PH Sensitive Hydrogel Valves - Sydney Foster

pH Sensitive Hydrogel Valves

Mechanical and electrical valving techniques rely on large amounts of power, complex physical configuration, and control algorithms for fluid control. Some electrical actuators generate excess heat which can unduly affect sensitive organics flowing through the actuators. The need for autonomous flow control, cold valving and easy to manufacture valves can be addressed with pH sensitive hydrogel actuated valves. The valves can either be used to directly control flow of flows at pH range or can be controlled with separate flows of buffers which require no pressurizing.

For the purposes of this page, the valve types will be categorized as direct valving and indirect valving, whereby indirect valving relies on operator pH control to actuate either NC or NO valves and direct valving involves the complete autonomous control of a fluid flow through the swelling of the hydrogel within the stream being controlled.

Hydrogels

Hydrogels are complex crosslinked polymer networks which are characterized by hydrophilic functional groups along the polymer backbones. These loosely cross linked masses can come in a variety of meta-structures (sponge like, fibrous, planar, monolith) and the degree to which they swell can be controlled via pH, and temperature.Hydrogels can be used in all parts of a microfluidic device, depending on their formulation and toughness, and the ability of some hydrogels to change size, shape or rigidity due to stimulus makes them useful for flow control and manipulation.

Ionic Division

Cationic hydrogels swell when introduced to acidic environments and are commonly based off of amine functionalized polymers. These bared ammonium ions become deprotonated in low pH, allowing hydrogen bonding, and thus swelling, to occur.

Anionic hydrogels contain negatively charged ions when dissociated (like carboxylic acid). The bare anionic moieties induce hydrogen bonding from water molecules nearby and swelling is induced.

Structural Variants

Super Porous

Super Porous Hydrogels (SPHs) are hydrogels with microscale pore structures spread throughout the mass. The hydrogels react quickly (5-20 seconds) and have large absorbance, however they are not durable and do not block fluid flow. The unique characteristics are created during cross linking, where pH conditions can be utilized along with additives, to generate a fine foam which later becomes the pores of the hydrogel mass. Because these hydrogels are not durable and can be very easy to permanently deform under relatively low levels of stress, they are not frequently used in valving set ups.[GEM]

Cryo SEM of poly(methacrylic acid-co-acrylamide) Gupta et al

Cryo SEM of poly(acrylamide-co-acrylic acid) while full swollen(scale 1.0 mm) Gemienhart et al

Monolithic

These hydrogels are synthesized via standard sol-gel conditions, in solution, and can be poured or spin coated. Due to the lack of pores or distinguishable subdivisions, these hydrogel structures can be very slow to react to stimulus, as they rely on Fickian Diffusion(link). The physical durability is what makes these hydrogels useful, the tough and defined mass allows for high definition channels and repeated usability.

Fibrous

A new form of hydrogel, these polymer structures are composed of micrometer scale fibers of hydrogel, which can be layered or made into entanglements. Commonly, planar expansion occurs rapidly (<5 seconds) while swelling throughout the mass acts more slowly (1-3 minutes).[JIN] Very little work has been done on using these hydrogels in valves, but their unique swelling character over short periods of time have potential.

SEM image of crosslinked PAA hydrogel fibers Jin et al

Swelling

The mechanism for swelling has been extensively examined by material scientists and chemists alike, and the ratio of swell, from wet to dry is often used to characterize the degree to which a gel swells in a buffered solution. Swell ratios can be mass based:

[math]\displaystyle{ Q=\frac{w_s}{w_d} }[/math]

[math]\displaystyle{ Q=\frac{M_s-M_d}{M_d} }[/math] gupta

[math]\displaystyle{ Q=1+(\frac{w_s}{w_d}-1)\frac{\rho_p}{\rho_w} }[/math] jeanine

Or the ratio can be a relation of volumes:

[math]\displaystyle{ q=\frac{V_s}{V_d} }[/math]park

[math]\displaystyle{ q^{5/3} = \frac{(1/2-\chi)2M_c}{V_1\rho_0\nu^{2/3}(1-3M_c/M_n)} }[/math] Jin

• q is the volume swelling ratio
• χ is the interaction parameter between polymer and solvent
• V₁ is the molar volume of the liquid
• ρ₀ is the density of the dry polymer
• n is the volume fraction of the polymer after cross-linking
• M_c is the molecular weight of the segment between crosslinking junctions
• M_n is the molecular weight of polymer chain before cross-linking.

Valve Types

Direct Valving

Direct valving relies upon the responsive swelling of hydrogels placed in the desired stream to be controlled. These valves act as passive flow controllers, reacting to the changing pH in the subject stream and either swelling or shrinking. These types of valves commonly come as multiple pillars arranged in-line, perpendicular to the flow of the stream.

• 

  Three hydrogel rings are placed upon pillars of PDMS inside the flow channel

• 

  Once pH has been suitably changed, the hydrogel swells, restricting the flow in the channel

Indirect Valving

Indirect hydrogel valve: green fluid is used to control swell of hydrogel(pink) and blue fluid is the flow being controlled by the valve.Case A is the valve open, Case B is the valve closed.

Indirect valving is dependent on a controlled flow of specified pH. Hydrogel pads are placed above points of restriction in a microfluidic device, entrapped, and their swelling action acts to push more durable and less permeable membranes to restrict flow across the valve. The valves are almost always designed as NO type valves, unless the hydrogel is bonded to a NC valve while swollen.

References

1. Zhang, Y, et al. “pH-Sensitive Hydrogel for Micro-Fluidic Valve”. J. Funct. Biomater. 2012, 3, 464-479 [1]
2. Mahdavinia, G.R. , et al. “Modified chitosan 4. Superabsorbent hydrogels from poly(acrylic acid-co-acrylamide) grafted chitosan with salt- and pH-responsiveness properties”. Eu. Poly. J. 2004, 40, 1399-1407 [2]
3. Jeannine, E., et al. “Structure and swelling of poly(acrylic acid) hydrogels: effect of pH, ionic strength, and dilution on the crosslinked polymer structure”. Poly. 2004, 45, 1503-1510 [3]
4. He, T., et al. “Modeling deformation and contacts of pH sensitive hydrogels for microfluidic flow control”. Soft Matter, 2012, 8, 3083 [4]
5. Jin, X., et al. “pH-responsive swelling behavior of poly(vinyl alcohol)/poly(acrylic acid) bi-component fibrous hydrogel membranes”. Poly. 2005, 46, 5149-5160 [5]
6. Arbabi, N., et al. “Study on pH-sensitive hydrogel micro-valves: A fluid–structure interaction approach”. J. Int. Mater. Sys. Struct. 2016, 1-14 [6]
7. Park, J.Y., et al. “A polymeric microfluidic valve employing a pH-responsive hydrogel microsphere as an actuating source”. J. Micromech. Microeng. 2006, 16. 656–663 [7]
8. Gemeinhart, R., et al. “pH-sensitivity of fast responsive superporous hydrogels”. J. Biomater. Sci. Poly. 2000, 11, 1371-1380
9. Ayala, V.C., et al. “Design, Construction and Testing of a Monolithic pH Sensitive Hydrogel-Valve for Biochemical and Medical Application”. Journal of Physics: Conference Series 2007, 90 [8]
10. Gupta, N.V., et al. “Investigation of Swelling Behavior and Mechanical Properties of a pH-Sensitive Superporous Hydrogel Composite”. Iranian Journal of Pharmaceutical Research (2012), 11, 481-493 [9]