TIBTEC-1152; No. of Pages 5
Opinion
Biosensing with cell phones Pakorn Preechaburana1, Anke Suska2, and Daniel Filippini2 1 2
Department of Physics, Faculty of Science and Technology, Thammasat University, Pathumthani 12121, Thailand Optical Devices Laboratory, Department of Physics, Chemistry, and Biology (IFM), Linko¨ping University, Linko¨ping 58183, Sweden
Continued progress in cell-phone devices has made them powerful mobile computers, equipped with sophisticated, permanent physical sensors embedded as the default configuration. By contrast, the incorporation of permanent biosensors in cell-phone units has been prevented by the multivocal nature of the stimuli and the reactions involved in biosensing and chemical sensing. Biosensing with cell phones entails the complementation of biosensing devices with the physical sensors and communication and processing capabilities of modern cell phones. Biosensing, chemical-sensing, environmental-sensing, and diagnostic capabilities would thus be supported and run on the residual capacity of existing cell-phone infrastructure. The technologies necessary to materialize such a scenario have emerged in different fields and applications. This article addresses the progress on cell-phone biosensing, the specific compromises, and the blend of technologies required to craft biosensing on cell phones. Cell phones for biosensing Modern cell phones are advanced multicore computers with sophisticated user interfaces and imaging capabilities. Default configurations include various physical sensors, such as magnetometers, accelerometers, and gyroscopes, which together with global positioning make smart phones a pervasively deployed physical-sensor platform. However, cell phones are not configured for chemical sensing or biosensing, which entail multivocal stimuli and reactions with target analytes that preclude a single permanent-sensor solution [1]. Biosensing [2] and diagnostics [3,4] typically involve detection of analytes in solution and sample conditioning, which can be integrated within lab-on-a-chip (LOC) devices [5]. The use of disposable fluidics with detection chemistry simplifies aspects such as periodic calibration [6] and can exploit strong chemical interactions that improve detection selectivity [7]. Although disposable devices are attractive for deployable solutions, the auxiliary systems for conditioning and readout [8] are typically dedicated, which limit their dissemination [9]. Biosensing with cell phones exploits the ubiquitous presence, physical-sensing capabilities, and processing power of phones to provide the auxiliary support required for advanced biosensing. For more than a decade [10,11], consumer electronic devices (CEDs), such as flatbed scanners [12], CD units Corresponding author: Filippini, D. ([email protected]). Keywords: cell phones; biosensing; optical sensing; lab-on-a-chip; point-of-care; diagnostics. 0167-7799/ ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tibtech.2014.03.007
[11], and web camera–screen combinations [10,13], have been adapted for chemical sensing and biosensing. Cell phones are the latest CED platforms that offer the necessary resources to complement biosensing. Additionally, cell phones are disseminated at a scale unmatched by any other CED. They are also inherently part of the communication network, mobile, and continuously renewed by improved versions. The central challenge to exploiting this ubiquitous resource for biosensing is to craft smart interfaces between the biosensors and the phones. Modern cell phones can be connected to digital acquisition boards (see the GitHub code repository, https:// github.com/ytai/ioio/wiki) or dedicated electronics can be configured for interfacing to chemical sensors (see ‘NASA Ames Scientist Develops Cell Phone Chemical Sensor’, http://www.nasa.gov/centers/ames/news/features/2009/ cell_phone_sensors.html). These approaches are equivalent to classical computer-controlled instrumentation and offer a vast range of possibilities, which fall outside the scope of this article. Here recent progress and perspectives on biosensing with minimal or no dedicated interfacing electronics are discussed, and in this context two strategies can be identified. One strategy uses auxiliary reusable devices (ARDs) to link the biosensing assay to the cell phone [4,14]. ARDs can be of varied complexity and sophistication, and are specifically designed for certain phone brands and models. The second approach uses auxiliary disposable devices (ADDs), in which not only the biosensing part but also the coupling system is disposable. ADDs have generic designs that are compatible with diverse phone brands and models [10,15,16]. In this work, the different cell-phone biosensing strategies, target determinations, and detection principles in ARDs and ADDs are examined to address the feasibility and status of modern biosensing merged with cell-phone readouts. Parallel history of cell phones and biosensing The evolution of mobile phones has brought with it an evolution in their compatibility with biosensing (Table 1). The first consumer cell phones appeared in the 1980s, with the most iconic example represented by the bulky 1988 Motorola DynaTAC. CED chemical sensing emerged at the beginning of the 2000s [10–12], but popular and advanced phones at that time were unsuitable for biosensing. The diverse form factors and configurations that existed at the time, and the lack of generic operating systems (OSs), restricted potential biosensing solutions to specific advanced phones in the best case. The introduction of the iPhone in 2007 produced three important contributions to biosensing on cell phones. The iPhone had an OS that Trends in Biotechnology xx (2014) 1–5
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Table 1. Selection of cell-phone platforms for their historic relevance, and first configurations demonstrated for biosensing in ARD and ADD modes Phones Motorola DynaTACb [18] Nokia 3310 c Nokia 9210 d Nokia N73e [15] iPhone 4f [22] Samsung Galaxy Note 3 g
Year 1988 2000 2001 2006 2010 2013
Price a 9,337 278 1,321 750 599 730
Front/rear camera No/No No/No No/No 0.3 MP/3.15 MP 0.3 MP/5 MP 2 MP/13 MP
OS – – – – iOS 4 to 7 Android 4.3
a
Prices in US$ corrected for inflation to 2013.
