Materials Science and Technology of Polymers at the University of Twente

Towards Smart “Lining” for Microfluidic Channels with Stimulus Responsive Polymers

Lionel Dos Ramos thesis cover: Towards Smart “Lining” for Microfluidic Channels with Stimulus Responsive Polymers

In this Thesis, the development and the characterization of polymeric films with the ultimate goal of switching the properties of functionalized microchannel surfaces is presented. The coatings were mainly based on two types of stimulus responsive polymers, temperature responsive poly(N-isopropylacrylamide) (PNIPAM) and redox-responsive poly(ferrocenylsilane) (PFS). These polymers allow one to control wettability, adhesion force, and friction which are important variables for preparing designer microchannels. Surface switching was primarily studied by colloid probe AFM. The motivation for the choice of PNIPAM and PFS is rationalized by their responsiveness and by the opportunities for anchoring these polymers to solid substrates, allowing one to functionalize the inner walls of microfluidic devices and tune the surface properties of these walls to control flow. As a proof of concept, we present a study of PFS functionalized channels for switchable delay valves.

The motivation for the reported research are presented in Chapter 1.

Chapter 2 starts with some definitions of commonly used terms associated with surface energetics. Then, a brief introduction to the theories behind contact angle measurements and AFM, allowing one to connect the contact angle values or the adhesion hysteresis and friction to the surface properties of a material, is given. Finally, we offer an overview of recent polymeric microvalves and micropumps, used in microfluidic devices to control the flow. The examples are divided in two categories: first, polymeric systems which use surface energy changes to influence capillary action, and second, polymeric systems based on movement, such as expansion and collapse, to manipulate the flow behavior inside microchannels. The first category is separated into the passive elements which cannot be actuated, and the stimulus responsive systems which permit an active switch. The movement-based actuators are classified in single and dual stimuli responsive systems.

In Chapter 3, we propose an additional literature overview of the opportunities offered by anionic polymerization of silaferrocenophanes for synthesizing well-defined PFS patterns. Microcontact printing and block polymer microphase separation techniques show immense potential for creating well-ordered patterns at the nanoscale. PFS-based patterns are shown to be perfect candidates for maskless lithography. The creation of patterned surfaces may be easily applied in microfluidic fabrication or simply to alter surface properties.

In Chapter 4, we develop a novel method to detach polymer brushes from a titanium dioxide surface via photocatalysis. Nitrodopamine was used as anchor to immobilize an atom transfer radical polymerization initiator for the surface-initiated growth of PNIPAM brushes. Detachment via UV-light irradiation resulted in both, a full cleavage of the brushes, and a controllable, partial and homogeneous detachment, depending on the exposure time. A full detachment of the PNIPAM brushes from the surface allowed the precise molar mass and polydispersity characterization via gel permeation chromatography (GPC). Then, the partial photocatalytic cleavage permitted us to tune the grafting density of the polymer brush from 0.43 to 0.01 chain/nm2. Having established a well-characterized system, we were able to study the effect of the grafting density on the thermally induced switching of adhesion hysteresis and friction. At the LCST, a particular behavior of the brush which held long range interactions with the PS colloid, induced a stretching of the brushes and an increase of the adhesion hysteresis and the friction. This phenomenon was maximized at an intermediate grafting density of 0.28 chain/nm2

In Chapter 5, we describe the synthesis and the characterization of redox-responsive and surface-anchored PFS layers which allow switching of the wettability and the adherence. First, a poly(ferrocenyl(3-iodopropyl)methylsilane) (PFS-I) layer permited a reversible change in contact angle values from 80º (reduced state) to 70º (oxidized state), over repeated cycles. Then, this PFS-I layer was functionalized, at the side groups, with alkylsulfonate amphiphilic chains (PFS-SO3) of which the conformation was changed by attracting or repulsing the polar head depending on the redox state of the PFS. The resulting “backbiting” caused an inverse switch of the wetting compared to the PFS-I layer, since the contact angle values changed from 59º (reduced state) to 77º (oxidized state). Nanoscale adherence with a PS colloid AFM probe confirmed the two different switching mechanisms for both layers.

In Chapter 6, as a proof of concept, we present a study of PFS-I functionalized channels for switchable delay valves. The redox response of the surface-anchored PFS-I and its ability to switch reversibly the surface energy was used to functionalize a gold coated microfluidic device. The switchable surface energy allowed a reversible control of the capillary filling speed of water through a microchannel. The velocity of the meniscus was reversibly switched between 1.8 ± 0.1 mm/s (reduced state) and 3.4 ± 0.1 mm/s (oxidized state).

Finally, in Chapter 7, we give an outlook of the possibilities of using a combination of the redox-response of PFS and the swelling response of a polymeric system, such as a brush or hydrogel. For example, we have successfully managed the functionalization of surface-anchored PFS layers with an ATRP initiator at the side groups. This system would allow one to control the swelling of the polymeric layer by using external and electrical stimuli.