Elastomeric PDMS Planoconvex Lenses ... - ACS Publications

Jul 15, 2016 - Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Patumwan,. Bangkok ...
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Elastomeric PDMS Planoconvex Lenses Fabricated by a Confined Sessile Drop Technique S. Ekgasit,*,† N. Kaewmanee,† P. Jangtawee,† C. Thammacharoen,† and M. Donphoongpri‡ †

Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Patumwan, Bangkok 10330, Thailand ‡ Central Institute of Forensic Science (CIFS), 8th Floor, B Building, The Government Complex, Chaengwatthana Road, Laksi, Bangkok 10210 Thailand S Supporting Information *

ABSTRACT: The ubiquity of high quality smartphones at affordable prices not only accelerated the social penetration in the global population but also promoted nontraditional usage of smartphones as point-of-care medical diagnostic devices, sensors, and portable digital microscopes. This paper reveals a simple, rapid, cost-effective, and template-free technique for mass-scale production of an elastomeric PDMS (ePDMS) planoconvex lens capable of converting a smartphone into a portable digital microscope. By taking advantage of the resistance to spreading of liquid by a sharp edge, highly stable spherical cap of viscous liquid PDMS (lPDMS) on a smooth PMMA circular disk was fabricated. The axisymmetric spreading of lPDMS under the gravitational force and interfacial tension force enable the formation of spherical cap with a certain radius of curvature. A thermal treatment at 80 °C for 30 min cured the spherical cap lPDMS into a bubble-free ePDMS planoconvex lens. Lenses with focal lengths of 55.2− 3.4 mm could be reproducibly fabricated by adjusting the volume of dispensed lPDMSs and diameter of PMMA disks. Highresolution panoramic microscope images without a distortion of small cylindrical object could be constructed on-the-fly using the imbedded smartphone app. Applications of the smartphone digital microscope equipped with an ePDMS planoconvex lens for imaging of micro printings, gun shot residues, cylindrical objects, and bullet toolmarks were explored. KEYWORDS: PDMS lens, smartphone microscope, sessile drop, Gibbs inequality condition, spreading resistance and microscope lens30−32 were employed for coupling the magnified image into the optical system of the smartphone without a hardware modification. The image quality and magnification are strongly dependent on the lens and attachment design. Smartphone microscope images with resolutions of 1.2 μm by an objective lens,21 1.5 μm by a planoconvex lens,9 1.5 μm by a ball lens,24 1.1 μm by a PDMS elastomeric lens,28 and 3.9 μm by a hanging drop PDMS elastomeric lens26 were successfully acquired. Due to its optical transparency from UV to NIR regions and nonfluorescence property, optical grade liquid polydimethylsiloxane (lPDMS: Sylgard 184 from Dow Corning and RTV 615 from Bayer Silicones) has been used for elastomeric optics (lenses, mirrors, and gratings), microfluidic devices, and microelectromechanic system (MEMS) fabrications. PDMS elastomer is highly chemically resistant and inert. It can withstand dimensional alteration (compression or expansion) without losing or degrading optical properties. A thermal curing at moderate temperature (80−200 °C) enables rapid fabrication of planoconvex PDMS elastomeric lenses for

1. INTRODUCTION The worldwide availability of compact and lightweight smartphones with powerful processors, high quality displays, large volume storage, connectivity (3G, 4G, Wi-Fi, bluetooth, near field communication), ease to use apps, and state-of-theart sensors (camera, proximity sensor, magnetometer, accelerometer, gyroscope, microphone, fingerprint sensor, thermometer, and light sensor) at affordable prices make smartphones the future point-of-care devices. There were 87 million new global subscriptions of mobile phones in the third quarter of 2015 alone. At the end of 2015, the number of mobile phone subscriptions was greater than that of the global population with a greater number of smartphone subscriptions compared to that of the basic phones.1 Lab-on-phone devices that take advantage of smartphone functionalities as biosensors,2−10 spectrometer,11−14 and colorimetric sensors15−20 have been fabricated. Powerful image processing with high pixel density image sensors encouraged researchers to explore its potential as an affordable mobile digital microscope capable of capturing and sharing detailed microscopic images anywhere, anytime by attaching an external lens onto the smartphone camera. Various types of lenses including objective lens,21−23 ball lens,23,24 planoconvex lens,9 iPhone lens module,23−25 PDMS elastomeric lens,26−29 © 2016 American Chemical Society