b
First consumer mobile phone (795 g, 30 min talk time).
c
Representative popular model (126 million sold, 133 g), http://nds1.nokia.com/phones/files/guides/3310_usersguide_en.pdf.
d
Representative advanced phone from 2001 (32 bit 66 MHz ARM9-CPU, 640200 pixel display), http://nds1.nokia.com/phones/files/guides/9210_usersguide_en.pdf.
e
First phone used for fluorescence microscopy.
f
Representative contemporaneous smart phone with all capabilities for ADD, http://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf.
g
Representative smart phone from 2014 (32 bit, Quad-core processor 2.3 GHz), http://www.samsung.com/global/microsite/galaxynote3-gear/spec.html.
enabled the migration of applications to later models and upgraded OS releases. The iPhone also pioneered an influential design that standardized the appearance of subsequent smart-phone generations. The glass front encasing the screen and front camera combined the required mechanical and optical coupling of biosensing solutions, and allowed generic designs to be compatible with multiple brands and models. The introduction of touch-screen user interfaces (UIs) facilitated software UI configuration and created a vibrant market for third-party applications. In general, advanced phones from 2006 onwards were already capable of ARD biosensing [14], and front-camera phones from 2009 already sufficed for ADD biosensing [15]. The imaging of colorimetric devices without controlled illumination was achievable with even older phone cameras [6]. Interfacing biosensing ARDs typically incorporate auxiliary light sources (Figure 1), which are necessary for fluorescence detection and microscopy. Most ARDs also provide mechanical support for the biosensing part, together with a well-defined configuration for aligning the sample. Although phone designs have converged to a glass-front configuration, the dimensions and optical specifications vary across brands and models, and ARDs are typically designed for specific phone models [15,17], allowing for dedicated solutions that can be developed relatively quickly. In recent years, numerous biosensing and diagnostic targets have been achieved with ARD systems [4,15,17], including viral imaging [18]. However, the major disadvantage of the ARDs is that they employ reusable accessories to complement the phone, which are scarcer than phones, and limit the reach of these solutions. By contrast, ADD systems (Figure 2) aim at developing deployable disposable devices able to match the diverse smart phone brands and models. Reusable accessories are excluded, and coupling and conditioning elements are integrated with the biosensor stage in a single disposable component. Achieving generic solutions of this type is demanding, and there are fewer examples of ADD systems [15,16,19]. A representative ARD system (Figure 1A), configured for rapid diagnostic test evaluation on a Galaxy SII smart phone [17], accommodates commercial immunochromatographic assays that respond with a test line, in which 2
contrast is proportional to the analyte concentration in a blood sample. The system in this example was used to detect tuberculosis, HIV, and malaria [17], and the ARD provided mechanical fitting to the phone, a three-LED light source for reflection and transmission configurations, two AAA batteries, and a test cartridge securing the assay alignment within the ARD. In a representative ADD system (Figure 2A,B), surface plasmon resonance (SPR), the benchmark for biosensing and for the study of biomolecular interactions [20], was implemented [16] with biosensing and coupling elements in a single disposable component. SPR experiments condition visible illumination for resonant energy transfer to plasmons on a thin metal film, which results in characteristic reflectance features that can be optically captured [20] and are sensitive to the metal surface conditions, where chemical detection takes place. The ADD-SPR concept used a disposable optical coupling element with embedded fluidics, in which the biosensing assay was implemented [16]. The coupler provided temporary optical and mechanical fitting to the phone front, conditioned the illumination (provided by the phone screen), and guided the reflection towards the front camera of the phone. The acquisition software was a time-lapsed imagecapture program, and the system was tested on Nokia, Android, and Apple devices with a commercial b2 microglobulin (b2M) assay [16], an established marker for cancer, inflammatory disorders, and kidney disease. Microscopy is relevant for the diagnosis of many diseases, and several cell-phone implementations focus on this technique. The first reported ARD example [14] was a compact bright-field and fluorescence microscope (Figure 1B), which had its own light source and was designed to couple to a Nokia N73 3.2 megapixel rear camera for image acquisition. This device was tested for the identification of tuberculosis, malaria, and sickle-cell anemia [14]. The ARD could be detached and the phone used for its original purpose; this could be considered an advantage over other ARD techniques that require removing the phone camera lens, such as lens-free microscopy [21]. However, although it is desirable to keep the phone intact, preserving the integrity of the phone is not a high priority for ARD systems. Because phones are constantly being replaced by newer models, thus providing a pool of unwanted, older cell phones for use with ARD systems,
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(A)
Auxiliary reusable device (ARD)
(B)
(D) LED
LED
Specimen Specimen Phone camera (E)
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Phone camera (C)
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(F)
Posioning
LED Specimen
Aperture
Ball lens
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Figure 1. (A) Auxiliary reusable device (ARD) system for rapid test evaluation [17]. (B) ARD bright-field and fluorescence microscope configuration [14]. (C) ARD ball lens magnification [25]. (D) ARD configuration used in item (A). (E) ARD configuration for fluorescence assays [21]. (F) ARD lens-free microscopy arrangement [21]. Part (A) is reproduced, with permission, from [17].
