Abstract
Nanofluidic structures are often the key element of many lab-on-chips for biomedical and environmental applications. The demand for these devices to be able to perform increasingly complex tasks triggers a request for increasing the performance of the fabrication methods. Soft lithography and poly(dimethylsiloxane) (PDMS) have since long been the basic ingredients for producing low-cost, biocompatible and flexible devices, replicating nanostructured masters. However, when the desired functionalities require the fabrication of shallow channels, the “roof collapse” phenomenon, that can occur when sealing the replica, can impair the device functionalities. In this study, we demonstrate that a “focused drop-casting” of h-PDMS (hard PDMS) on nanostructured regions, provides the necessary stiffness to avoid roof collapse, without increasing the probability of deep cracks formation, a drawback that shows up in the peel-off step, when h-PDMS is used all over the device area. With this new approach, we efficiently fabricate working devices with reproducible sub-100 nm structures. We verify the absence of roof collapse and deep cracks by optical microscopy and, in order to assess the advantages that are introduced by the proposed technique, the acquired images are compared with those of cracked devices, whose top layer, of h-PDMS, and with those of collapsed devices, made of standard PDMS. The geometry of the critical regions is studied by atomic force microscopy of their resin casts. The electrical resistance of the nanochannels is measured and shown to be compatible with the estimates that can be obtained from the geometry. The simplicity of the method and its reliability make it suitable for increasing the fabrication yield and reducing the costs of nanofluidic polymeric lab-on-chips.
Highlights
We demonstrate that a “focused drop-casting” of hard poly(dimethylsiloxane) (h-PDMS) on nanostructured regions, provides the necessary stiffness to avoid roof collapse, without increasing the probability of deep cracks formation, a drawback that shows up in the peel-off step, when h-PDMS is used all over the device area
We verify the absence of roof collapse and deep cracks by optical microscopy and, in order to assess the advantages that are introduced by the proposed technique, the acquired images are compared with those of cracked devices, whose top layer, of h-PDMS, and with those of collapsed devices, made of standard PDMS
Solid-state nanochannels are produced on Si, SiO2 or glass substrates by using high-resolution nanopatterning techniques such as focused ion beam (FIB) milling, e-beam lithography (EBL) or laser machining [20,21,22,23,24,25]
Summary
Biomedical and environmental applications of fluidic chips for the detection of nanoplastics [1,2], proteins [3,4,5], viruses [6,7,8], bacteria [9,10] and DNA [11,12,13] rely on micro and nanofeatures that are the core elements of these devices for their capability to allow in-situ and real-time analysis of nanosize objects.In order to achieve the desired functionality, these devices must be fabricated with nanometer-scale structures, i.e., dimensions close to the objects of interest. A paradigmatic example is DNA sensing: here biological nanopores such as α-haemolysin and Phi connector channel represent a good choice for their suitable dimension and bio-compatibility, but they are extremely fragile and susceptible to the environmental conditions [15,16]. For this reason, the integration of these pores in complex devices is difficult and not durable. Solid-state nanochannels are produced on Si, SiO2 or glass substrates by using high-resolution nanopatterning techniques such as focused ion beam (FIB) milling, e-beam lithography (EBL) or laser machining [20,21,22,23,24,25]. These techniques provide excellent control of the geometry and high reproducibility but are usually expensive and not scalable to a production stage
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