrates uses PECs to adhere to their protective exoskeletons, ensuring a strong and reliable bond.

Properties

PECs prove to be generally very robust materials. By employing physical crosslinking through coulombic charge pairing, PECs are extremely durable materials compared to many polymer samples. The charges form stronger bindings between chains approaching the strength and thermal/reactive resistance of chemically cross-linked polymer networks, while also maintaining the "recyclable" aspects of physical crosslinking. This crosslinking provides many of the interesting phenomena and characteristics associated with the PECs.

Plasticity

Due to the ionic nature of PECs there is a strong electrostatic force that acts between all of the constituent polymer chains and any surrounding materials. In an aqueous environment, these complexes have a ratio of open ionic sites and shielded sites; this ratio is dependent on the number of available counter ions (often from simple ionic salts) and the number of ionic sites along the polymer chains.

Figure 1 ^{[#(Wang and Schlenoff)]}: The salt concentration increases from left to right. The PEC forms a solid when the salt concentration is really low, a coacervate at a medium salt concentration, and a single phase solution at high salt concentration.

Salt Content

Increasing the concentration of salt in aqueous mixture of Polyelectrolytes decreases the viscosity of the complex, up to the point where the PEC becomes full dissolved in the solvent. Removing the salt in a PEC encourages ionic site pairing, drastically increasing the viscosity of the PEC until it becomes a rigid solid which is no longer soluble in water.

This phenomenon has been called "Saloplasticity"

Talk about how salt content impacts the polymer structure ("conformation") and physical properties. We MUST have polymer conformation data backed up by logic involving ionic site behavior, perhaps talking about the work done on doping levels and the doping constant of different salts by schaff and schlenoff

Solvent Quality

Solvent work seems to be less widespread, but good solvents make the PEC act like it has a high counter ion content. Good solvents for PECs tend to be polar and have high dielectric constants(water, oxolane)

an important concept to explain is that good solvents, like high salt, increase polymer molecular clumping,i.e. nano bead formation while poor solvents or low salt content increases the interaction between neighboring polymer chains. This means that poor solvents and low salt allow for more smooth and homogenized molecular distribution.

"Self Healing"

Exploiting the "saloplastic" properties of PECs a self healing property is created. Self healing usually implies a sense of memory in the material, but in the case of PECs the salt content allows for this property. The addition of salt, as previously explained, shields the ionic sites in the polyelectrolyte which removes the physical crosslinking. This creates a more elastic complex and this elastic behavior allows for self healing in the material. This property also implies a method of adhering PECs; by pressing two PECs layers together with a thin layer of salt between the two layers and then removing the salt, the layers, in theory, will form crosslinks and form a single PEC sample.⁴

Use in Microfluidic Devices

Although both the field of microfluidics and use of polyelectrolyte complexes has not yet matured, polyelectrolyte complexes have potential to be used for manufacturing of microfluidic devices. The

Fabrication

The fabrication of PEC microfluidic devices has yet to be perfected, but the most promising technique combines soft lithography to create a PDMS master mold and then pressing the PEC coacervate into the mold. The final step is critical because without an external force the coacervate will not take the shape of the mold and maintain a high resolution. The use of a PDMS mold plays a vital role because its soft malleable structure can peel away from the PEC without damaging the PEC structures.

Problems

Due to the novelty of the material, there are many unknown characteristics and hurdles to overcome for the microfabrication process. The PEC coacervates are generally very sticky, which results in extreme difficulty when molding and pressing the devices. The PEC surfaces have also been shown to have a porous structure due to the shift from the complexes formed in the salt rich coacervate phase to the solid low salt concentration PEC material.³ These pores may interfere with the flow in the channels of the device, which could destroy the laminar flow- the defining and most useful characteristic of microfluidic devices-within the device.

Future Work

PECs are very promising materials, in theory, for the fabrication of microfluidic devices. Their impermeable, unreactive, and durable material properties seem ideal for microfludic devices, where the device scaffolding should provide little to no interference with the fluids within the channels. As a result, current experimentation will unveil whether or not this material is a feesible material for micros fludic devices. By defining the basic characteristics such as swelling properties and resolution will determine whether or not proceeding with microfabrication is doable. These extrapolations combined with data on the materials adhesive properties will provide insight on the best method for fabrication. Then, further analysis of the material will either determine whether the porous surface will provide an issue, and if so how to evade that problem.

References

1. Lankalapalli S, Kolapalli VRM. Polyelectrolyte Complexes: A Review of their Applicability in Drug Delivery Technology. Indian Journal of Pharmaceutical Sciences. 2009. 71(5): 481-487. http://dx.doi.org/10.4103/0250-474X.58165.

2. Wang Y, Kimura K, Dubin Paul: Effects of Micelle Surface Charge Density, Polymer Molecular Weight, and Polymer/Surfactant Ratio. Macromolecules. 2000; 33(9). 3324-3331. http://dx.doi.org/10.1021/ma991886y.

3. Wang and Schlenoff: The Polyelectrolyte Complex/Coacervate Continuum. Macromolecules. 2014. 47: 3108−3116. http://dx.doi.org/10.1021/ma500500q.

4.Reisch, A., Roger, E., Phoeung, T., Antheaume, C., Orthlieb, C., Boulmedais, F., Lavalle, P., Schlenoff, J. B., Frisch, B. and Schaaf, P. (2014), On the Benefits of Rubbing Salt in the Cut: Self-Healing of Saloplastic PAA/PAH Compact Polyelectrolyte Complexes. Adv. Mater., 26: 2547–2551. http://dx.doi.org/10.1002/adma.201304991

5.C.H. Porcel, J.B. Schlenoff*; Biomacromolecules, 10, 2968-2975 (2009), Compact Polyelectrolyte Complexes: “Saloplastic” Candidates for Biomaterials. http://dx.doi.org/10.1021/bm900373c