Received: May 26, 2016 Accepted: July 15, 2016 Published: July 15, 2016 20474

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

Research Article

ACS Applied Materials & Interfaces smartphone and MEMS applications.26,28,33 Lenses capable of transforming a smartphone into a microscope were successfully fabricated by an inkjet printing,28,33 hanging drop technique,26 replica molding,34 free surface shaping,35 and dielectrophoresis force.36 Although elastomeric optics will not offer the best optical performances as those of the optical glasses, new types of optical performances and novel devices at a relatively low cost are their major advantages.35 There were several successful techniques for PDMS milli-lens and microlens fabrications,26,28,33,36−38 however, methods capable of making PDMS planoconvex lenses with precisely controlled properties and reproducibility are required. The greater social penetration of high quality smartphones at affordable prices would certainly promote nontraditional usages of smartphones. Smartphone microscopy and its attachments will become common gadgets for general public with microphotography enthusiasm while serving as point-of-care devices for researchers.8,14,39−41 This paper explores the confined sessile drop technique as a simple, cost-effective, template-free, and highly reproducible approach for fabricating elastomeric PDMS (ePDMS) planoconvex lens with controllable lens parameters (size, curvature, focal length, and magnification). We exploit the surface tension of viscous liquid PDMS (lPDMS) for automatically forming a spherical cap with precisely controlled curvature as its axisymmetric spreading was stopped at the edge of a circular disk. The low-temperature-cured PDMS spherical cap free of air bubbles became a planoconvex lens capable of transforming the smartphone imaging system into a personal mobile digital microscope. The lens parameters can be manipulated by changing the volume of dispensed lPDMS and diameter of the PMMA disk. Our single step fabrication technique eliminates the requirement of complex multistep protocols and rapid curing at high temperature employed by other techniques. Any researchers with an access to lPDMS and a plastic substrate can replicate the technique and make their own PDMS lenses without a requirement of complex instruments.

bubbles inside the lens. By precisely controlling the volume of the dispensed lPDMS using a syringe pump (NE-1000 Programmable Single Syringe Pump, New Era Pump System, Inc., U.S.A.), ePDMS planoconvex lenses with reproducible curvature, focal length, imaging area, field of view, and magnification could be fabricated. 2.2. Reproducibility of the Fabrication Process. To demonstrate a potential of the confined sessile drop technique for mass-scale production of ePDMS planoconvex lenses with high reproducibility and controllability, series of lenses were fabricated. The focal length and contact angle of the fabricated lenses were measured and statistically analyzed. The contact angles of the lens curvature were measured using the photographic capability of a standard goniometer (Model 200-F1, Ramé-Hart Instrument Co., U.S.A.). The focal length were measured using a protocol given elsewhere.28 A variation of the lens parameters would indicate the reproducibility of the fabrication technique as the uniform focal length and contact angle of lenses with the same diameter imply the same curvature. To demonstrate a capability for fabricating lenses of the same diameters with different focal lengths, series of 6 mm lenses were fabricated using 5, 10, 15, 20, 25, 30, 35, and 40 μL lPDMS. Note: a dispensed volume to 45 μL would cause an overflow of the lPDMS from the 6 mm circular PMMA disk. Additionally, to demonstrate a capability for fabricating lenses of certain diameter with the highest curvatures (i.e., shortest focal length and highest magnification), a series of lenses with diameter and volume of 2 mm (3 μL), 3 mm (6 μL), 4 mm (15 μL), 5 mm (30 μL), 6 mm (40 μL), 7 mm (60 μL), 8 mm (70 μL), 9 mm (90 μL), and 10 mm (100 μL) were fabricated. The specified volumes are the highest possible volumes of lPDMS that can be dispensed onto the corresponding PMMA disks before the overflow. 2.3. Focal Length Manipulation. To manipulate the magnification and utility of the ePDMS planoconvex lens, a 6 mm lens was glued on top of a 6 mm PDMS disk using a drop of 5 μL lPDMS as a binder. The lPDMS binder was solidified by heating on a hot plate at 150 °C for 1 min. The thickness of the PDMS disk was adjusted (1, 2, 3, and 4 mm) in order to manipulate the focal length.

