Impact of PDMS-Based Microfluidics on Belousov–Zhabotinsky Chemical Oscillators

Abstract

Sub-nanoliter volumes of the Belousov−Zhabotinsky (BZ) reaction are sealed in microfluidic devices made from polydimethylsiloxane (PDMS). Bromine, which is a BZ reaction intermediate that participates in the inhibitory pathway of the reaction, is known to permeate into PDMS, and it has been suggested that PDMS and bromine can react (J. Phys. Chem. A. 108, 2004, 1325−1332). We characterize the extent to which PDMS affects BZ oscillations by varying the volume of the PDMS surrounding the BZ reactors. We measure how the oscillation period varies with PDMS volume and compare with a theoretical reaction−diffusion model, concluding that bromine reacts with PDMS. We demonstrate that minimizing the amount of PDMS by making the samples as thin as possible maximizes the number of oscillations before the BZ reaction reaches equilibrium and ceases to oscillate. We also demonstrate that the deleterious effects of the PDMS−BZ interactions are somewhat mitigated by imposing constant chemical boundary conditions through using a light-sensitive catalyst, ruthenium, in combination with patterned illumination. Furthermore, we show that light can modulate the frequency and phase of the BZ oscillators contained in a PDMS matrix by 20−30%.

Partition, Reaction, and Diffusion Coefficients of Bromine in Elastomeric Polydimethylsiloxane

“…The decay rate is a constant, α = 7 × 10 –3 s –1 , independent of bromine concentration, as expected when the reaction is first-order in bromine concentration. This result is qualitatively consistent with the presence of a reaction between bromine and PDMS determined from modeling the period of the BZ oscillation as a function of PDMS mass …”

“…Its main advantages are low cost, optical transparency, and ease of processing, such as high-fidelity molding, toughness, and tensile strength. However, PDMS is permeable to liquids with low dielectric constants, which limits its use in microfluidic applications to aqueous solutions. − This material is used to fabricate microfluidic devices holding networks of Belousov–Zhabotinsky (BZ) chemical oscillators, − where the oscillators in the aqueous compartments (linear dimensions of 30–100 μm) interact via intermediates that diffuse across the PDMS barriers (wall width of 30–100 μm). In a related system, we showed that collective dynamics are governed by the diffusion of the activator, bromous acid (HBrO 2 ), and bromine (Br 2 ) which participates in the inhibitory pathway. − Due to its low dielectric constant, bromine readily permeates PDMS, separating the BZ compartments, leading to a network of inhibitory coupled chemical oscillators.…”

“…PDMS has been used to trigger the nucleation sites of chemical waves by locally disinhibiting the BZ reaction. Permeation of bromine into PDMS and the reaction of bromine with this could also result in the loss of bromine from the BZ compartments, suppressing oscillations if the ratio of the volume of PDMS to the aqueous compartment is large. ,, In order to design BZ microfluidic networks in PDMS with predictable properties, the permeation and reaction constants of bromine in PDMS must be known. Developing a quantitative reaction–diffusion model accounting for the transport of bromine through PDMS is the purpose of this work.…”

“…The coupling strength between two chemical reactors has been calculated to be proportional to the product of the partition coefficient ( P ) and the diffusion constant ( D ) of bromine in the medium separating the reactors . Experiments of BZ reactors in PDMS and finite element models of experiments of BZ chemical waves in PDMS channels yielded an estimate of PD = 0.4 × 10 –9 m 2 /s . Assessment of the values of P from the measurements of PD requires the independent measurement of D .…”

“…Additionally, our recent work with BZ oscillators in PDMS microfluidic devices indicated that bromine reacts with PDMS, which would have an effect on the previous measurement of the partition coefficient which assumed mass conservation . Other works have shown that PDMS reacts with some chemicals. ,, Therefore, it is imperative that the partition constant be re-examined.…”