The use of polyurethane as an elastomer in thermoplastic microfluidic devices and the study of its creep properties

Abstract

We report using polyurethane (PU) as an elastomer in microvalves integrated with thermoplastic microfluidic devices. Elastomer-based microvalves have been used in a number of applications and the elastomer often used is PDMS. Although it is a convenient material for prototyping, PDMS has been recognized to possess shortcomings such as solvent incompatibility and unfavorable manufacturability. We investigated the use of PU as an elastomer to address the challenges. A reliable method was developed to bond hybrid materials such as PU and cyclic olefin copolymer. The film thickness from 3.5 to 24.5 μm was studied to identify an appropriate thickness of PU films for desirable elasticity in microvalves. We integrated PU with thermally actuated, elastomer-based microvalves in thermoplastic devices. Valve actuations were demonstrated, and the relationship between the valve actuation time and heater power was studied. We compared PU with PDMS in terms of their microvalve performance. Valves with PDMS failed to function after two weeks since the thermal-sensitive solution evaporated through porous PDMS membrane, whereas the same valve with PU functioned properly after eight months. In addition, we evaluated the creep and creep recovery of PU, which is a common phenomenon of viscoelastic materials and is related to the long-term elastic property of PU after prolonged use.

Keywords: Bonding; Creep; Elastomer; Microvalves; Polyurethane.

Figure 1

(a) Schematic of a pneumatically actuated valve consisting of three layers. The channel layer consists of flow channels; the middle valve layer consists of the control channels; and the bottom layer is glass support. When a pressure is supplied through the control channels, the elastomeric film is deformed into the flow channel to close valve as shown on the right. Adapted from references [2, 3]. (b) Schematic of a thermally actuated valve consisting of four layers: the channel layer at the top, an elastomer, a valve layer containing cavities housing a temperature-sensitive fluid, and the bottom layer with resistive micro-heaters patterned on a cover film. When the heater is turned on, the temperature-sensitive fluid expands, deflecting the elastomer into the channel to close valve as shown on the right.

Figure 2

The bonding strength, indicated by the peeling force of bonded PU/COC, is dependent on the temperature of the annealing step that was employed to promote the bonding between chemical treated COC surfaces and activated PU surfaces. Each data point is the average of three repeat experiments and the error bars indicate one standard deviation.

Figure 3

(a) The process flow of preparing PU samples for their thickness measurement. (b) The thickness of PU films as a function of the spin-coating speed. Each data point represents the average of three PU samples and the error bars indicate one standard deviation.

Figure 4

(a) Exploded view of the valve region consisting of Au serpentine heaters, a cavity for a temperature-sensitive fluid (FC40), and a microfluidic channel in different layers. The filling hole is for dispensing FC40 into the cavity. A dye solution was in the channel when valve was open and heaters were off. (b) Same image of (a) when heaters were on. The dye solution in the valve region was forced out by the deflected PU film, indicating that the valve was closed. (c) Temporal profiles of electric current through the microchannel (the Y axis on the left) and of the heater temperature (the Y axis on the right) when the valve was actuated. Three cycles of valve open/close were shown to indicate the decreasing actuation time with increasing power from 42 mW to 48 mW.

Figure 5

Valve actuation time as a function of input heater power. The error bars indicated the standard deviation of multiple tests.

Figure 6

(a) Strain-time diagram of PU film under a constant stress of 210 KPa. The testing temperature was 25°C or 50°C as indicated. (b) Temporal profiles of strain (the Y axis on the left) and of stress (the Y axis on the right). A strain of 10% was kept the same in the first 20 minutes and the corresponding stress required over the period was measured. Creep recovery was studied in the second 20 minutes by reducing the stress to 0 and measuring the strain change over the period. The testing temperature was 50°C.