3. RESULTS AND DISCUSSION 3.1. Fabrication and Testing of ePDMS Planoconvex Lenses. To explore the potential of the confined sessile drop technique for ePDMS planoconvex lens fabrication, we investigated the spreading of lPDMS droplet on flat surfaces of poly(methyl methacrylate) (PMMA) sheet, PDMS film, polycarbonate (PC) sheet, and glass slide (Figure 1). As the 15μL lPDMS droplets reached the surfaces, it spread axisymmetrically in a stick−slip manner under the influence of gravitational force while the interfacial tension force made the surface of the lPDMS atomically smooth.35 The lPDMS droplet transformed into a hemispherical cap within 1 s and a large spherical cap of low curvature within 5 s. At 10 s, the 15-μL lPDMS droplets reached spreading diameters of 5.6 mm on PMMA sheet, 5.1 mm on PDMS film, 5.3 mm on PC sheet, and 5.8 mm on glass slide. The contact angle decreased from 105° to 31° (glass), 112° to 32° (PMMA), 108° to 28° (PC), and 128° to 49° (PDMS). The spreading continued and the droplet transformed into a thin film with zero contact angle in 30 min (data not shown). The lPDMS spread faster on the solid surfaces in the order glass > PMMA > PC > PDMS. Water droplet with high interfacial tension (72.8 dyn/cm) retains its hemispherical shape on polymeric surfaces with certain contact angel governed by Young’s law. The lPDMS with low interfacial tension (20−23 dyn/cm), however, spread into a thin film with equilibrium (thermodynamic) contact angle of 0°.33,43 However, within a few seconds after discharging from syringe, one can cure the lPDMS into a working ePDMS planoconvex lens as the droplet attain an optimal curvature with near 90° contact angle. By thermally

2. EXPERIMENTAL SECTION 2.1. Fabrication of ePDMS Planoconvex Lens. lPDMS precursor (Sylgard 184, Dow Corning, U.S.A.) was prepared by mixing the PDMS base and curing agent according to the recommended weight ratio of 10:1. The homogeneous mixture was vacuumed for 30 min to de-gas and to remove solvents42 before filling it into a 10 mL syringe. A laser-cut 1 mm thick PMMA disk with a diameter of 2−10 mm was employed as a substrate for precisely controlling the diameter of the ePDMS planoconvex lens. Other reasons for using PMMA were (1) its wide availability, (2) optically smooth surface, (3) ease of fabrication into any shape, and (4) low cost. The flat PMMA sheet has a surface roughness Ra in the range of ∼2 nm (Figure S1 of the Supporting Informaiton, SI). The nanorough surface could be replicated by PDMS elastomer but does not cause an optical interference. Ones can also use other polymeric films and sheets with smooth surface as a substrate. The circular disk can be made by any means such as cutting or punching. We employed a laser cutter for a precise fabrication of a large number of circular disks with different diameters. To fabricate an ePDMS planoconvex lens, an aliquot of lPDMS (5−40 μL) was syringe-pumped onto a PMMA disk with the tip positioned at ∼10 mm above the disk to avoid an impactinduced spreading of the droplet. The lPDMS axisymmetrically spread and was confined within the perimeter of the disk. The lPDMS was left undisturbed for at least 5 min to allow the droplet to transform into a stable spherical cap before a thermal curing at 80 °C for 30 min. We slowly cure the lPDMS spherical cap to avoid a development of air 20475

