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Stretchable Optoelectronic Circuits Embedded in a Polymer Network Dominic Ruh,* Patrick Reith, Stanislav Sherman, Michael Theodor, Johannes Ruhhammer, Andreas Seifert, and Hans Zappe* The demands of modern medicine are increasingly focused on the continuous monitoring of physiological parameters using external sensors which must be designed to provide maximum functionality with minimal limitation on patient mobility and virtually no discomfort. The measurement of parameters such as heart rate, pulse-shape, blood oxygen saturation or blood pressure in a point-of-care environment is of particular relevance. Considerable progress in this area has been made following the introduction of “electronic skin”,[1] a flexible and stretchable polymer foil incorporating electronics with which, for example, an electrocardiogram (EKG) can be measured using a sensor no more invasive or uncomfortable than an adhesive plaster. Key to the success of such a sensor is the realization of a stretchable and flexible substrate onto which simple electronic components can be integrated.[2–4] Two primary strategies have been employed to allow the fabrication of stretchable electronics. The first, termed “materials that stretch”, is based on the integration of micro- or nanosized conductive fillers into elastic polymer matrices. These conductive fillers may be, for example, single-walled carbon nanotubes,[5] silver particles bound to carbon nanotubes,[6] gold nanoparticles,[7] or graphene films,[8] while polydimethylsiloxane (PDMS) represents the most widely used elastic polymer matrix into which these are embedded. Alternatively, “structures that stretch” rely on shaping inorganic conductive materials into stretchable geometries. A variety of spring-like architectures for many different materials that allow accommodation of large strain have been designed and demonstrated, including metallic thin films defined as a meandering sequence of horseshoe-shaped bends, which are widely used for stretchable interconnect technology.[9] By enclosing the horseshoe-shaped metallic thin films in polyimide, the formation and propagation of cracks inside the
D. Ruh, P. Reith, S. Sherman, M. Theodor, Dr. A. Seifert, Prof. H. Zappe Gisela and Erwin Sick Chair of Micro-optics Department of Microsystems Engineering University of Freiburg, 79110, Freiburg, Germany E-mail: [email protected]; [email protected] J. Ruhhammer Laboratory for the Design of Microsystems Department of Microsystems Engineering University of Freiburg 79110, Freiburg, Germany
DOI: 10.1002/adma.201304447
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metallic thin films could be minimized, so that elongations of more than 250% have been demonstrated.[10] “Structures that stretch” have also been used to fabricate rudimentary flexible optoelectronic networks. Previous work has employed horseshoe-shaped, polyimide-reinforced, metallic thin-films as interconnects to establish two-dimensional stretchable and flexible light-emitting-diode (LED) arrays. The approach employed was based on an interconnect chain linked by flip-chip-bonded commercially available surface-mount LEDs[11] and is known as the CINE (Combination of Interconnects and Electronics) process. We significantly extend this concept here by miniaturizing the interconnect process to include multilevel wiring, enabling top- and bottom-side contacting of optoelectronic chips, and by integrating chip-level optoelectronic circuitry into stretchable polyimide foils which show no degradation or shift in their optical characteristics under dilation. The result is a miniaturized, flexible, and stretchable optoelectronic circuit, an “optoelectronic skin”. This optoelectronic skin includes two narrow-band light sources as well as detectors, and thus incorporates two independent optoelectronic circuits with all the mechanical degrees of freedom of the arbitrary surface, such as the epidermis, on which it is deployed. It is shown that the optoelectronic circuitry is suitable for continuous extracorporal measurement of photoplethysmograms, from which parameters such as pulse, blood oxygenation and blood pressure may be determined. The realization of such highly integrated, miniaturized, flexible, and stretchable optoelectronic circuits requires a versatile substrate technology, as is schematically outlined in Figure 1. Based on a silicone matrix as the substrate, high-density wiring is realized using polyimide-reinforced metallic thin films, whose stretchability is achieved using design rules for horseshoe-shaped interconnect.[10,12] Interconnnects are defined on multiple levels, with micrometer-sized wires defined by photolithography combined with spin-coating of subsequent polymer layers that are structured into sequences of horseshoe-shaped wires. Integration of chiplevel optoelectronics into these polyimide-reinforced stretchable interconnects is realized by mounting the optoelectronic chips onto opened contact pads inside polymeric islands between two adjacent interconnects to contact the reverse side of the optoelectronic chip, while the front contact is robustly achieved using “microflex interconnect” (MFI) technology.[13] We consider the design rules employed for flexible and stretchable interconnect; the demands on the optoelectronic components; and subsequently the circuit design of the system. The stretchability of the interconnect used in the optoelectronic skin was realized through its structure as a sequence of horseshoe-shaped
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Figure 1. Schematic representation of the miniaturized, flexible, and stretchable optoelectronic circuit. The lower left inset shows two polyimide-reinforced, parallel Pt-Au-Pt metallic thin film interconnects (ch1, ch2) separated from a lower ground (GND) wiring level by another polyimide layer. In addition to electric isolation, polyimide (Pi) reduces crack formation and propagation inside the metal. The upper right inset shows the integration of chip-level optoelectronics using a microflex connection.
