Sunday, December 20, 2015

Robust micropatterned superhydrophobic/superhydrophilic polymer surfaces for inkjet printing and lab-on-a-chips

The ability for water to wet a surface and/or for a surface to repel water has important technological implications, ranging from microfluidics to cell microarrays.  A subfield of applied research has focused on the creation of stable patterns of superhydrophobic and superhydrophilic areas.  To be clear, hydrophobic is the technical term for "water-repelling"; phobic is in the name after all!  Of course, hydrophilic means attracted to water.  When scientists add the "super" to the name, you know these properties are in the upper echelons of the scale.

Microfluidics is important in inkjet printing, DNA chips, and lab-on-a-chip devices.  These technologies play very important roles in biotechnology, especially concerning clinical pathology, where immediate diagnosis of disease is critical.  Microfluidics is divided in many subcategories, so let's focus on one: droplet-based microfluidics–the generation of micro droplets in more ways than one would normally care to remember.

Microdroplets can be used as incubators for single cells.  Cell behavior is typically observed as populations in bulk assays.  However, medical fields such as immunology are described at the single-cell level, which means a technique that can enable one to observe one cell would yield important insights for that field.  Recently, a team in Holland devised a method that uses microdroplets of agrose to encapsulate T-cells.  These cells secrete cytokine, which then binds to beads already present in the microdroplets.  This method could be automated and performed multiple times simultaneously. This can detect differences or variations among individual cells, and map subsets within cell populations with specific functions.

Micro-patterns of superhydrophobic and superhydrophilic areas are created by modifying the surface of a superhydrophilic substrate through a mask to reverse the hydrophilicity  of the exposed areas.  However, this usually necessitates harsh conditions, risks irreversible modifications, and requires the entire substrate to perform the modification.

Here, researchers in Germany report an easy method for printing superhydrophilic patterns on a superhydrophobic substrate.  The "ink" is an ethanol solution containing phospholipid and is deposited onto a superhydrophobic porous polymer surface.  Lipids may sound familiar from high school biology because you may remember it's a fancy word for fats.  The key property about these molecules is their amphiphilic nature, meaning they have hydrophobic and hydrophilic parts.  Like attracts like, so the hydrophobic segment of the phospholipid should attach to the superhydrophobic substrate.  This leaves a free hydrophilic phosphate in that spot.  To put it another way, the amphiphilic lipid is the ink that creates superhydrophilic patterns on the superhydrophobic surface.

Figure 1. (A) Schematic representation of switching from superhydrophobicity to superhydrophilicity by applying an “ink” containing a phospholipid. SEM images of the microporous structure of the superhydrophobic (B) and superhydrophilic (C) polymer film. (D) SEM images and images of water droplets on the BMA-EDMA surfaces with different morphologies (scale bars 1 μm; average sizes of polymer globules are indicated under SEM images). (E) Static water contact angles on BMA-EDMA surfaces with different morphologies before and after modification with the POPG lipid. Average sizes of polymer globules are indicated.

The figure above is taken from the paper itself.  1(A) is the schematic of how the normally superhydrophobic surface becomes superhydrophilic, and vice-versa.  The best part about these materials is how easy it is to switch between the two states of water interaction.  You add an ethanol solution with phospholipid to the surface, and the hydrophilic ends of the phospholipid molecules stick up.  You then just add methanol to the surface, and the lipid is washed away, rendering the substrate superhydrophobic again.  This switch was done 30 times by the researchers without performance decay.

Key to this performance is the porosity of the polymer surface.  1(B) and 1(C) are scanning electron micrographs (SEMs) of the cross-sectioned polymer film (left panels) and closeup of polymer surface.  Porosity is indicate by average polymer particle in 1(E), meaning that it's harder for larger particles to close holes.  Thus, polymer surfaces with the largest average particles performed best.  Best performance means ease of switching between superhydrophobic and superhydrophilic without decrease in performance.  Smaller particles just didn't respond as well to the repeated washing.  A nonporous did the worst.  When water is applied to a porous substrate with superhydrophilic regions, the pores might protect the lipids from the mechanical action of the applied water flow.  Without pores, there is little for the lipids to hang on to, and are easily washed away by water.

Robust, easily fabricated micro-patterned polymer substrates could open the floodgates to new applications.  Easy switching between superhydrophobicity and superhydrophilicity makes for easy incorporation into well-established techniques for printing, including microcontact printing, dip-pen nano lithography, or inkjet printers.  Naturally, the question now is it possible to fabricate a superhydrophilic substrate that is available for printing a micro-pattern consisting of ordered superhydrophobic regions.  Time to dive deeper into the Obscura.

Source:
  1. Printable Superhydrophilic–Superhydrophobic Micropatterns Based on Supported Lipid Layers

    Junsheng S. Li, Erica Ueda, Asritha Nallapaneni, Linxian X. Li, and Pavel A. Levkin
    Langmuir 2012 28 (22), 8286-8291
    DOI: 10.1021/la3010932