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

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ACS Applied Materials & Interfaces

Figure 1. Time-dependent photographic images of 15-μL lPDMS sessile drops on flat surfaces of (A) PMMA sheet, (B) PDMS film, (C) PC sheet, and (D) glass slide (see also Figure S2). Scale bars indicate 2 mm.

curing the sessile drop within 2 s (i.e., by dispensing the droplet on to a preheated substrate), the solidified PDMS spherical cap becomes a planoconvex lens as the PDMS elastomer is transparent in the UV/visible region with a refractive index of ∼1.42.33 However, due to high viscosity and temperaturedependent curing rate of lPDMS, a rapid curing at 180−200 °C makes the fabrication of PDMS elastomeric lenses with reproducible curvature, focal length, and diameter impossible. The viscosity of the liquid PDMS increases rapidly within a short storage period, especially at high temperature and high concentration of hardener.44 Air bubbles always developed within the PDMS elastomeric lens when curing at temperature greater than 150 °C (Figure S3). A production loss due to irregular curvatures and air bubbles is common under a high temperature curing. As a result, PDMS microlenses and PDMS milli-lens were normally fabricated by controllable approaches such as lithography, thermal-reflow process, or hanging drop technique.26,37,38 However, those techniques require a complex instrument or multistep production protocol. To fabricate the ePDMS planoconvex lenses with a more efficient, controllable, and reproducible manner, we developed a confined sessile drop technique that takes advantage of the resistance to spreading by sharp edge for reproducibly fabricating spherical cap of lPDMS on a circular PMMA disk. A commercially available 1 mm thick PMMA sheet with flat surfaces was laser cut into a disk (2−10 mm in diameter) and was employed as a substrate for reproducibly making lens with controllable size and focal length. As the lPDMS droplet touched a PMMA disk, it axisymmetrically spread over the entire 3 mm PMMA disk within 2 s (Figures 2A and S4). The three phase contact line (lPDMS/Air/PMMA) radially expanded with a concomitant decreasing of contact angle. When the lPDMS reached the edge of the PMMA disk, the spreading stop with a local contact angle of θP. Unlike those in Figure 1A, where the lPDMS kept spreading and eventually became a thin film, the spherical cap in Figure 2 was stabilized without flowing over the subtended edge. The lPDMS spherical caps were highly stable as the curvature was retained without an

Figure 2. Time-dependent images of 15-μL lPDMS confined sessile drops spreading on 3 mm PMMA (see also Figure S4). The droplets were deposited at (A) 0, (B) 0.5, and (C) 1 mm from the center of the disk. Microscopic images of microprintings on the front side of a new USD100 banknote (Figure S5) acquired by a smartphone microscope equipped with the fabricated lenses under (D−F) normal photography, and (G−I) 4X digital zoom. Scale bars indicate 1 mm (A−C), 500 μm (D−F), and 100 μm (G−I).

observable change for at least 24 h. The liquid PDMS does not flow across the sharp edge of the disk as long as θP is smaller than the critical contact angle for spreading over the edge (θC) defined by the Gibbs inequality equation as follows:43,45−47 θC = (180 − ϕ) + θe

(1)

where ϕ is the subtended angle at the edge (in this case ϕ = 90°), θe is the equilibrium (thermodynamic) contact angle (in this case θe = 0°).43,47 From our experiment, the lPDMS spread 20476