bends, as is seen in the diagrams of Figure 1 and 2. The degrees of freedom in the design are the radius of curvature r; the opening angle θ; and the width of the polyimide surrounding the Pt-AuPt layer w. Based on the considerations of previous work in this area,[14,15] the ratio of radius of curvature to total track width was chosen to be r/w ≥ 7.5; the width of the surrounding polyimide layer to be equal to the metallic wiring width; and the opening angle to be θ = 30°, as a trade-off between plastic strain inside the conducting path and its electrical resistance. Since the horseshoe-shaped wiring accommodates stress inplane by out-of-plane motion, the Young’s modulus and thickness of the encapsulating PDMS (i.e., the polymer substrate material) should be as low as possible to allow the structure to buckle in all directions.[14] As a result, silicone RTV 23 (Altropol Kunstoff GmbH, Germany), with a Young's modulus of 60 kPa and a thickness of 1 mm, was chosen as a substrate material. A summary of typical wiring dimensions used in various horseshoe configurations is given in Table 1. The optoelectronic circuit realized on the stretchable substrate, shown in Figure 2, consisted of two spectrally distinct LEDs and the corresponding spectrally-selective photodiodes along with transimpedance amplifiers (Texas Instruments OPA140) for signal conditioning. The LEDs (ELC-650-24 and ELC-875-22, from Jenoptik AG, Germany) emit at wavelengths of λred = 655 ± 10 nm and λir = 875 ± 15 nm and have chip sizes
Figure 2. Layout of the two-channel optoelectronic circuit: channels Ch1 and Ch5 drive the LEDs; Ch2 and Ch4 connect the outputs of the photodiodes to their transimpedance amplifiers. All the chips use a common ground (GND, blue) routed through a separate layer at the bottom of the substrate.
of 310 μm × 310 μm × 250 μm and 650 μm × 650 μm × 180 μm, respectively. The corresponding photodetectors (EPC-660-0.9-1 and EPC880-0.9-1, also Jenoptik AG) show a spectrally-limited sensitivity of λ1 = 660 ± 40 nm and λ2 = 880 ± 58 nm, with peak sensitivities of s1 = 0.45 A/W and s2 = 0.55 A/W and chip sizes of 860 μm × 860 μm × 260 μm and 860 μm × 860 μm × 300 μm, respectively. The nonzero sensitivity in the red wavelength range of the infrared photodetector causes a maximum theoretical crosstalk of 0.72% and the maximum theoretical crosstalk for the infrared channel is below 0.04%. As the circuit layout of Figure 2 shows, the two photodetectors and the two LEDs share a common ground (GND, blue), while the currents for the LEDs can be set individually using the wires labeled Ch1 and Ch5. The photocurrents generated by the two photodetectors are directed to two transimpedance amplifiers via the wires labelled Ch2 and Ch4. A high level of integration is achieved by routing the different channels in parallel as well as through the use of two separate signal and ground layers. For electrically bonding the top surfaces of the optoelectronic chips, MFI technology[13] is used to replace fragile wire bonds with small flexible polymeric cables that route the top contact of the chips to the substrate. The fabrication of the optoelectronic skin is summarized in Figure 3. For generation of the polyimide support for the metallic thin film interconnect, a 4 μm-thick polyimide (U-VARNISH-S-UBE) layer is spin-coated onto a polished
Table 1. Design parameters for various horseshoe interconnect configurations. Radius of curvature, r [μm] 562.5
Metal track width, wtr [μm]
Polyimide width, wpi [μm]
Opening angle, θ [°]
RTV 23 thickness, te [mm]
25
25
30
1
1125
50
50
30
1
2250
100
100
30
1
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Figure 3. Fabrication process of the optoelectronic skin: a) spin-coating of the polyimide; b) lift-off structuring of the Pt-Au-Pt metallization; c) second spin-coating step of the polyimide; d) RIE structuring and opening of the contact pads; e) chip assembly including MFI steps for contacting the topsides of the optoelectronic chips; f) casting in PDMS and epoxy encapsulation of the optoelectronic chips.
