## 4 Conclusions  
The present study shows the fabrication, characterization and functional application of two spiral-shaped microdevices. The PDMS microdevices have different surface to volume ratios and differ for the total internal surface available for the capture of biomarkers. The microfluidic device presenting a higher S/V was demonstrated to capture a higher amount of microRNAs. This aspect is especially important since biomarkers, in particular microRNAs, are present in little amounts in body fluids and therefore, a smart method for concentrating and detecting is welcome. The microdevices presented here satisfy this requirement, being able to purify different microRNAs considered as cancer biomarkers from a biological fluid, such as human plasma. The performances of the microdevices approached or even exceeded those of a similar PDMS-based microdevices previously reported in the literature. This consideration fosters the idea of a real use of PDMS-based microdevices as diagnostic tools to identify low abundant microRNA biomarkers in clinical setting.  
Liquid biopsy is indeed emerging as a precious source of biomarkers, easily accessible and therefore useful for mass screening and for biomarkers profiling of patients during specific therapies. Microfluidic devices could simplify liquid biopsy, allowing the spread of these methodologies not only for diagnostic purposes, but also for following the progress of a disease. Moreover, the detection of low amounts of biomarkers could take to earlier diagnosis of pathologies with a possible better outcome.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/11/3867/s1](http://www.mdpi.com/2076-3417/10/11/3867/s1)]([http://www.mdpi.com/2076-3417/10/11/3867/s1](http://www.mdpi.com/2076-3417/10/11/3867/s1)), Figure S1: PCR assay to test PDMS compatibility; Figure S2: optimization of silanization time; Figure S3: fluorescent microRNAs adsorption on microdevices.  
Conceptualization, L.L., C.P. ([PERSON]), M.C., S.L.M.; methodology, L.L., C.P. ([PERSON]), S.L.M.; software, L.L.; validation, L.L. and C.P. ([PERSON]); formal analysis, L.L.; investigation, L.L., F.B., G.S., L.V., C.P. ([PERSON])); resources, C.F.P. and C.P. ([PERSON]); data curation, L.L. and C.P. ([PERSON]); writing--original draft preparation, L.L. and C.P. ([PERSON]), writing--review and editing, all authors; visualization, L.L. and C.P. ([PERSON]); supervision, L.L., C.P. ([PERSON]), M.C., S.L.M., C.F.P., C.P. ([PERSON]); project administration, L.L., C.P. ([PERSON]), M.C., S.L.M.; funding acquisition, C.F.P. and C.P. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by DEFLeCT (\"advanced platform for the early detection of not small cells lung cancer\") project, financed by Piedmont Region in the framework of \"Health and Well-Being\" Platform project.  
The authors want to acknowledge [PERSON] and [PERSON] for their help on micro fabrication process.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.