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

Research Article

ACS Applied Materials & Interfaces into a thin film (θe = 0°) on PMMA sheet within 30 min. According to eq 1, θC in our system equals 90°. A unique advantage of the confined sessile drop technique was realized as we could fabricate the exact same spherical caps even though the droplets were deposited off-center of the PMMA disks (Figures 2B and 2C). The edge effect prevent the viscous liquid to flow down the subtended corner, while the interfacial tension force pulled the lPDMS to the center and form into a spherical cap within 6 s. The lPDMS spherical caps can be thermally cured (80 °C, 30 min) into ePDMS planoconvex lenses. The 3 mm ePDMS planoconvex lenses in Figure 2A−C has a focal length of 5.0 mm with a magnification of ∼50X and viewing area of 5.0 × 3.8 mm2. The magnification power is calculated according to the expression: Magnification X = 250/FL, where FL is the focal length in mm.24,48 Figure 2D−F shows images of microprinting on a new USD100 banknote (series 2009 A) acquired by an iPhone 6s plus equipped with the fabricated lenses. Details and textures of the microprinting could be clearly imaged. Moreover, by taking advantages of the digital zoom capabilities, images with a higher magnification (∼200X) could be acquired (Figure 2G−I). The high pixel density photographic capability of smartphones enables a microscopic imaging without the limitation of an electrical power supply.49 To demonstrate efficiency, reproducibility, and controllability of the confined sessile drop technique, we produced 6 mm ePDMS planoconvex lenses of various focal lengths by dispensing different volumes of lPDMS onto 6 mm PMMA disks. As shown in Figure 3, a larger dispensed volume produced lens with a shorter focal length. The focal length decreased from 34.2 mm to 13.0 mm while the contact angle increased from 22° to 95° as the dispensed volume was increased from 5 to 40 μL (Figure 3). The narrow distributions of the focal length implied a high degree of reproducibility of the fabrication technique. Due to the nonvolatile and dimensional stability nature of the lPDMS, the contact angles of the spherical caps equaled those of the lenses. The contact angle of the ePDMS planoconvex lens in Figure 3 agrees with the Gibbs inequality condition as θP increased from 22° to 95° when the dispensing volumes were increased from 5 to 40 μL. However, at a dispensing volume of 45 μL, θP is much greater than 90° and the lPDMS flow over the edge (Figure S6). The confined sessile drop of lPDMS was highly stable and self-cured into an ePDMS planoconvex lens in 24 h at room temperature (30 °C. It is possible to make PDMS elastomeric lens with contact angle slightly greater 90° by curing the lPDMS spherical cap before it flows over the edge (Figure 3I). The detailed printing of Benjamin Franklin’s eyes on the front side of a new USD100 banknote could be clearly imaged with a smartphone digital microscope (iPhone 6s Plus) equipped with the fabricated ePDMS planoconvex lenses (Figure 3B−I). We could also control focal length of the ePDMS planoconvex lens by using PMMA disk of different diameters (i.e., 2−10 mm disk with 1 mm increment). The maximum possible volume of lPDMS was dispensed on a PMMA disk to fabricate a lens with the shortest focal length. The focal length decreases as the diameter of the disk becomes shorter. The focal length of the ePDMS planoconvex lens decreases from 55.2 to 3.4 mm as the lens diameter is decreased from 10 to 2 mm (Figure 4). The narrow distributions of the focal length confirmed a high degree of reproducibility of the fabrication technique. The detailed microscopic images of George Washington’s statue at the rear of Philadelphia’s Independence

Figure 3. (A) 6 mm ePDMS planoconvex lens of various focal lengths fabricated by dispensing 5, 10, 15, 20, 25, 30, 35, and 40 μL lPDMS on 6 mm PMMA disks. The standard deviation (SD) bar of each lens set was calculated from 20 lenses. (B−I) Microscopic images of the Benjamin Franklin’s eyes on the front side of a new USD100 banknote were taken by an iPhone 6S plus coupled with the fabricated lenses. The viewing area (VA, 4:3 format) and contact angle (CA) were indicated in the images. The insert is a normal photographic image taken at a focal length of 78 mm without an ePDMS planoconvex lens.

