RESUMEN
Poly(vinyl alcohol) (PVA) has ice binding and ice nucleating properties. Here, we explore the dependence of the molecular size of PVA on its ice nucleation activity. For this purpose, we studied ice nucleation in aqueous solutions of PVA samples with molar masses ranging from 370 to 145 000 g mol-1, with a particular focus on oligomer samples with low molar mass. The experiments employed a novel microfluidic setup that is a follow-up on the previous WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) design by Reicher et al. The modified setup introduced and characterized here, termed nanoliter Bielefeld Ice Nucleation ARraY (nanoBINARY), uses droplet microfluidics with droplets (96 ± 4) µm in diameter and a fluorinated continuous oil phase and surfactant. A comparison of homogeneous and heterogeneous ice nucleation data obtained with nanoBINARY to those obtained with WISDOM shows very good agreement, underpinning its ability to study low-temperature ice nucleators as well as homogeneous ice nucleation due to the low background of impurities. The experiments on aqueous PVA solutions revealed that the ice nucleation activity of shorter PVA chains strongly decreases with a decrease in molar mass. While the cumulative number of ice nucleating sites per mass nm of polymers with different molar masses is the same, it becomes smaller for oligomers and completely vanishes for dimer and monomer representatives such as 1,3-butanediol, propan-2-ol, and ethanol, most likely because these molecules become too small to effectively stabilize the critical ice embryo. Overall, our results are consistent with PVA polymers and oligomers acting as heterogeneous ice nucleators.
RESUMEN
Numerous microfluidic separation applications have been shown in the past years providing a fast analysis of biological samples like DNA or proteins. Microfluidic separation based on dielectrophoresis (DEP), that is the migration of a polarizable object in an inhomogeneous electric field, provides numerous advantages. However, the main drawback of DEP separation devices is that they are not sufficient for large-scale sample purification due to the lack of high sample throughput. In this work, we present for the first time a microfluidic device with two parallelized dielectrophoretic separations of (biological) samples smaller than 1 µm. The separation is carried out by means of insulator-based DEP, that is an insulating ridge reduced the flow through height and thus created a nanoslit at which the selective DEP forces occur. The device consists of a cross injector, two parallel operation regions and separate harvesting reservoirs where the samples are collected. Each DEP operation region contains an insulating ridge. We successfully demonstrate the separation of 100 and 40 nm beads and 10 and 5 kbp DNA with a separation purity of more than 80%. This states the proof-of-concept for up-scaling of dielectrophoretic separation by parallelization. As the present technique is virtually label-free, it offers a fast purification, for example in the production of gene vaccines.