phone integrity is not necessarily crucial for ARD solutions. A different situation arises for ADD solutions, which are intended for phones in current use. In this case, the cell phone should remain intact, which precludes techniques such as lens-free microscopy. Simpler ARD microscopy configurations have been demonstrated [22] using a glass sphere as a ball lens in contact with the camera surface, and ambient light as illumination (Figure 1C). With this configuration, focusing requires sample positioning, and multiple acquisitions, to
achieve 1.5 mm resolution. A compact ARD monochromator, for visible-absorption spectroscopy, was also demonstrated with the same phone [22]. Visible-absorption spectroscopy [13] and excitation emission-matrix spectroscopy [23] have been previously demonstrated for other CEDs, and those configurations can be adapted for cell phones. ADD magnification on cell phones has also been explored [15] (Figure 2C). Rather than microscopy, the goal in this case was to visualize details in the detection areas of disposable LOC devices. Additional requirements included using a short focal distance that was compatible with a compact configuration, placing the LOC at a fixed distance, applying adaptive focusing in order to serve diverse phones, and using technology that was suitable for integration with disposable LOCs. A confined sessile drop changing its curvature during natural evaporation was used to scan 3.5 mm in back focal distance, which made the solution adaptable to different cameras. Fluorescence detection schemes benefit from lateral illumination systems, such as the ARD configuration in Figure 1E [24]. This ARD comprised a battery-powered LED illumination with a glass substrate as waveguide that was used to excite a fluorescence-labeled specimen. A focusing lens and excitation filter completed the ARD, which was capable of 20 mm resolution. A modified version of this concept has been demonstrated for flow cytometry [25], in this case using the liquid within a polydimethylsiloxane (PDMS) microchannel as waveguide. In both cases, the ARD was configured for an unmodified Sony-Eriksson UIO I Aino from 2009. In addition to classical refractive optics, compact ARD configurations can be realized using lens-free computational imaging concepts [26]. Cell-phone lens-free microscopy has also been demonstrated with partially coherent LED illumination [21,27] using an in-line holography configuration (Figure 1F). Microscopes based on this principle are compatible with red- and white-blood-cell counting [28], and for microorganism identification in water quality assessment [29]. The configurations that were used for the evaluation of lateral-flow rapid diagnostic tests (Figure 1A,D) were also adapted for food-allergen detection [30]. The allergens were measured in vials, which required an additional modular component in the ADR. Modular components complementing a core ADR design are a sensible choice for these types of systems, which share common aspects with dedicated instrumentation. However, this alternative is unsuitable for ADD systems, which aim at a single, crossplatform, disposable device for each target. Commercial lateral-flow tests have also been evaluated with ADD systems [19]. NT-ProBNP (N-terminal of the prohormone brain natriuretic peptide) is used in routine monitoring of patients with heart failure via commercial quantitative immunologic blood tests, which give a contrast response that is proportional to NT-ProBNP concentration. In order to improve contrast detection, screen illumination is used with simple front-camera cell phones (Nokia 6710 classic) to create a high dynamic range (HDR [31]) time-lapse acquisition of the assay response, which more than doubles the detection resolution [19] (Figure 2D). These alternatives and those explored in biosensing with CEDs [10], or adapted from optical techniques [4] 3
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Auxiliary disposable device (ADD)
(B)
10 mm LOC / PDMS PDMS
Au 45 nm n = 1.5 77°
PDMS
67°
PDMS
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Ambient light
Specimen Sessile drop Phone camera (D)
Phone camera Screen illuminaon Phone
Commercial test TRENDS in Biotechnology
Figure 2. (A) Auxiliary disposable device (ADD) system for surface plasmon resonance detection of b2 microglobulin [16]. (B) ADD system used in item (A). (C) ADD adaptive magnification for lab-on-a-chip (LOC) visualization [15]. (D) ADD approach for high-dynamic-range imaging of commercial NT-ProBNP cardiac tests [19]. Parts (B) and (C) are reproduced, with permission, from [16].
