Flexible SAW Microfluidic Devices as Wearable pH Sensors Based on ZnO Nanoparticles.

Luigi Piro,Antonio Balena,Francesco Rizzi,Leonardo Lamanna,Massimo De Vittorio,Massimo Mariello,Francesco Guido

doi:10.3390/nano11061479

Abstract

In this work, a new flexible and biocompatible microfluidic pH sensor based on surface acoustic waves (SAWs) is presented. The device consists of polyethylene naphthalate (PEN) as a flexible substrate on which aluminum nitride (AlN) has been deposited as a piezoelectric material. The fabrication of suitable interdigitated transducers (IDTs) generates Lamb waves (L-SAW) with a center frequency ≈500 MHz traveling in the active region. A SU-8 microfluidics employing ZnO nanoparticles (NPs) functionalization as a pH-sensitive layer is fabricated between the IDTs, causing a shift in the L-SAW resonance frequency as a function of the change in pH values. The obtained sensitivity of ≈30 kHz/pH from pH 7 to pH 2 demonstrates the high potential of flexible SAW devices to be used in the measurement of pH in fluids and biosensing.

Highlights

The development of biocompatible and flexible micro-electro-mechanical system (MEMS) sensors has increased over the last decade [1]
surface acoustic waves (SAWs) devices are fabricated on thick piezoelectric crystals as LiNbO3 or LiTaO3 [11]
aluminum nitride (AlN)-based SAW devices were fabricated on 125 μm thick polyethylene naphtalate (PEN), purchased from Teonex (Wilmington, DE, USA) with one pretreated surface, which improves the growth adhesion for material deposition

Summary

Introduction

The development of biocompatible and flexible micro-electro-mechanical system (MEMS) sensors has increased over the last decade [1]. Surface acoustic wave (SAW)based devices represent one of the main building blocks in MEMS and have applications in many fields, mostly the development of biosensors for many different purposes (pH, heavy metals concentrations) [2,3], and different physical (light intensity, temperature, deformation) [4,5,6,7] and biological (proteins, small molecule or cell concentrations) [8,9,10]. SAW devices are fabricated on thick piezoelectric crystals as LiNbO3 or LiTaO3 [11]. These materials show excellent piezoelectric properties, but their high stiffness and expensive cost make them unsuitable for wearable applications. The demand for flexible and skin-compliant materials has increased in the last decade, and new flexible materials such as Kapton [12]

Methods

Results

Conclusion