Research paperA dielectric coating for improved performance of capacitive sensors in all-polymer microfluidic devices
Graphical abstract
Introduction
Capacitive elements have been established as an important means of sensing in microfluidic devices. Interdigitated electrodes as well as plate capacitors are widely used for material or droplet detection [1]. Also, the sensing in biological processes, e.g., the monitoring of bacterial growth [2], the detection of viruses [3], DNA sensing [4] or chemical processes [5] are often performed via capacitive sensor elements. Yet, most of the aforementioned sensors are fabricated in CMOS processes, which are time-consuming and relatively expensive.
In that regard polymer-based microfluidic devices have the advantage of being suitable for rapid prototyping and cost-efficient fabrication due to their ease of processing. Sensor fabrication processes compatible with polymeric microchannels are screen printing and spin coating of polymer-based pastes and solutions containing filler particles. When compared to CMOS fabricated devices, however, these processes result in rather thick layers and therefore reduce the sensitivity of capacitive sensing elements drastically.
For combining the advantages of polymeric fabrication technologies with high sensitivity, it is beneficial to increase the relative permittivity of polymeric layers to achieve suitable sensitivities even with thicker dielectric layers.
In this contribution a dielectric coating made from poly(methyl methacrylate) (PMMA) and barium titanate (BaTiO3) for the use as a passivation layer for capacitive elements in microfluidic devices is presented. Barium titanate is already commonly used in capacitors due to its high relative permittivity (εr,BaTiO3 ≈ 500–6900) [6]. Additionally, barium titanate has been investigated regarding its biocompatibility and found to exhibit no toxicity, making it suitable for biomaterial applications [7]. As a polymeric base material, PVDF is commonly used for high permittivity polymer-ceramic composites due to its advantageous dielectric properties [8,9]. In this work, PMMA is chosen. as a polymeric component due to its processing properties, e.g., solubility, and its common use in microfluidic systems making this composite compatible with pre-existing systems [10,11]. Also PMMA exhibits favorable mechanical properties, therefore combining it with BaTiO3 results in a robust coating with improved dielectric properties compared to pure PMMA. The effective relative permittivity from mixing two materials with different dielectric constants does not scale linearly with the ratio of the two materials. Unfortunately, well known mixing rules (e.g. Maxwell-Garnett rule) are often only suitable for low concentrations of the high permittivity component. In order to find the optimal ratio between polymer and ceramic filler, a rigorous analysis of dielectric properties for a large range of volume percentages of BaTiO3 is provided. In contrast to previous studies, this work investigates the dielectric properties of the chosen composite over a wide range from 0 to 90 vol% [12,13]. The obtained results therefore give insight to the limits of dielectric and processing possibilities, rendering the results helpful for engineering applications. To show the usability of the proposed coating, a microfluidic demonstrator device with an embedded capacitive sensor used for differentiating between liquids with different relative permittivity is presented. It was fabricated with the custom made dielectric layer and, for the sake of comparison, also with pure PMMA to demonstrate the increase in sensitivity due to the inclusion of BaTiO3 particles.
Section snippets
Dielectric coating and sample preparation
To determine the dielectric properties of different PMMA/BaTiO3 mixtures, parallel plate capacitors were fabricated featuring the mixture as dielectric. To do so, the dielectric was spin coated from solution. Fig. 1 shows a schematic of the fabrication process of the capacitors.
First, the coating was prepared: A 1:10 PMMA in anisole solution was produced by dissolving PMMA powder in the solvent overnight on a magnetic stirrer plate. The coating was then prepared by grinding BaTiO3 particles in
Results
The relative permittivity and the loss tangent spectra were determined for the ten different BaTiO3 concentrations in Table 1. The measurement results are shown in Fig. 3. The graphs show the average of the five fabricated samples for each concentration. It is apparent that the relative permittivity as well as the dielectric loss decrease with increasing frequency. This is due to the fact that, with increasing frequency, the capability of the dipoles to change orientation according to the
Fabrication
In order to evaluate the usability of the proposed coating and to assess the improvement when compared to pure PMMA, two demonstrator devices for detecting fluids with different relative permittivities were fabricated. One of them featured an insulation layer made purely from PMMA, while the other one was prepared using the 40 vol% PMMA + 60 vol% BaTiO3 dielectric. Fig. 8 shows a picture of the fabricated device featuring the white PMMA/BaTiO3 coating.
A top and bottom piece was cut from a PMMA
Conclusion
The preparation, fabrication, and evaluation of a coating based on acrylic with mixed in barium titanate particles with improved dielectric properties was presented. It was demonstrated that it can be used for capacitive sensors in microfluidic devices. Characterization of the dielectric properties over a large frequency range and for different mixing ratios of PMMA and BATiO3 showed an optimal proportion between the polymer matrix and the ceramic filler of 40 vol% PMMA and 60 vol% BaTiO3.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been supported by the COMET-K2 “Center for Symbiotic Mechatronics” of the Linz Center of Mechatronics (LCM) funded by the Austrian Federal Government and the Federal State of Upper Austria.
References (22)
- et al.
Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation
Lab Chip
(2004) - et al.
Plastic microfluidic systems made by imprinting against an epoxy stamp
Microelectron. Eng.
(May 2010) - et al.
Dielectric, mechanical and thermal properties of polymer/BaTiO3 composites for embedded capacitor
Compos. Part B Eng.
(2013) - et al.
Detection of microdroplet size and speed using capacitive sensors
Sensors Actuators A Phys.
(Nov. 2011) - et al.
Lab-scale prototyping of polymer based microfluidic devices using gallium as phase-changing sacrificial material
Microelectron. Eng.
(Apr. 2019) - et al.
Bacteria growth monitoring through a differential CMOS capacitive sensor
IEEE Trans. Biomed. Circuits Syst.
(Aug. 2010) - et al.
Si-based sensor for virus detection
IEEE Sensors J.
(Jun. 2005) - et al.
CMOS DNA sensor Array with integrated A/D conversion based on label-free capacitance measurement
IEEE J. Solid State Circuits
(Dec. 2006) Integrated chemical microsensor systems in CMOS-technology
- et al.
History and challenges of barium titanate: part I
Sci. Sinter.
(2008)
Biocompatible evaluation of barium titanate foamed ceramic structures for orthopedic applications
J. Biomed. Mater. Res. Part A
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Both authors contributed equally to this work.