Feature Article

Detecting a cerebrovascular event
Cerebrovascular disease, commonly called a “stroke,” currently is the most common life-threatening neurological event and the leading cause of serious, long-term disability in the developed world. Although strokes are challenging to identify, the development of a rapid diagnostic instrument inspires researchers because early intervention during a stroke has a huge impact on treatment efficacy. Supported by the European Commission Seventh Framework (ICT Call 4), the P3SENS project aims to develop and validate an innovative photonic technology responding to the needs for emergency-medicine diagnostic devices. The technology combines high sensitivity, rapid response and easy-to-use multiparametric detection at a low price point. 

The P3SENS biosensor
The objective of the P3SENS project is to design and validate a label-free disposable optical biosensor with up to 50 channels for the detection of minute concentrations of analytes (<1 ng/ml) in solution. In particular, the project focuses on the ability to rapidly detect biochemical markers (antibodies) in a patient’s blood. The presence of well-identified biomarkers S100b, H-FABP, NSE, Troponin I, BNP and CRP is the signature of an imminent or ongoing stroke. If the test is performed sufficiently early, it can significantly improve the patient’s chance of survival and subsequent quality of life.

Figure 1: Schematic of the photonic crystal device used for the optical detection of proteins.

P3SENS technology involves coupling photonic crystal (PhC) microcavities to a planar waveguide optical distribution circuit (Figure 1). The light from a laser source at a given wavelength is coupled to the circuit to detect the presence, at low concentrations, of the aforementioned analytes in a biological liquid.1, 2 This is made possible by exploiting the extremely high sensitivity of the photonic structure with respect to a small variation of the refractive index at the sensitive surface of the device, which is produced by the antigen/antibody binding.

The photonic chip is coupled to a microfluidic device that transports the sample liquid (blood or plasma) to the sensing area. The manipulation of fluids is a key issue in terms of the final application of the device. In the P3SENS approach, the photonic and fluidic functionalities are fully integrated in a single low-cost, disposable chip. Moreover, sample manipulation is implemented by complex microfluidic structures that perform sample preparation tasks such as mixing, dilution, transportation and separation. The goal is to design, develop and produce reliable and robust microfluidic systems that enable fast and cost-effective sample transport whilst performing some simple sample preparation functions, such as mixing or dilution.

Material selection plays a key role in terms of the structural integrity and performance of the finished device, as it will have an impact on achieving a leak-free bonding process. The most appropriate polymers for the fabrication of simple microfluidic systems are PDMS and the negative photoresist SU-8. These materials are relatively inexpensive and can be easily structured by a combination of microlithography and polymer technology. Figure 2 provides an overview of the fabrication and integration of the biosensor.

Reducing the cost of photonic chips
To reduce production costs, the photonic chip can be patterned by means of nanoimprint lithography (NIL). NIL has been successfully demonstrated on a silicon substrate; P3SENS seeks to show that this technique also can be applied to the fabrication of a photonic chip on a low-cost polymer substrate. It is anticipated that the fabrication processes can be adapted to roll-to-roll type processing to achieve low-cost, high-throughput production.

The photonic crystal’s optical circuitry, which routes light around the chip and enables its interaction with the medium being sensed, is defined via arrays having feature sizes on the order of 100 nm. The design and manufacture of an effective polymer-based photonic crystal device is challenging, primarily because of the low refractive index of commercially available polymers. This is especially striking when it is compared with the refractive index of standard photonics materials such as silicon. Specially engineered materials are required to successfully implement the optical guiding properties on a polymer chip. In the P3SENS project, this is achieved through the controlled modification of the material’s optical properties by the integration of nanoparticles. The nanocomposite polymers are optimised both for their inherent optical properties and processability via NIL.

Figure 2: Panel A shows a schematic of the fabrication steps undertaken in P3SENS for the fabrication and integration of the photonic crystal device. The detection mechanism (panel B) relies on the interaction of the evanescent wave with the analytes under test and multiplexed wavelength-based reading of the optical signal.

The controlled inclusion of nanoparticles in the bulk material imparts the desired modification of optical properties (or refractive index) in the material. The material, therefore, is engineered to respond to the desired optical properties. For instance, careful design of the nanocomposite material results in a sufficiently high refractive index contrast for guiding the light in the photonic device even in the presence of water (i.e., when the biological sample is under test). The optical properties of the polymers are defined by the nature of the nanoparticles within them: the use of metal oxides allows modification of the refractive index to optimise the inherent sensitivity of the biosensor.

A range of polymer host materials and nanoparticles have been screened to identify suitable systems for the formation of nanocomposites and their subsequent processing via NIL. It has been shown that the refractive index of a polymer can be increased significantly by incorporation of the correct nanoparticles. Thin polymer films measuring less than 500 nm and having a refractive index up to 1.74 have been prepared. Initial tests indicate that the high-index nanocomposites can be structured by NIL, which proved to be successful for patterning 1-µm deep nanostructures in polyimide. In parallel, progress also has been made in the development of polymer layer stacks needed for implementation of the optical sensor.

The way forward
The completion of these significant milestones, showing development of new materials suitable for the production of polymer photonic crystals, is rapidly advancing the P3SENS consortium towards its goal: the production of a biosensor platform and demonstration of its applicability to a point-of-care diagnosis of a cerebrovascular event. The consortium continues its multidisciplinary approach with parallel activities driving forward developments in polymer materials, photonic crystal fabrication, optical and fluidic systems and biomedical proteomics. The device under development by P3SENS is specifically intended for stroke-related applications, but it also can be used to screen a number of proteins related to a larger panel of diseases.
 
References
1.    J. Cooper et al., “Photonic Crystal Structures in Molecular Biosensing,” OPN Optics & Photonics News, 21, 9, 27-31 (2010).
2.    C.F. Carlborg et al., “A packaged optical slot-waveguide ring resonator sensor array for multiplexed label-free assays in labs-on-chips,” Lab on Chip, 10, 281-290 (2010).

Domenico Giannone, PhD,
is P3SENS Project Coordinator, Multitel
Research Centre, Applied Photonics
Department, Parc Initialis, 7000 Mons, Belgium
tel. +32 6 534 2849
e-mail: domenico.giannone@multitel.be
www.p3sens-project.eu