Independent Optical Control of Microfluidic Valves Formed from Optomechanically Responsive Nanocomposite Hydrogels

Abstract

The ability to actively manipulate fluid-flow patterns through microfluidic devices is important to many current applications and vital to the development of more complex systems in the future. A typical actively controlled valve developed to date consists of a flexible diaphragm coupled to an electromagnetic, electrostatic, or thermopneumatic actuator.[1]Design of an air-driven membrane valve has also been reported.[2] Despite this progress, there remains a significant need for materials that allow easy and controlled manipulation of valves and actuators in microfabricated devices, and ideally, independent control over multiple components within a single device. Recently, pH-responsive hydrogel materials have been used as valves in microfluidics devices. [3,4] This has allowed control of flow based on local environmental conditions (e.g., pH), which is attractive for some applications due to the simplicity of the system design. This work demonstrated the feasibility of forming hydrogel valves in microfluidic devices via photopolymerization and of utilizing hydrogels that undergo shape changes in response to stimuli as controllable valves or actuators. However, using pH as a control mechanism limits the number of components that can be independently controlled and requires that all components of the system be compatible with the requisite changes in pH. Many microfluidics applications will not be compatible with a wide range of pH conditions. Furthermore, for some applications it would be advantageous to have independent external control of valve operation, which would be difficult to achieve with pH-responsive materials. In the current system, dramatic changes in material size and shape can instead be induced by exposure to specific wavelengths of light. The use of light as a stimulus is particularly attractive as it is easily directed to specific locations within a channel network, possibly allowing further control.To accomplish this, we have developed materials that undergo pronounced and reversible changes in shape and size in response to different wavelengths of light. These materials are composites of a thermally responsive polymer (poly[N-isopropylacrylamide-co-acrylamide] with a 95:5 comonomer ratio) and particles that have distinct and strong optical-absorption profiles (gold colloids and nanoshells). The gold colloid was formed using the method of Duff et al., [5] while gold nanoshells with a 110 nm diameter silica core and 10 nm thick gold shell were synthesized as described by Oldenburg et al. [6] The absorption spectra of these two nanomaterials are shown in Figure 1. The composites were formed by mixing the nanoparticles with the monomer solution, thus entrapping the particles within the hydrogel matrix after polymerization. These nanocomposite materials respond to different wavelengths of light (Fig. 2); in this example, one nanocomposite material collapses in response to green light (gold-colloid nanocomposite hydrogel) while the other collapses in response to near-IR light (gold-nanoshell nanocom...

Selective Release of Multiple DNA Oligonucleotides from Gold Nanorods

“…Others have utilized similar concept to independently control microfluidic valves using two different nanoparticles, gold colloids and gold nanoshells. 20 Here we demonstrate selective release of two distinct DNA strands from two different NRs by matching laser excitation wavelength to the NRs' SPR long (Scheme 1). We first demonstrate selective melting of two different NRs.…”

Biomimicry, Biofabrication, and Biohybrid Systems: The Emergence and Evolution of Biological Design

“…Similar microfluidic valves have also been demonstrated by West and co-workers, who demonstrated optical control over swelling in gold-colloid composite hydrogels. [23] This allowed for greater spatiotemporal precision and more independent external control of valve opening and closing. Smart hydrogel components for microfluidic flow control have also been engineered to respond to other forms of external control, such as electrical and thermal stimuli.…”

Environmentally responsive polymeric "intelligent" materials: the ideal components of non-mechanical valves that control flow in microfluidic systems

“…Satarkar et al (2009) were able to couple metallic nanostructures with environmentally responsive hydrogels and build a non-mechanical actuator that controlled flow in response to both temperature and an electromagnetic field. A similar valve was constructed by Sershen et al (2005) by synergizing the optical properties of gold-silicon nanoshells with the thermal response capabilities of a thermally responsive polymer hydrogel.. In particular, coupling thermally responsive polymers to metallic nanoparticles has allowed the development of materials that can be triggered both optically and thermally since the incorporated metallic nanoparticles act as amplifiers and converters of light of a specific wavelength into heat (Chou et al, 2005).…”

“…Furthermore, these hydrogels have been incorporated successfully into microfluidic systems and achieved optical flow control. (Sershen et al, 2001;Sershen et al, 2002;Jones and Lyon, 2003;Sershen et al, 2005) The synthesis of these systems is advantageous since it is simple to physically entrap nanoparticles within the hydrogel; however, the downside is the long time to trigger the response of these systems and the fact that they require high power lasers. This challenge has motivated the development of synthetic routes that couple polymer chains, instead of the bulky hydrogels, to metallic nanoparticles and obtain stable nanocomposites.…”

“…Hence, the necessity to synthesize and design materials that can respond to localized stimuli, without changing the bulk conditions of the system. Recently there has been interest in developing materials that present localized response to light, (Liu et al, 2005;Sershen et al, 2005), or through the implementation of nanofabricated sources containing chemical reagents that are dispensed to the system through pressure , or simple diffusion (Abhyankar et al, 2006). However, even though up until now the use of pressurized reagent sources has been the most successful, they require complicated circuitry and do not offer the capability of stimulating highly specific sites; therefore, light responsive materials are the preferred option since their response can be easily triggered at highly localized sites (using lasers) without requiring physical contact.…”