Paper-based microfluidic devices: On-site tools for crime scene investigation

Highlights

• A critical review on paper-based microfluidics devices developed for forensic science issues.

• Several strategies for developing paper-based microfluidics devices for forensic applications have been proposed.

• Paper-based microfluidics technology can be used as a fast, reliable, and cost-efficient tool in forensic sciences.

• Current challenges and future trends in paper-based microfluidics for forensic issues are discussed.

Abstract

The use of paper-based microfluidics devices (μPADs) for designing rapid, easy, and on-site devices has undergone a dramatic increase in the last few years and the development of applications in the field of forensic science has had concomitant advances. The use of μPADs at the crime scene is finalized not only to obtain a rapid response but also to have crucial information on evidence collection.

Several authors have proposed μPADs as alternative approaches to existing tests. These devices are designed to be used on-site without, or minimal, power sources. Furthermore, the technology provides prompt-readouts using a small sample volume, preserving important evidence for instrumental analysis.

This review aims to provide a critical evaluation of developed μPADs. The document includes a brief overview on fabrication methods and on detection strategies, and it assesses advantages and limitations of applications related to forensic matters. A final section is devoted to future developments.

Introduction

The initial investigation of a crime scene is of utmost importance to police and forensic examiners for collecting relevant evidence and for verifying the actions of potential suspects [1]. Technological advancements have enabled the detection of substances with great accuracy and precision, providing results with high sensitivity and reliability. However, instrument-based approaches that can only be implemented in laboratories, are expensive, not user-friendly and require highly skilled personnel for the complex analytical procedure and the result interpretation. Consequently, these technological approaches needing well equipped laboratories, are impractical for application at the crime scene.

Instead, crime scene investigators are bound to use “presumptive tests” to detect substances of forensic interest, ranging from explosives to clandestine preparation of drugs or toxicants either in the environment or in body fluids. However, these tests show clear shortcomings terms of sensitivity and specificity, which limit greatly their usefulness for the investigative actions. An important body of literature has piled on this subject, as witnessed by several reviews, but these papers often have not a specific focus on forensics [2], including different approaches [1,3] and analytical techniques [6]. On the other hand, in some cases, the reports are focused only on selected applications [4,5]. On these grounds, the present work has been primarily intended as a detailed review on developments in paper-based microfluidic technology in the context of the forensic investigation, and particularly in rapid evidence testing to be carried out the crime scene.

The first example of a paper microfluidic tool was reported in 1949 by Müller et al. The authors proposed the use of paraffin-patterned paper microfluidic channels as a method to hinder lateral diffusion of bands in paper chromatography [7]. However, this pioneering work did not receive much attention until the early years of this century when other researchers understood the usefulness of forming microchannels in paper sheets for analytical purposes. A well-known report which has boosted in paper microfluidics appeared in 2007 by the Whitesides group [8]. Paper-based microfluidics devices (μPADs) were developed to provide diagnostic devices for use in developing countries or remote areas, but recently have attracted the attention of several researchers working in forensic sciences.

Paper-based technology aims to produce microanalysis in compartments by delimiting hydrophilic areas through the use of barriers made of hydrophobic compounds. Paper has an inherent ability to provide fluidic flow as a result of capillarity action without the need for pumps or external actuation. Also, paper is inert, low-cost, abundant, and biodegradable.

The ASSURED criteria, (Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free and Deliverable to end-users), works as a benchmark for evaluating the reliability of the disease-based analytical tests [9], but similarly is necessary also in the initial examination of a crime scene. Since the first applications of paper-based microfluidics for forensic purposes [18,19], several devices have been proposed. To date, a wide range of drugs and cutting agents can be identified by μPADs [[12], [13], [14], [16], [17], [18], [19], [20]]. These devices are also used in forensic serology to distinguish blood, urine, and semen [21] as well as in thanatochemistry to estimate time since death [22,23]. μPADs may also be used to detect warfare agents, such as pre-blast and post-blast explosives and nerve agents [11,[24], [25], [26], [27], [28]]. More recently, Azuaje-Hualde et al. have developed a proof-of-concept device for identifying the sex of a person from a biological sample [29].

In this paper we review reports on the use of paper-based devices for forensic aims, organizing the information into two sections. The first section is a short overview of the processes utilized in the fabrication of μPADs and the main detection methods, highlighting the advantages and limitations in the forensic context. The second section presents a critical view of devices intended for applications in the field, discussing both the advantages and disadvantages of these devices and describing their approaches towards measurements that can be performed directly at the crime scene.