A Smart Microfluidic Affinity Chromatography Matrix Composed of Poly(N-isopropylacrylamide)-Coated Beads

The efficient upstream processing of complex biological or environmental samples for subsequent biochemical analysis remains a challenge in many analytical systems. New microfluidic platforms that provide multidiagnostic capabilities on single chips face a similar challenge in getting specific analytes purified or contaminants removed in different fluid streams. Here, stimuli-responsive polymers have been used to construct "smart" beads that can be reversibly immobilized on microfluidic channel walls to capture and release targets. The 100-nm latex beads were surface-modified with the temperature-sensitive polymer poly(N-isopropylacrylamide) (PNIPAAm). At room temperature, a suspension of these beads flows through a microfluidic channel constructed of poly(ethylene terephthalate). However, when the temperature in the channel is raised above the lower critical solution temperature (LCST) of PNIPAAm, the beads aggregate and adhere to the walls of the channel. The adhered beads are stable for long durations on the channel walls (demonstrated up to 70 min) in the presence of flow. The beads were further modified with the affinity moiety biotin, which tightly binds streptavidin. The dual-modified beads were adhered to the channel walls and functioned as a chromatographic affinity separation matrix, capable of binding streptavidin that was flowed through the microfluidic channel. Upon the reverse thermal stimulation to below the PNIPAAm LCST, the beads and captured streptavidin were observed to quickly dissolve and elute from the channel walls. This temperature-responsive affinity chromatography matrix can thus be flowed into a column and aggregated via temperature change, followed by the controlled release of affinity-captured targets back into the microfluidic flow stream.