silicon handle-wafer at 4500 rpm for 30 s followed by a solvent (n-methyl-2-pyroridon) evaporation step at 120 °C for 3 min and curing for 10 min at 450 °C in nitrogen atmosphere to avoid bubble formation and delamination (Figure 3a). Wiring is defined using a 7 μm-thick layer of negative photoresist AZ nLOF 2070 (MicroChemicals GmbH) spun onto the cured polyimide for 30 s at 3000 rpm and soft-baked at 100 °C for 7 min. After i-line exposure through a foil mask at 4.5 mW cm-2 for 90 s and a 1 min-long post-exposure-bake at 110 °C, development in AZ 726 (MIF) (MicroChemicals GmbH) is performed for 3.5 min. Subsequently, a Pt-Au-Pt layer stack with film thicknesses of tPt = 50 nm, tAu = 400 nm and tPt = 200 nm is evaporated (Figure 3b) and structured by lift-off in acetone. Platinum serves as adhesion promoter to polyimide and as an etch stop for later processes, whereas Au provides high electrical conductivity. Second and third polyimide layers are deposited by subsequent spin-coating steps (Figure 3c). These are structured by reactive ion etching (RIE) using a 28 μm thick layer of AZ 9260 photoresist deposited in a two-step spin-coating process followed by multiple exposure. At the same time, the substrates as a whole are patterned into individual stripes and a single-oxygen RIE step (200 W, 50 sccm, 30 mTorr) is used to open the contact pads and separate the substrates (Figure 3d). After removing the remaining resist in acetone and ultrasonic cleaning, the thus diced substrates can be easily peeled off the handle wafer by tweezers. The optoelectronic chips are then positioned on the substrate using a die-bonder and attached using a conductive epoxy glue based on Araldite 2020 (Huntsman Advanced Materials) onto the contact pads opened using RIE. The top side contact is routed to the polyimide substrate using MFI technique (Figure 3e) in which a small polyimide strip containing a Pt-AuPt wire, forming a loop of 60 μm diameter, is cut out of the contact pad for the backside contact and bent upwards using tweezers and fixed to a gold bump deposited by a ball-wedge bonder. A biocompatible, optical epoxy adhesive (EPO-TEK 302) is used to encapsulate the optoelectonic chips and a final spin-coating process at 600 rpm for 30 s embeds the entire stretchable optoelectronic circuit in a 1 mm thick RTV 23 PDMS layer (Figure 3f). The thus-fabricated optoelectronic skin was characterized with respect to electrical and optical behavior as a function of externally applied strain. The measurement setup employed
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a linear stage (PI M-413.32S, Physik Instrumente) to stretch the samples and a digital multimeter to implement four point resistance measurements (Model 27000, Keithley Instruments). The optical characterization of the optoelectronic components used an optical multimeter (ILX Lightwave OMM6810B/ OMH6722B) to measure the total optical power and an optical spectrum analyzer (Ando AQ-6315A) to investigate spectral variations. A laser diode controller (ILX Lightwave LDC-3724B) was used to drive the LEDs under test. The resistance of the stretchable interconnect was first characterized in its unstretched state to elucidate process variations among different samples and to compare the measured resistances with theoretically expected values. The theoretical sheet resistance is RB = 53.3 mΩ, as calculated using using film thicknesses given in the fabrication section (see Figure 3b) and the bulk resistivity ρPt = 1.06 × 10−7 Ωm and ρAu = 2.44 × 10−8 Ωm.[16] The difference between the theoretical (RB) and the measured (R ) sheet resistance averaged over all fabricated samples is (R − RB)/RB = 162 ± 14%. As expected, the evaporated thin-film sheet resistance is higher than the estimated bulk sheet resistance, while the small standard deviation indicates a stable process with a small process variability. In the stretched state, the normalized resistance of the stretchable interconnects becomes a function of the applied strain. The behavior of the normalized resistance of the stretchable interconnects was investigated in two different experiments: the first examined the variation