and point-of-care instrumentation [3], offer a comprehensive toolbox for biosensing implementations that are suitable for migration to cell phones. Concluding remarks and future perspectives Current cell phones are more powerful than the computers on which chemical sensing with CEDs was originally demonstrated in 2000. The lifespan of complementary software for biosensing, especially in the case of ADD systems that operate on phones in current use, is strongly benefited by the existence of advanced operating systems, such as iOS and Android, that enable migration through subsequent phone generations and OS upgrades. 4
Although numerous sophisticated physical sensors are default configurations in modern cell phones, ADD/ADR biosensing examples are dominated by optical detection. This is supported by the continuously increasing performance of phone cameras and the numerous and wellestablished optical biosensing and LOC solutions that can be adapted to ARD and ADD configurations. ARD systems are efficient to rapidly produce dedicated solutions and are especially well suited to the recycling of replaced, but still viable, phones into specialized instruments. ARD units are phone model specific and are not designed to match the large variety of cell phones; thus, they may not be appropriate for large-scale deployment. By contrast, ADD systems are conceived as disposable, generic accessories that are aimed at empowering diverse phones in current use with biosensing capabilities, and they have the potential to be deployable at a large scale. However, ADD systems are less mature, and autonomous solutions will require further development in assay integration. Demonstrated and prospective concepts indicate that well-crafted ARD and ADD solutions can deliver robust performances, even compatible with diagnostic uses. Examples such as the SPR-detection experiment require external systems to control the sequence of sample delivery, washing steps, and optional amplification. In the case of ARD solutions, commercial lateral-flow configurations are used, or separate sample preparation is required. Both examples highlight the challenges ahead for biosensing with cell phones, which entail marrying autonomy [32] with sophistication in a way that maximizes the exploitation of resources available in phones. The ultimate version of either ARD or ADD approaches implies entirely autonomous operation with uncompromising sample conditioning, including active transport. Recent reviews on point-of-care instrumentation [3] and optical instrumentation for diagnostics [4,10] depict general autonomous systems, which suggest viable migration to cell phones. Autonomous LOC tests [33] with integrated multiple steps sample conditioning have been demonstrated, as well as diverse passive transport systems for LOC configurations [34]. The emerging use of 3D printers in LOC microfabrication [35,36] suggests that there will be an acceleration of innovations in the design of LOCs for cell phones. 3D printers enable affordable fast prototyping of sophisticated LOC configurations [36], which will facilitate experimentation with and custom solutions for the LOC elements; in the same ways that have already contributed to create ARD accessories [17]. Smart phones are capable of complementing biosensing performance, and future models will only improve on this aspect. However, the obstacles to integrating permanent chemical and biosensors in phones persist, and possible solutions are limited in scope or compromise the phone’s long-term reliability. Rather than replacing versatile ARD or ADD solutions with integrated permanent biosensors, a positive scenario would involve phone manufacturers and independent ARD/ADD developers contributing to a thriving community of software and hardware/LOC applications. In the same way that prevalent industrial design contributes to the re-use of ADDs on diverse phone models, other physical systems could be incorporated to facilitate
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Opinion advanced LOC command and readout beyond optical and imaging principles, without compromising the phone’s reliability. Current initiatives, such as modular phone projects [see Project Ara (http://www.projectara.com) and Phonebloks (https://phonebloks.com)], offer the opportunity to create standardized modules for signal acquisition and active sample transport, which could support uncompromised autonomous LOC biosensing on cell phones. Acknowledgments This work has been supported by grants from the Linko¨ping Centre for Life Science Technologies (LIST) (D.F.) the Swedish Research Council (VR) (D.F.) and Thammasat University, Thailand (P.P.).