Hall on the flip side of a new USD100 banknote can be acquired. By changing the volume of the dispensed lPDMS and the diameter of the PMMA disk, we could systematically manipulate the size, focal length, and magnification of the ePDMS planoconvex lens. A confined sessile drop on a smaller disk produces ePDMS planoconvex lens with higher curvature, shorter focal length, and higher magnification. The lenses can be efficiently fabricated with controllable and reproducible manner. Three sets of independent ePDMS elastomeric lenses with narrow distribution of contact angle and curvature confirm the reproducibility of the developed technique (Figure S7). Moreover, several sets of 100 5 mm lenses were fabricated and distributed for educational purposes (Figure S8). The ePDMS planoconvex lenses can be rapidly fabricated with a relatively low cost due to a small production lost. The material cost for making an ePDMS planoconvex lens shown in Figure 3I is ∼0.01 USD (i.e., the cost included 10% material lost). Note: a 1.1 kg Kit Sylgard 184 was offered by a local vendor at 260 USD. The total cost for making an ePDMS planoconvex lens is approximately 0.10 USD (i.e., the cost included liquid PDMS, PMMA sheet, electricity, and labor). 20477

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

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simplify the fabrication process while achieving controllability and reproducibility, we developed the lens-on-base fusing technique. A newly fabricated ePDMS planoconvex lens was glued onto a PDMS disk of desired thickness and diameter by lPDMS (∼5 μL). After a thermal curing at 150 °C for 1 min, the ePDMS lens and PDMS base fused into a single PDMS lens with base. The fused layer was transparent without air bubbles. The radius of curvature R of a planoconvex lens (Figure 5) is

Figure 5. Focal length of 6 mm PDMS elastomeric lens with base of different thickness. On the basis of the spherical cap fitting, the 6 mm lenses fabricated from a confined sessile drop of 40-μL lPDMS have a radius of curvature R of 4.0 mm.

defined by the radius of a circle with the longest arch superimposed on the envelop of the lens.51−54 The 6 mm lens fabricated from 40-μL lPDMS has a radius of curvature R of 4.0 mm and a focal length of 13.0 mm. The transparent PDMS base slightly increased the focal length of the lens from 13.0 mm (without PDMS base) to 15.0 mm (with 3 mm PDMS base). However, the focal length slightly decreased to 12.9 mm when a 4 mm thick PDMS base was employed. A thicker base is not recommended as it makes the lens too heavy and rigid. A large PDMS base is particularly beneficial for a small lens as it increases the contact area while improving the adhesion of lens with the flat surface of camera window. The PDMS base prevents lens damage and tearing during handling but insignificantly affects the focal length. 3.2. Field Applications of ePDMS Planoconvex Lenses. One particular promising application of the smartphone microscope is in the area of forensic science. Smartphone microscopy is a novel tool that can efficiently assist the screening and collection of meaningful evidence. The compact and lightweight smartphone with high-resolution display, high pixel density optical sensor, long battery life, powerful processor, ease to use apps, and broadband Internet connection make the smartphone microscope an excellent imaging device for forensic analysis both in the field and in the laboratory. The ePDMS elastomeric lens functioned as an external accessory interfacing with a smartphone in order to exploit its optical capabilities.39,55 Since the smartphone digital microscope is running on a battery, the microscopic investigation can be performed in the field without a restriction of electrical power supply. This unique advantage is very useful for samples or specimens that cannot be moved or transferred to a laboratory. Figure 6, show microscopic images of gun shot residue (GSR) on a cotton fabric generated by a point-blank shooting test. The images were taken at the shooting facility. GSRs on surfaces such as pillows, blankets, and cloths are common evidence.

Figure 4. (A) ePDMS planoconvex lenses fabricated by dispensing the highest possible volume of lPDMS on PMMA disks: (B) 2 mm (4 μL), (C) 3 mm (6 μL), (D) 4 mm (15 μL), (E) 5 mm (30 μL), (F) 6 mm (40 μL), (G) 7 mm (55 μL), (H) 8 mm (65 μL), (I) 9 mm (80 μL), and (J) 10 mm (100 μL). The SD bar of each lens set was calculated from 20 lenses. (B−J) Microscopic images of George Washington’s statue on the flip side of a new USD100 banknote were taken by an iPhone 6S plus coupled with the fabricated lenses. The viewing area (VA, square format) and the contact angle (CA) were indicated. The insert is a normal photographic image taken at a focal length of 78 mm without an ePDMS planoconvex lens.