in electrical resistance as the substrate is stretched to catastrophic failure of the interconnect (i.e., rupture of the wiring) and the second evaluated the variation in electrical resistance by repetitively switching between two strain levels, thereby examining the drift in resistance in an accelerated lifetime test. For the first test, the conducting stripes were stretched in discrete steps of Δε = 2.5% until the wires ruptured. It was seen that at around 40% strain the silicone begins to delaminate from the polymide support. Whereas this delamination does not affect the electrical performance, it is a failure mechanism. As strain was increased further, the horseshoe shaped interconnect ruptured at the transition between the stretchable bend and the straight connector. The maximum strain which could be applied before this occurred was almost 100%, at which point the resistance changed by ΔR = 1.14%. Since human skin, on which these substrates are to be placed as sensors, is
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well-known that variations in drive current cause spectral shifts in p-n junctions due to variations in chip temperature, using a constant current source as is done here would result in greater spectral stability. The good stability of the electrical and optical behavior of the components integrated into the flexible foil implies that this circuitry is suitable for use in extracorporal sensing, for example in photoplethysmography. This technique relies on the change in optical transmission or reflection from blood vessels due to the pulse in the cardiovascular system.[17] Using a single wavelength allows the determination of the pulse and the characteristics of the pulse wave; two spatially separated sensors may be used to measure Figure 4. Optical spectrum of the infrared LED (λε,ir = 876 nm for four strain states showing blood pressure. In addition, two or more virtually no spectral shift with applied strain to the substrate, see right inset. The left inset wavelengths may be employed to measure shows optical power Popt and voltage V as function of drive current I. blood composition, including oxygenation and hemoglobin concentration.[17–19] To demgenerally subject to strains smaller than 10%, the technology is onstrate this functionality, the stretchable optoelectronic skin suitable for conforming to the movement of the host tissue in incorporating two LEDs and the corresponding photodetectors medical applications. on an approximately 4 cm × 2 cm polyimide foil embedded in In the second experiment, the long-term resistance/strain RTV 23 (see Figure 2), was applied externally to the epidermis behavior of the stretchable interconnect was investigated. Since for a photoplethysmographic measurement. The photoplethysstrains up to 10% are of interest for the applications considered mograms were recorded by driving the LEDs with a laser diode here, long-term stability was measured by operating the LEDs controller (ILX Lightwave LDC-3724B), measuring the photoand varying the strain cyclically 105 times between 10% and currents through transimpedance amplifiers (Texas Instru25%. It was seen that the value of the resistance averaged over ments OPA140) and finally performing analog to digital conall cycles remained virtually constant at R10% = 153.22 ± 0.01 Ω version with a data acquisition card (National Instruments NI and R25% = 153.31 ± 0.01 Ω. Since both resistances differ only USB-6251 BNC). The recorded photoplethysmograms in the slightly and the resistance values themselves showed only a red and infrared spectral ranges, shown in Figure 5, clearly small variation over all 100 000 cycles, we believe that these show the relevant features of the pulse, including heart rate, stretchable interconnects are suitable for driving optoelecsystole, diastole, and the pulse wave reflection. tronics over longer periods of time. The advantage of this system over standard methods, of The optomechanical behavior was evaluated by measurement which the finger clip is the most popular, is quite clearly in its of the output spectrum of the LEDs. Previous work has shown size and ability to conform to the shape and movement of the that the spectral characteristics of LEDs in a SMD package attached to a flexible substrate can shift significantly when subject to a strain of ε = 67%.