Water-resistant flexible GaN LED on a liquid crystal polymer substrate for implantable biomedical applications

Abstract

A flexible GaN light-emitting diode (LED) has been fabricated on a liquid crystal polymer (LCP) substrate for biomedical applications. The bending radius and fatigue tests demonstrated the mechanically and optically stable characteristics of the GaN LEDs on the flexible substrates. A white light-emitting phosphor-coated GaN LED showed its potential as a next-generation flexible light source. A water-resistant and biocompatible polytetrafluoroethylene (PTFE) coated flexible LED detected prostate-specific antigen (PSA), which is expected to be used in future biomedical devices.

Introduction

Light-emitting diodes (LEDs) have superior characteristics, such as long-term stability, high efficiency, and strong brightness compared to conventional incandescent lamps. With these advantages, LEDs have been developed and explored for not only consumer electronics such as energy efficient light lamp, back-light unit (BLU), and active matrix organic LED (AMOLED) but also medical applications, including body composition detectors (e.g., hemoglobin [1], human body fat [2], prostate-specific antigen (PSA) [3], and cholesterol [4]), and therapy devices (e.g., sterilization [5], skin homeostasis [6], and operation lamps [7]). In particular, flexible LEDs, which can conformally contact on curvilinear surface, have intrigued researchers in the field of biosensors and micro-sized surgery robots for non-bleeding diagnoses and treatment.

Organic LEDs (OLEDs) have been studied actively over the last two decades with the feasibility of flexible devices [8], [9], [10], [11], [12]. However, OLEDs have drawbacks such as short lifetime (∼10,000 h) [13] low efficiency, low brightness, and low stability in humidity condition compared to inorganic LEDs (ILEDs) [14], [15]. Although the first flexible GaN materials were demonstrated in 2005 by the author [16], the first flexible III–V LEDs were presented by Kim et al. using micro-structured GaAs (μs-GaAs) [17], [18], [19], [20]. Their μs-GaAs LEDs printed on a polyethylene terephthalate (PET) substrate were applied to detect glucose concentrations from the variation of transmitted light intensities. However, the GaAs-based LED, which emits only infrared and red light, ultimately limits the possible applications due to its narrow wavelength range. Recently in 2011, Kim et al. established the flexible GaN ILED by micro-structure transferring process [21]. Although they proposed the feasibility for flexible LED applications in display, it did not provide the opportunity for biomedical devices.

Herein, for the first time, single crystal micro-structured GaN (μs-GaN) was employed for the water-resistance and biomedical applications. The superb properties of the GaN material in terms of its wide band gap and high efficiency enable the dramatic extension of not only consumer electronic applications (13 billion dollar market of GaN itself in 2011) [22] but also the biosensing scale [23], [24], [25], [26], [27], [28]. In this study, we describe four works on flexible ILEDs, as follows: (1) establishment of the flexible GaN ILED that can be applied to full color RGB elements [24] and controllable micro-patterned LED arrays; (2) a flexible white LED created through a combination of a blue GaN LED and yellow phosphor, which can be utilized for the promising flexible BLU [25]; (3) a water-resistant and flexible GaN LED on a liquid crystal polymer (LCP) substrate packaged by biocompatible polytetrafluoroethylene (PTFE), promising candidate materials that enables robustness in body-implanted conditions [29], and (4) diagnosis of diseases (e.g., PSA) by detecting antigen–antibody reactions using the flexible GaN LED and photodiode.