Elsevier

Microelectronic Engineering

Volume 223, 15 February 2020, 111220
Microelectronic Engineering

Research paper
A dielectric coating for improved performance of capacitive sensors in all-polymer microfluidic devices

https://doi.org/10.1016/j.mee.2020.111220Get rights and content

Highlights

  • Evaluation of permittivity increase by embedding BaTiO3 particles into PMMA.

  • Composite coatings with a wide range of concentrations (0–90 vol%) were fabricated.

  • Maximum permittivity was found at 60 vol% ceramic filler.

  • Sensitivity enhancement by a factor of 5 in demonstrator device for fluid detection

Abstract

Increasing the relative permittivity of dielectric materials can be useful in many applications, including capacitive sensing in microfluidics. In order to be able to efficiently integrate capacitive sensors with high sensitivity into all-polymer microfluidic devices, polymeric layers with high dielectric constants are required. In this contribution, a dielectric coating made from a polymeric base with mixed in ceramic particles, which exhibits enhanced dielectric properties compared to the polymer itself, is presented. Poly(methyl methacrylate) is chosen as a polymeric base material due to its processing properties. Its relative permittivity is increased by mixing in barium titanate particles at concentrations ranging from 0 vol% to 90 vol%. The dielectric properties of each fabricated mixture are investigated and the results are compared to theoretical values derived from standard mixing rules. To demonstrate the sensitivity enhancement of sensors due to the use of the fabricated dielectric coating, a microfluidic device is presented featuring a capacitive sensor for detection of fluids with different dielectric constants. It is shown that the sensitivity of the capacitive sensor is significantly increased by using the custom dielectric mixture when compared to pure poly(methyl methacrylate).

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.

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    Both authors contributed equally to this work.

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