[11] Measurements of this characteristic on components using the new technology presented here were undertaken in the red and infrared wavelength ranges in the unstrained and in three different strained states, 10%, 50% and 75%. In contrast to earlier work, spectral measurements on both LEDs do not show a measurable spectral shift at different strain states, as is clearly seen in Figure 4. The observed spectral peaks averaged over all four strains were λε,red = 688.44 ± Figure 5. Photoplethysmograms in the red and the infrared spectral ranges, recorded using 0.29 nm and λε,ir = 876.25 ± 0.23 nm. One possible explanation for the greatly the stretchable sensor circuit attached conformally to the epidermis. a) The photoplethysmoenhanced stability seen here may be that graphic sensor shown wrapped conformally around a finger. b) Photoplethysmogram recorded in the red spectral range; the red dots show the unfiltered photoplethysmogram, while the black the technology presented results in comcurve results from low-pass filtering at 3 Hz. c) The filtered and unfiltered photoplethysmoplete support of the optoelectronic chip with grams measured in the infrared spectral range. Both photoplethysmograms (b and c) clearly polyimide, thereby reducing stress on the show the characteristics of a photoplethysmogram, the heart rate, the systole, the diastole, and semiconductor chip. In addition, since it is the pulse wave reflection.
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skin. Optoelectronic skin allows continuous measurement of parameters using a sensor whose mechanical properties make it no less uncomfortable than a plaster, such that long-term, continuous medical diagnostics with no impediment to the patient are possible. In conclusion, we have shown for the first time that chiplevel optoelectronics can be assembled onto flexible and stretchable foils with no reduction in performance and interconnected with stretchable wiring of high robustness. The electrical and spectral stability of the components under repeated strain is sufficiently high to allow medically relevant measurements, and the mechanical properties are such that this circuitry may be conformally applied to arbitrary surfaces, even those subject to movement and stretching.
Acknowledgements The authors thank Dr. Juan Ordonez and Dr. Martin Schuettler of the Laboratory of Biomedical Microtechnology for their help with microfabrication. Received: September 4, 2013 Revised: September 25, 2013 Published online: November 27, 2013
[1] D.-H. Kim, N. Lu, R. Ma, Y.-S. Kim, R.-H. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J. Yu, T.-i. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H.-J. Chung, H. Keum, M. McCormick, P. Liu, Y.-W. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, J. A. Rogers, Science 2011, 333, 838–843. [2] J. A. Rogers, T. Someya, Y. Huang, Science 2010, 327, 1603–1607.
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[3] D.-H. Kim, N. Lu, Y. Huang, J. A. Rogers, MRS Bull. 2012, 37, 226–235. [4] D.-H. Kim, R. Ghaffari, N. Lu, J. A. Rogers, Annu. Rev. Biomed. Eng. 2012, 14, 113–128. [5] N. Lu, C. Lu, S. Yang, J. Rogers, Adv. Funct. Mater. 2012, 22, 4044–4050. [6] K.-Y. Chun, Y. Oh, J. Rho, J.-h. Ahn, Y.-j. Kim, H. R. Choi, S. Baik, Nat. Nanotechnol. 2010, 5, 853–857. [7] Y. Kim, J. Zhu, B. Yeom, M. Di Prima, X. Su, J.-G. Kim, S. J. Yoo, C. Uher, N. A. Kotov, Nature 2013, 500, 59–63. [8] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.-M. Cheng, Nat. Mater. 2011, 10, 424–432. [9] D. H. Kim, J. Xiao, J. Song, Y. Huang, J. A. Rogers, Adv. Mater. 2010, 22, 2108–2124. [10] Y.-y. Hsu, M. Gonzalez, F. Bossuyt, J. Vanfleteren, I. De Wolf, IEEE Trans. Electron. Devices 2011, 58, 2680–2688. [11] X. Hu, P. Krull, B. de Graff, K. Dowling, J. Rogers, W. J. Arora, Adv. Mater. 2011, 23, 2933–2939. [12] M. Gonzalez, F. Axisa, M. V. Bulcke, D. Brosteaux, B. Vandevelde, J. Vanfleteren, Microelectron. Reliability 2008, 48, 825–832. [13] T. Stieglitz, H. Beutel, J. Meyer, J. Intelligent Mater. Syst. Structures 2000, 11, 417–425. [14] M. Gonzalez, F. Axisa, F. Bossuyt, Y.