The ePDMS planoconvex lens detached from the PMMA disk can be reattached onto the protecting window of the smartphone camera. However, a lens with a short diameter (i.e., 2−4 mm) is difficult to handle and easily fall off. One approach to making an ePDMS planoconvex lens easily handled while improving surface adhesion is to fabricate a lens with base.33,34,50 PDMS lens with base was directly fabricated on a PDMS film via an up-right approach,33 the hanging drop approach,26 and the mold replication technique.34,50 To 20478

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

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images of the entire cylinder were not in focused (Figure 7C). However, by rotating the cylinder (i.e., by our homemade device) while taking photo under a panorama app of iPhone 6s Plus, the microscopic image of the cylindrical paper strip was reconstructed on-the-fly.56−60 To trigger an image stitching, the entire image within the field of view must be moved. As a result, a smaller cylindrical object required lens with higher magnification. The microscopic panorama image was reconstructed in real-time during the rotation (Figure S9). Interestingly, the details of printed image as well as texture of the paper were efficiently captured without a detectable image distortion (Figure 7D). The high pixel density of the camera sensor enabled a graining-free digital zoom of the panorama microscope image. Although the microscope photography has a shallow depth of field (Figure 7C), the panoramic image is in focus without a detectable influence of the curved surface. The out-of-focus at the beginning and the end of the panorama image was due to the initial and final images used in the stitching algorithm.56−62 One of the challenging tasks in forensic analysis is the imaging of toolmarks on bullets. To demonstrate potential applications of panoramic microscopy images in toolmarks analysis, a 9 mm Luger bullets fired from a Glock 17 semiautomatic pistol was investigated. A full metal jacket (FMJ) bullet was fired into and retrieved from a Kevlar cast tube. The bullet was positioned on the rotating stage and imaged under the panorama app of an iPhone 6s Plus (Figure 8A). The panoramic microscopy image revealed detailed

Figure 6. (A) A photographic image of GSRs generated by a pointblank shooting on a cotton fabric (4X digital zoom). A microscopic image of a GSR taken by (B) normal photography and (C−E) 4X digital zoom. (F) A microscopic image of a clean cotton fabric. The microscope images were captured by an iPhone 6s Plus coupled with a 6 mm ePDMS planoconvex lens.

GSRs invisible to the naked eye can be clearly imaged with a high resolution microscope. In general, the whole pillow or blanket must be collected for thorough investigations in the lab. The bulky pillow form was taken out before GSR analysis. With a high magnification smartphone microscope, the GSR residues can be imaged without the removal of pillow form or the analysis can be performed on-site. 3.3. Panorama Microscope Imaging by Smartphone Microscope. In addition to high pixel density imaging, smartphones are equipped with powerful apps for video, slow motion, time-lapse, and panorama photography. Although those imbedded apps were designed for normal photography, they work equally well in the microscopic imaging. The panorama app with its powerful image-stitching algorithm can unfold the detailed structural information on curved surface of a cylindrical object. The curved surface makes image analysis of cylindrical object more challenging. To explore the potential of smartphone panorama microscopy for surface imaging of a small cylindrical object, we use paper strip (3.3 × 1.4 mm2, Figure 7A) wrapping around a 10 mm cylinder as a model (Figure 7B). Due to a shallow depth-of-field of microscopic imaging, especially that at high magnification, microscopic

Figure 8. (A) Photographic images of an FMJ bullet. The image was cropped and display by the bullet actual size. (B) Panoramic microscopy image of the bullet and (C) the corresponding microscopic images of the bullet taken by an iPhone 6S plus coupled with a 3 mm ePDMS planoconvex lens. Note: the grazing angle illumination used to enhance the toolmarks made the illuminating side highly reflective.