References 1 Ro¨ck, F. et al. (2008) Electronic noses: current status and future trends. Chem. Rev. 108, 705–725 2 Turner, A.P.F. (2013) Biosensors: sense and sensibility. Chem. Soc. Rev. 42, 3184–3196 3 Gubala, V. et al. (2012) Point of care diagnostics: status and future. Anal. Chem. 84, 487–515 4 Zhu, H. et al. (2013) Optical imaging techniques for point-of-care diagnostics. Lab Chip 13, 51–67 5 Kovarik, M. et al. (2012) Micro total analysis systems for cell biology and biochemical assays. Anal. Chem. 84, 516–540 6 Martinez, A.W. et al. (2008) Three-dimensional microfluidic devices fabricated in layered paper and tape. Proc. Natl. Acad. Sci. U.S.A. 105, 19606–19611 7 Askim, J. et al. (2013) Optical sensor arrays for chemical sensing: the optoelectronic nose. Chem. Soc. Rev. 42, 8649–8682 8 Meagher, R. et al. (2008) An integrated microfluidic platform for sensitive and rapid detection of biological toxins. Lab Chip 8, 2046– 2053 9 Whitesides, G. (2006) The origins and the future of microfluidics. Nature 442, 368–373 10 Preechaburana P. et al. (in press) Interfacing diagnostics with consumer electronics. In Mobile Point-of-Care Monitors and Diagnostic Device Design (Karlen, W. and Iniewski, K. eds), CRC Press. 11 Potyrailo, R. (2013) Ubiquitous devices for chemical sensing. In Autonomous Sensor Networks: Collective Sensing Strategies for Analytical Purposes (Filippini, D., ed.), pp. 237–264, Springer 12 Rakow, N. and Suslick, K. (2000) A colorimetric sensor array for odour visualization. Nature 406, 710–713 13 Filippini, D. et al. (2003) Computer screen as a programmable light source for visible absorption characterization of (bio) chemical assays. Chem. Commun. 2, 240–241 14 Breslauer, D. et al. (2009) Mobile phone based clinical microscopy for global health applications. PLoS ONE 4, e6320
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15 Preechaburana, P. et al. (2012) Embedded adaptive optics for ubiquitous lab-on-a-chip readout on intact cell phones. Sensors 12, 8586–8600 16 Preechaburana, P. et al. (2012) Surface plasmon resonance chemical sensing on cell phones. Angew. Chem. Int. Ed. Engl. 51, 11585–11588 17 Mudanyali, O. et al. (2012) Integrated rapid-diagnostic-test reader platform on a cellphone. Lab Chip 12, 2678–2686 18 Wei, Q. et al. (2013) Fluorescent imaging of single nanoparticles and viruses on a smart phone. ACS Nano 7, 9147–9155 19 Preechaburana, P. et al. (2011) HDR imaging evaluation of a NTproBNP test with a mobile phone. Biosens. Bioelectron. 26, 2107–2113 20 Homola, J. (2008) Surface plasmon resonance sensors for detection of chemical and biological Species. Chem. Rev. 108, 462–493 21 Tseng, D. et al. (2010) Lensfree microscopy on a cellphone. Lab Chip 10, 1787–1792 22 Smith, Z. et al. (2011) Cell-phone-based platform for biomedical device development and education applications. PLoS ONE 6, e17150 23 Gatto, E. et al. (2008) Polychromatic fingerprinting of excitation emission matrices. Chem. Eur. J. 14, 6057–6060 24 Zhu, H. et al. (2011) Cost-effective and compact wide-field fluorescent imaging on a cell-phone. Lab Chip 11, 315–322 25 Zhu, H. et al. (2011) Optofluidic fluorescent imaging cytometry on a cell phone. Anal. Chem. 83, 6641–6647 26 Greenbaum, A. et al. (2012) Imaging without lenses: achievements and remaining challenges of wide-field on-chip microscopy. Nat. Methods 9, 889–895 27 Garcia-Sucerquia, J. et al. (2006) Immersion digital in-line holographic microscopy. Opt. Lett. 31, 1211–1213 28 Seo, S. et al. (2010) High-throughput lens-free blood analysis on a chip. Anal. Chem. 82, 4621–4627 29 Mudanyali, O. et al. (2010) Detection of waterborne parasites using field-portable and cost-effective lensfree microscopy. Lab Chip 10, 2419–2423 30 Coskun, A. et al. (2013) A personalized food allergen testing platform on a cellphone. Lab Chip 13, 636–640 31 Reinhard, E. et al. (2010) High Dynamic Range Imaging: Acquistion, Display, and Image-based Lightning. (2nd edn), Morgan Kaufmann 32 Bharadwaj, R. and Singh, A. (2013) Autonomous lab-on-a-chip technologies. In Autonomous Sensor Networks: Collective Sensing Strategies for Analytical Purposes (Filippini, D., ed.), pp. 217–236, Springer 33 Wang, J. et al. (2010) A self-powered, one-step chip for rapid, quantitative and multiplexed detection of proteins from pinpricks of whole blood. Lab Chip 10, 3157–3162 34 Zimmermann, M. et al. (2007) Capillary pumps for autonomous capillary systems. Lab Chip 7, 119–125 35 Kitson, P. et al. (2012) Configurable 3D-printed millifluidic and microfluidic ‘lab on a chip’ reactionware devices. Lab Chip 12, 3267–3271 36 Comina, G. et al. (2014) PDMS lab-on-a-chip fabrication using 3D printed templates. Lab Chip 14, 424–430
5
Opinion
Biosensing with cell phones Pakorn Preechaburana1, Anke Suska2, and Daniel Filippini2 1 2
Department of Physics, Faculty of Science and Technology, Thammasat University, Pathumthani 12121, Thailand Optical Devices Laboratory, Department of Physics, Chemistry, and Biology (IFM), Linko¨ping University, Linko¨ping 58183, Sweden
Continued progress in cell-phone devices has made them powerful mobile computers, equipped with sophisticated, permanent physical sensors embedded as the default configuration. By contrast, the incorporation of permanent biosensors in cell-phone units has been prevented by the multivocal nature of the stimuli and the reactions involved in biosensing and chemical sensing. Biosensing with cell phones entails the complementation of biosensing devices with the physical sensors and communication and processing capabilities of modern cell phones. Biosensing, chemical-sensing, environmental-sensing, and diagnostic capabilities would thus be supported and run on the residual capacity of existing cell-phone infrastructure. The technologies necessary to materialize such a scenario have emerged in different fields and applications. This article addresses the progress on cell-phone biosensing, the specific compromises, and the blend of technologies required to craft biosensing on cell phones. Cell phones for biosensing Modern cell phones are advanced multicore computers with sophisticated user interfaces and imaging capabilities. Default configurations include various physical sensors, such