-Y. Hsu, B. Vandevelde, J. Vanfleteren, Circuit World 2009, 35, 22–29. [15] R. Verplancke, F. Bossuyt, D. Cuypers, J. Vanfleteren, J. Micromech. Microeng. 2012, 22, 015002. [16] R. A. Serway, Principles of Physics, 2nd ed., Saunders College Publishing, Philadelphia, USA 1998. [17] J. T. B. Moyle, Pulse Oximetry, BMJ Books, London, UK 2002. [18] M. Theodor, D. Ruh, J. Fiala, K. Förster, C. Heilmann, Y. Manoli, F. Beyersdorf, H. Zappe, A. Seifert, Biomed. Microdevices 2013, 15, 1–10. [19] J. Fiala, P. Bingger, D. Ruh, K. Foerster, C. Heilmann, F. Beyersdorf, H. Zappe, A. Seifert, Biomed. Microdevices 2013, 15, 73–81.
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Stretchable Optoelectronic Circuits Embedded in a Polymer Network Dominic Ruh,* Patrick Reith, Stanislav Sherman, Michael Theodor, Johannes Ruhhammer, Andreas Seifert, and Hans Zappe* The demands of modern medicine are increasingly focused on the continuous monitoring of physiological parameters using external sensors which must be designed to provide maximum functionality with minimal limitation on patient mobility and virtually no discomfort. The measurement of parameters such as heart rate, pulse-shape, blood oxygen saturation or blood pressure in a point-of-care environment is of particular relevance. Considerable progress in this area has been made following the introduction of “electronic skin”,[1] a flexible and stretchable polymer foil incorporating electronics with which, for example, an electrocardiogram (EKG) can be measured using a sensor no more invasive or uncomfortable than an adhesive plaster. Key to the success of such a sensor is the realization of a stretchable and flexible substrate onto which simple electronic components can be integrated.[2–4] Two primary strategies have been employed to allow the fabrication of stretchable electronics. The first, termed “materials that stretch”, is based on the integration of micro- or nanosized conductive fillers into elastic polymer matrices. These conductive fillers may be, for example, single-walled carbon nanotubes,[5] silver particles bound to carbon nanotubes,[6] gold nanoparticles,[7] or graphene films,[8] while polydimethylsiloxane (PDMS) represents the most widely used elastic polymer matrix into which these are embedded. Alternatively, “structures that stretch” rely on shaping inorganic conductive materials into stretchable geometries. A variety of spring-like architectures for many different materials that allow accommodation of large strain have been designed and demonstrated, including metallic thin films defined as a meandering sequence of horseshoe-shaped bends, which are widely used for stretchable interconnect technology.[9] By enclosing the horseshoe-shaped metallic thin films in polyimide, the formation and propagation of cracks inside the
D. Ruh, P. Reith, S. Sherman, M. Theodor, Dr. A. Seifert, Prof. H. Zappe Gisela and Erwin Sick Chair of Micro-optics Department of Microsystems Engineering University of Freiburg, 79110, Freiburg, Germany E-mail: [email protected]; [email protected] J. Ruhhammer Laboratory for the Design of Microsystems Department of Microsystems Engineering University of Freiburg 79110, Freiburg, Germany
DOI: 10.1002/adma.201304447
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metallic thin films could be minimized, so that elongations of more than 250% have been demonstrated.[10] “Structures that stretch” have also been used to fabricate rudimentary flexible optoelectronic networks. Previous work has employed horseshoe-shaped, polyimide-reinforced, metallic thin-films as interconnects to establish two-dimensional stretchable and flexible light-emitting-diode (LED) arrays. The approach employed was based on an interconnect chain linked by flip-chip-bonded commercially available surface-mount LEDs[11] and is known as the CINE (Combination of Interconnects and Electronics) process. We significantly extend this concept here by miniaturizing the interconnect process to include multilevel wiring, enabling top- and bottom-side