toolmarks on the surface of the bullet in a single photo (Figure 8B). The image did not show any sign of distortion and light reflection on the curve surface. The normal microscopic images suffered from the reflection of the curved surfaces due to the illumination at the grazing angle of incidence (Figure 8C). Exact same patterns of toolmarks were observed while detail grooves of toolmarks on bullet surface were effectively captured in the panorama microscope image. To our knowledge, this is the first report on panorama microscope imaging using smartphone. The reconstructed panoramic microscopy images were distortion free. Due to the stitching algorithm, the curve surface appeared at the beginning and the end of the panoramic image (Figures 7C and 8B). The microscopy imaging and panoramic microscopy imaging by a smartphone can be applied to other applications that benefit from microscopic imaging. The smartphone microscope could drastically decrease the cost for microscopic investigation while making the point-of-care analysis widely available

Figure 7. Actual size photographic images of a printed-paper strip as it (A) was laid on a flat surface and (B) was wrapped around a 10 mm cylinder. (C) Microscopic images of the curved paper strip and (D) the corresponding panorama microscopic image of the curved paper strip taken by an iPhone 6S plus coupled with a 3 mm lens (see Figure S9). 20479

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The mobile digital microscope equipped with ePDMS planoconvex lens exploits the state-of-the-art imaging capabilities of smartphones. The advanced features of smartphones (i.e., lightweight, high-resolution display, high density optical sensor, long battery life, powerful processor, and multiplatform wireless communication) make smartphone microscopy highly mobile. The availability of low cost smartphones with high quality cameras and advanced image processing apps makes smartphone digital microscopy more accessible and ease to use. Since smartphones run on a battery, the microscopic investigation can be conducted anywhere, anytime without a restriction of electrical power supply. The smartphone microscope could drastically decrease the cost for microscopic investigation while making the point-of-care analysis widely available

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.E.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Research University Project, Office of Higher Education Commission and Chulalongkorn University (WCU-58-008-FW).

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4. CONCLUSIONS

REFERENCES

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We have successfully developed a simple, rapid, cost-effective, and template-free technique for mass-scale production of an ePDMS planoconvex lens of controllable size and focal length. By taking advantage of the resistance to spreading of liquid by a sharp edge, a stable spherical cap lPDMS with certain curvature and contact angle on a PMMA disk could be fabricated. A thermal treatment at 80 °C for 30 min cured the lPDMS into a bubble-free ePDMS planoconvex lens capable of turning a smartphone into a portable digital microscope. Elastomeric PDMS planoconvex lenses with focal lengths of 55.2−3.4 mm and magnifications of 4.5X to 73.5X (without a digital zoom) could be reproducibly fabricated by dispensing 5−40 μL lPDMS on 2−10 mm PMMA disks. The smartphone digital microscope could image small objects invisible to the naked eye at high resolution. Applications of the smartphone digital microscope for imaging of microprinting, gun shot residue, and bullet toolmarks were demonstrated. High-resolution panoramic microscopy images without distortion of surfaces of cylindrical objects can be reconstructed on-the-fly by an imbedded app.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06305. (1) AFM images of a 1 mm thick PMMA disk. (2) Series of still images show spreading of 15-μL lPDMS on flat surfaces of PMMA, PDMS, PC, and glass. (3) PDMS elastomeric lens with air bubbles when cured at temperature greater than 150 C. (4) Series of still images show spreading of 15-μL lPDMS on 3 mm PMMA disks as the droplet was dispensed at 0.0, 0.5, and 1.0 mm from the center of the disk. (5) Position of the microprinting on a new USD 100 banknote. (6) Overflow of 45-μL lPDMS on a 6 mm PMMA disk. (7) Contact angles and curvatures of ePDMS planoconvex lens. (8) A set of 100 ePDMS planoconvex lenses (5 mm diameter). (9) A series of still images show an experiment setup for panorama microscope imaging. (10) High-resolution microscopic images of Figure 3B−I. (11) High-resolution panorama microscopic images of Figure 7D (PDF) 20480

DOI: 10.1021/acsami.6b06305 ACS Appl. Mater. Interfaces 2016, 8, 20474−20482

Research Article

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