as magnetometers, accelerometers, and gyroscopes, which together with global positioning make smart phones a pervasively deployed physical-sensor platform. However, cell phones are not configured for chemical sensing or biosensing, which entail multivocal stimuli and reactions with target analytes that preclude a single permanent-sensor solution [1]. Biosensing [2] and diagnostics [3,4] typically involve detection of analytes in solution and sample conditioning, which can be integrated within lab-on-a-chip (LOC) devices [5]. The use of disposable fluidics with detection chemistry simplifies aspects such as periodic calibration [6] and can exploit strong chemical interactions that improve detection selectivity [7]. Although disposable devices are attractive for deployable solutions, the auxiliary systems for conditioning and readout [8] are typically dedicated, which limit their dissemination [9]. Biosensing with cell phones exploits the ubiquitous presence, physical-sensing capabilities, and processing power of phones to provide the auxiliary support required for advanced biosensing. For more than a decade [10,11], consumer electronic devices (CEDs), such as flatbed scanners [12], CD units Corresponding author: Filippini, D. ([email protected]). Keywords: cell phones; biosensing; optical sensing; lab-on-a-chip; point-of-care; diagnostics. 0167-7799/ ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tibtech.2014.03.007
[11], and web camera–screen combinations [10,13], have been adapted for chemical sensing and biosensing. Cell phones are the latest CED platforms that offer the necessary resources to complement biosensing. Additionally, cell phones are disseminated at a scale unmatched by any other CED. They are also inherently part of the communication network, mobile, and continuously renewed by improved versions. The central challenge to exploiting this ubiquitous resource for biosensing is to craft smart interfaces between the biosensors and the phones. Modern cell phones can be connected to digital acquisition boards (see the GitHub code repository, https:// github.com/ytai/ioio/wiki) or dedicated electronics can be configured for interfacing to chemical sensors (see ‘NASA Ames Scientist Develops Cell Phone Chemical Sensor’, http://www.nasa.gov/centers/ames/news/features/2009/ cell_phone_sensors.html). These approaches are equivalent to classical computer-controlled instrumentation and offer a vast range of possibilities, which fall outside the scope of this article. Here recent progress and perspectives on biosensing with minimal or no dedicated interfacing electronics are discussed, and in this context two strategies can be identified. One strategy uses auxiliary reusable devices (ARDs) to link the biosensing assay to the cell phone [4,14]. ARDs can be of varied complexity and sophistication, and are specifically designed for certain phone brands and models. The second approach uses auxiliary disposable devices (ADDs), in which not only the biosensing part but also the coupling system is disposable. ADDs have generic designs that are compatible with diverse phone brands and models [10,15,16]. In this work, the different cell-phone biosensing strategies, target determinations, and detection principles in ARDs and ADDs are examined to address the feasibility and status of modern biosensing merged with cell-phone readouts. Parallel history of cell phones and biosensing The evolution of mobile phones has brought with it an evolution in their compatibility with biosensing (Table 1). The first consumer cell phones appeared in the 1980s, with the most iconic example represented by the bulky 1988 Motorola DynaTAC. CED chemical sensing emerged at the beginning of the 2000s [10–12], but popular and advanced phones at that time were unsuitable for biosensing. The diverse form factors and configurations that existed at the time, and the lack of generic operating systems (OSs), restricted potential biosensing solutions to specific advanced phones in the best case. The introduction of the iPhone in 2007 produced three important contributions to biosensing on cell phones. The iPhone had an OS that Trends in Biotechnology xx (2014) 1–5
1
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Opinion
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Table 1. Selection of cell-phone platforms for their historic relevance, and first configurations demonstrated for biosensing in ARD and ADD modes Phones Motorola DynaTACb [18] Nokia 3310 c Nokia 9210 d Nokia N73e [15] iPhone 4f [22] Samsung Galaxy Note 3 g
Year 1988 2000 2001 2006 2010 2013
Price a 9,337 278 1,321 750 599 730
Front/rear camera No/No No/No No/No 0.3 MP/3.15 MP 0.3 MP/5 MP 2 MP/13 MP
OS – – – – iOS 4 to 7 Android 4.3
a
Prices in US$ corrected for inflation to 2013.
b
First consumer mobile phone (795 g, 30 min talk time).
c
Representative popular model (126 million sold, 133 g), http://nds1.nokia.com/phones/files/guides/3310_usersguide_en.pdf.
d
Representative advanced phone from 2001 (32 bit 66 MHz ARM9-CPU, 640200 pixel display), http://nds1.nokia.com/phones/files/guides/9210_usersguide_en.pdf.
e
First phone used for fluorescence microscopy.
f
Representative contemporaneous smart phone with all capabilities for ADD, http://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf.
g
Representative smart phone from 2014 (32 bit, Quad-core processor 2.3 GHz), http://www.samsung.com/global/microsite/galaxynote3-gear/spec.html.
