Microfluidics for Biologists Fundamentals and Applications

coupled with electrophoretic separation in a PMMA-integrated microdevice
[ 31 ]. Chen et al., reported a self-contained, integrated, disposable, PC microfluidic
cassette for nucleic acid—based detection of pathogens at the point of care[ 32 ]
(Fig.6.7). The system, in its current state, successfully and reproducibly detected
down to 1000 pathogen particles in the sample.
Covalently modified surfaces are generally more stable in thermoplastics than in
PDMS. For example, after treatment with oxygen plasma, the surface of PMMA
retains hydrophilicity for up to a few months [ 33 ]. Also, they can be easily
integrated with electrodes for flexible circuits; one related application is digital
microfluidics, which can manipulate droplets by electrowetting. In general, they
show a slightly better solvent compatibility than the PDMS elastomer. Unfortu-
nately, they are incompatible with most organic solvents, such as ketones and
hydrocarbons. In contrast to elastomers, thermoplastics are normally purchased
solid and fabricated by thermomolding. Thermomolding can produce thousands of
replicas at a high rate and low cost, but it requires templates in metal or silicon for
use at high temperatures (to allow ample plastic flow); it is excellent for commercial
production but not economical for prototypic use.
From a manufacturing point of view, the main advantage of thermoplastics is
their stability and ability to be melted and reshaped against a mold, enabling
production of thermoplastic parts with high throughput. From a lab-on-chip per-
spective, the availability of many commercial, medical grade formulations is a
great advantage. Their stiff mechanical properties also provide structural support
and protection for the sensor and the microfluidic network. However, many solvents
common in chemical analysis and separation dissolve thermoplastics. Nevertheless,
most commercial lab-on-chips are made of thermoplastics.

Table 6.1 Summary of physical properties for common microfluidic thermoplastics [ 29 ]

Polymer Tg (C) Tm (C)

Water

absorption

(%)

Solvent

resistance

Acid/base

resistance

Optical

transmissivity

Visible UV

COC/

COP

70–155 190–320 0.01 Excellent Good Excellent Excellent

PMMA 100–122 250–260 0.3–0.6 Good Good Excellent Good

PC 145–148 260–270 0.12–0.34 Good Good Excellent Excellent

PS 92–100 240–260 0.02–0.15 Poor Good Excellent Excellent

PEEK 147–158 340–350 0.1–0.5 Excellent Good Poor Poor

PET 69–78 248–260 0.1–0.3

PE 
 30 120–130 0.01 Excellent Excellent Fair Fair

PVDC 0 76 0.10 Good Good Good Poor

PVC 80 180–210 0.04–0.4 Good Excellent Good Poor

PSU 170–187 180–190 0.3–0.4 Fair Good Fair Poor

Note: The qualitative metrics shown in this chart are neither comprehensive nor definitive and are
provided only as a general guide for material evaluation
In particular, note that solvent resistance can be highly dependent upon the solvent type,
e.g. hydrocarbons, alcohols, ketones, etc

6 Materials and Surfaces in Microfluidic Biosensors 155