Nanofluidic channels offer new opportunities for manipulating small molecules such as DNAs [1] and proteins.[2]However, the fabrication of nanofluidic channels aligned with microfluidic networks currently requires precise lithography and nanofabrication procedures. [2a] To address the need for a simple, low-cost fabrication method for forming close-packed nanofluidic channels, we explored an approach that produces shallow wrinkles on a poly(dimethyl siloxane) (PDMS) substrate. We found that the wrinkle pattern could be created by stretching a sheet of PDMS and then treating the surface with oxygen plasma to generate a stiff skin, largely composed of SiO x [3] the density of which is approximately half that of silica.[4a] Upon releasing the pre-stress, a sinusoidal wrinkle pattern forms with a well-defined wavelength and an amplitude that exhibits nanometer-scale dimensions. This method of wrinkle formation on a PDMS surface is similar to the method first demonstrated by Efimenko et al.[4a] The apparent advantage is that such wrinkles can be easily formed into wrinkle nanochannels (WNCs) by bonding on other substrate, at a high density without the need for a lithography, etching, or deposition processes, by using only surface treatments with UVO (ultraviolet/ozone), an ion beam, or an oxygen plasma.[4]Wrinkles were created with varied process values, followed by atomic force microscopy (AFM) measurements to build a design guideline. The process is illustrated in Figure 1. Above the critical stress associated with the onset of wrinkling in the thin stiff layer of approximate thickness t on a PDMS surface, the stiff layer under uniaxial compressive stress would wrinkle in a sinusoidal configuration. The wavelength (L) or width of wrinkle would be determined by the ratio of elastic moduli between the film (E f ) and substrate (E s ) according to [4,5] L=tHere, a is approximately 4.36 and E ¼ E=ð1 À n 2 Þ.[4a] The elastic moduli are kept constant since the modulus for the stiff layer would be fixed regardless of exposure time.[4] However, with exposure time, the thickness of a stiff surface can be expected to increase with the time. That makes the wavelength (L) to be linearly increasing, based on Equation 1. Note that wrinkle wavelength is decreased by reducing the thickness of the stiff layer or by reducing the ratio of elastic moduli, E f =E s , and that the wavelength and therefore the width of WNCs can be controlled by varying surface exposure time. An analytical prediction for the amplitude (W) or height of the wrinkles under uniaxial strain, is given as [5] W=t ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiwhere e and e c are the applied strain, and the critical strain ðe C ¼ 1=4ð3E s =E f Þ 2=3 Þ for the onset of wrinkling. Consequently, the height of the channel can be controlled by the stretching strain and the thickness of the stiff layer, or treatment time.Using the theory as a guide, the wavelength and amplitude of wrinkles can be independently controlled by varying strain and exposure time. B...