enabled the migration of applications to later models and upgraded OS releases. The iPhone also pioneered an influential design that standardized the appearance of subsequent smart-phone generations. The glass front encasing the screen and front camera combined the required mechanical and optical coupling of biosensing solutions, and allowed generic designs to be compatible with multiple brands and models. The introduction of touch-screen user interfaces (UIs) facilitated software UI configuration and created a vibrant market for third-party applications. In general, advanced phones from 2006 onwards were already capable of ARD biosensing [14], and front-camera phones from 2009 already sufficed for ADD biosensing [15]. The imaging of colorimetric devices without controlled illumination was achievable with even older phone cameras [6]. Interfacing biosensing ARDs typically incorporate auxiliary light sources (Figure 1), which are necessary for fluorescence detection and microscopy. Most ARDs also provide mechanical support for the biosensing part, together with a well-defined configuration for aligning the sample. Although phone designs have converged to a glass-front configuration, the dimensions and optical specifications vary across brands and models, and ARDs are typically designed for specific phone models [15,17], allowing for dedicated solutions that can be developed relatively quickly. In recent years, numerous biosensing and diagnostic targets have been achieved with ARD systems [4,15,17], including viral imaging [18]. However, the major disadvantage of the ARDs is that they employ reusable accessories to complement the phone, which are scarcer than phones, and limit the reach of these solutions. By contrast, ADD systems (Figure 2) aim at developing deployable disposable devices able to match the diverse smart phone brands and models. Reusable accessories are excluded, and coupling and conditioning elements are integrated with the biosensor stage in a single disposable component. Achieving generic solutions of this type is demanding, and there are fewer examples of ADD systems [15,16,19]. A representative ARD system (Figure 1A), configured for rapid diagnostic test evaluation on a Galaxy SII smart phone [17], accommodates commercial immunochromatographic assays that respond with a test line, in which 2
contrast is proportional to the analyte concentration in a blood sample. The system in this example was used to detect tuberculosis, HIV, and malaria [17], and the ARD provided mechanical fitting to the phone, a three-LED light source for reflection and transmission configurations, two AAA batteries, and a test cartridge securing the assay alignment within the ARD. In a representative ADD system (Figure 2A,B), surface plasmon resonance (SPR), the benchmark for biosensing and for the study of biomolecular interactions [20], was implemented [16] with biosensing and coupling elements in a single disposable component. SPR experiments condition visible illumination for resonant energy transfer to plasmons on a thin metal film, which results in characteristic reflectance features that can be optically captured [20] and are sensitive to the metal surface conditions, where chemical detection takes place. The ADD-SPR concept used a disposable optical coupling element with embedded fluidics, in which the biosensing assay was implemented [16]. The coupler provided temporary optical and mechanical fitting to the phone front, conditioned the illumination (provided by the phone screen), and guided the reflection towards the front camera of the phone. The acquisition software was a time-lapsed imagecapture program, and the system was tested on Nokia, Android, and Apple devices with a commercial b2 microglobulin (b2M) assay [16], an established marker for cancer, inflammatory disorders, and kidney disease. Microscopy is relevant for the diagnosis of many diseases, and several cell-phone implementations focus on this technique. The first reported ARD example [14] was a compact bright-field and fluorescence microscope (Figure 1B), which had its own light source and was designed to couple to a Nokia N73 3.2 megapixel rear camera for image acquisition. This device was tested for the identification of tuberculosis, malaria, and sickle-cell anemia [14]. The ARD could be detached and the phone used for its original purpose; this could be considered an advantage over other ARD techniques that require removing the phone camera lens, such as lens-free microscopy [21]. However, although it is desirable to keep the phone intact, preserving the integrity of the phone is not a high priority for ARD systems. Because phones are constantly being replaced by newer models, thus providing a pool of unwanted, older cell phones for use with ARD systems,
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Figure 1. (A) Auxiliary reusable device (ARD) system for rapid test evaluation [17]. (B) ARD bright-field and fluorescence microscope configuration [14]. (C) ARD ball lens magnification [25]. (D) ARD configuration used in item (A). (E) ARD configuration for fluorescence assays [21]. (F) ARD lens-free microscopy arrangement [21]. Part (A) is reproduced, with permission, from [17].
phone integrity is not necessarily crucial for ARD solutions. A different situation arises for ADD solutions, which are intended for phones in current use. In this case, the cell phone should remain intact, which precludes techniques such as lens-free microscopy. Simpler ARD microscopy configurations have been demonstrated [22] using a glass sphere as a ball lens in contact with the camera surface, and ambient light as illumination (Figure 1C). With this configuration, focusing requires sample positioning, and multiple acquisitions, to
achieve 1.5 mm resolution. A compact ARD monochromator, for visible-absorption spectroscopy, was also demonstrated with the same phone [22]. Visible-absorption spectroscopy [13] and excitation emission-matrix spectroscopy [23] have been previously demonstrated for other CEDs, and those configurations can be adapted for cell phones. ADD magnification on cell phones has also been explored [15] (Figure 2C). Rather than microscopy, the goal in this case was to visualize details in the detection areas of disposable LOC devices. Additional requirements included using a short focal distance that was compatible with a compact configuration, placing the LOC at a fixed distance, applying adaptive focusing in order to serve diverse phones, and using technology that was suitable for integration with disposable LOCs. A confined sessile drop changing its curvature during natural evaporation was used to scan 3.5 mm in back focal distance, which made the solution adaptable to different cameras. Fluorescence detection schemes benefit from lateral illumination systems, such as the ARD configuration in Figure 1E [24]. This ARD comprised a battery-powered LED illumination with a glass substrate as waveguide that was used to excite a fluorescence-labeled specimen. A focusing lens and excitation filter completed the ARD, which was capable of 20 mm resolution. A modified version of this concept has been demonstrated for flow cytometry [25], in this case using the liquid within a polydimethylsiloxane (PDMS) microchannel as waveguide. In both cases, the ARD was configured for an unmodified Sony-Eriksson UIO I Aino from 2009. In addition to classical refractive optics, compact ARD configurations can be realized using lens-free computational imaging concepts [26]. Cell-phone lens-free microscopy has also been demonstrated with partially coherent LED illumination [21,27] using an in-line holography configuration (Figure 1F). Microscopes based on this principle are compatible with red- and white-blood-cell counting [28], and for microorganism identification in water quality assessment [29]. The configurations that were used for the evaluation of lateral-flow rapid diagnostic tests (Figure 1A,D) were also adapted for food-allergen detection [30]. The allergens were measured in vials, which required an additional modular component in the ADR. Modular components complementing a core ADR design are a sensible choice for these types of systems, which share common aspects with dedicated instrumentation. However, this alternative is unsuitable for ADD systems, which aim at a single, crossplatform, disposable device for each target. Commercial lateral-flow tests have also been evaluated with ADD systems [19]. NT-ProBNP (N-terminal of the prohormone brain natriuretic peptide) is used in routine monitoring of patients with heart failure via commercial quantitative immunologic blood tests, which give a contrast response that is proportional to NT-ProBNP concentration. In order to improve contrast detection, screen illumination is used with simple front-camera cell phones (Nokia 6710 classic) to create a high dynamic range (HDR [31]) time-lapse acquisition of the assay response, which more than doubles the detection resolution [19] (Figure 2D). These alternatives and those explored in biosensing with CEDs [10], or adapted from optical techniques [4] 3
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Figure 2. (A) Auxiliary disposable device (ADD) system for surface plasmon resonance detection of b2 microglobulin [16]. (B) ADD system used in item (A). (C) ADD adaptive magnification for lab-on-a-chip (LOC) visualization [15]. (D) ADD approach for high-dynamic-range imaging of commercial NT-ProBNP cardiac tests [19]. Parts (B) and (C) are reproduced, with permission, from [16].
and point-of-care instrumentation [3], offer a comprehensive toolbox for biosensing implementations that are suitable for migration to cell phones. Concluding remarks and future perspectives Current cell phones are more powerful than the computers on which chemical sensing with CEDs was originally demonstrated in 2000. The lifespan of complementary software for biosensing, especially in the case of ADD systems that operate on phones in current use, is strongly benefited by the existence of advanced operating systems, such as iOS and Android, that enable migration through subsequent phone generations and OS upgrades. 4
Although numerous sophisticated physical sensors are default configurations in modern cell phones, ADD/ADR biosensing examples are dominated by optical detection. This is supported by the continuously increasing performance of phone cameras and the numerous and wellestablished optical biosensing and LOC solutions that can be adapted to ARD and ADD configurations. ARD systems are efficient to rapidly produce dedicated solutions and are especially well suited to the recycling of replaced, but still viable, phones into specialized instruments. ARD units are phone model specific and are not designed to match the large variety of cell phones; thus, they may not be appropriate for large-scale deployment. By contrast, ADD systems are conceived as disposable, generic accessories that are aimed at empowering diverse phones in current use with biosensing capabilities, and they have the potential to be deployable at a large scale. However, ADD systems are less mature, and autonomous solutions will require further development in assay integration. Demonstrated and prospective concepts indicate that well-crafted ARD and ADD solutions can deliver robust performances, even compatible with diagnostic uses. Examples such as the SPR-detection experiment require external systems to control the sequence of sample delivery, washing steps, and optional amplification. In the case of ARD solutions, commercial lateral-flow configurations are used, or separate sample preparation is required. Both examples highlight the challenges ahead for biosensing with cell phones, which entail marrying autonomy [32] with sophistication in a way that maximizes the exploitation of resources available in phones. The ultimate version of either ARD or ADD approaches implies entirely autonomous operation with uncompromising sample conditioning, including active transport. Recent reviews on point-of-care instrumentation [3] and optical instrumentation for diagnostics [4,10] depict general autonomous systems, which suggest viable migration to cell phones. Autonomous LOC tests [33] with integrated multiple steps sample conditioning have been demonstrated, as well as diverse passive transport systems for LOC configurations [34]. The emerging use of 3D printers in LOC microfabrication [35,36] suggests that there will be an acceleration of innovations in the design of LOCs for cell phones. 3D printers enable affordable fast prototyping of sophisticated LOC configurations [36], which will facilitate experimentation with and custom solutions for the LOC elements; in the same ways that have already contributed to create ARD accessories [17]. Smart phones are capable of complementing biosensing performance, and future models will only improve on this aspect. However, the obstacles to integrating permanent chemical and biosensors in phones persist, and possible solutions are limited in scope or compromise the phone’s long-term reliability. Rather than replacing versatile ARD or ADD solutions with integrated permanent biosensors, a positive scenario would involve phone manufacturers and independent ARD/ADD developers contributing to a thriving community of software and hardware/LOC applications. In the same way that prevalent industrial design contributes to the re-use of ADDs on diverse phone models, other physical systems could be incorporated to facilitate
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Opinion advanced LOC command and readout beyond optical and imaging principles, without compromising the phone’s reliability. Current initiatives, such as modular phone projects [see Project Ara (http://www.projectara.com) and Phonebloks (https://phonebloks.com)], offer the opportunity to create standardized modules for signal acquisition and active sample transport, which could support uncompromised autonomous LOC biosensing on cell phones. Acknowledgments This work has been supported by grants from the Linko¨ping Centre for Life Science Technologies (LIST) (D.F.) the Swedish Research Council (VR) (D.F.) and Thammasat University, Thailand (P.P.).
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