contacting of optoelectronic chips, and by integrating chip-level optoelectronic circuitry into stretchable polyimide foils which show no degradation or shift in their optical characteristics under dilation. The result is a miniaturized, flexible, and stretchable optoelectronic circuit, an “optoelectronic skin”. This optoelectronic skin includes two narrow-band light sources as well as detectors, and thus incorporates two independent optoelectronic circuits with all the mechanical degrees of freedom of the arbitrary surface, such as the epidermis, on which it is deployed. It is shown that the optoelectronic circuitry is suitable for continuous extracorporal measurement of photoplethysmograms, from which parameters such as pulse, blood oxygenation and blood pressure may be determined. The realization of such highly integrated, miniaturized, flexible, and stretchable optoelectronic circuits requires a versatile substrate technology, as is schematically outlined in Figure 1. Based on a silicone matrix as the substrate, high-density wiring is realized using polyimide-reinforced metallic thin films, whose stretchability is achieved using design rules for horseshoe-shaped interconnect.[10,12] Interconnnects are defined on multiple levels, with micrometer-sized wires defined by photolithography combined with spin-coating of subsequent polymer layers that are structured into sequences of horseshoe-shaped wires. Integration of chiplevel optoelectronics into these polyimide-reinforced stretchable interconnects is realized by mounting the optoelectronic chips onto opened contact pads inside polymeric islands between two adjacent interconnects to contact the reverse side of the optoelectronic chip, while the front contact is robustly achieved using “microflex interconnect” (MFI) technology.[13] We consider the design rules employed for flexible and stretchable interconnect; the demands on the optoelectronic components; and subsequently the circuit design of the system. The stretchability of the interconnect used in the optoelectronic skin was realized through its structure as a sequence of horseshoe-shaped
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Figure 1. Schematic representation of the miniaturized, flexible, and stretchable optoelectronic circuit. The lower left inset shows two polyimide-reinforced, parallel Pt-Au-Pt metallic thin film interconnects (ch1, ch2) separated from a lower ground (GND) wiring level by another polyimide layer. In addition to electric isolation, polyimide (Pi) reduces crack formation and propagation inside the metal. The upper right inset shows the integration of chip-level optoelectronics using a microflex connection.
bends, as is seen in the diagrams of Figure 1 and 2. The degrees of freedom in the design are the radius of curvature r; the opening angle θ; and the width of the polyimide surrounding the Pt-AuPt layer w. Based on the considerations of previous work in this area,[14,15] the ratio of radius of curvature to total track width was chosen to be r/w ≥ 7.5; the width of the surrounding polyimide layer to be equal to the metallic wiring width; and the opening angle to be θ = 30°, as a trade-off between plastic strain inside the conducting path and its electrical resistance. Since the horseshoe-shaped wiring accommodates stress inplane by out-of-plane motion, the Young’s modulus and thickness of the encapsulating PDMS (i.e., the polymer substrate material) should be as low as possible to allow the structure to buckle in all directions.[14] As a result, silicone RTV 23 (Altropol Kunstoff GmbH, Germany), with a Young's modulus of 60 kPa and a thickness of 1 mm, was chosen as a substrate material. A summary of typical wiring dimensions used in various horseshoe configurations is given in Table 1. The optoelectronic circuit realized on the stretchable substrate, shown in Figure 2, consisted of two spectrally distinct LEDs and the corresponding spectrally-selective photodiodes along with transimpedance amplifiers (Texas Instruments OPA140) for signal conditioning. The LEDs (ELC-650-24 and ELC-875-22, from Jenoptik AG, Germany) emit at wavelengths of λred = 655 ± 10 nm and λir = 875 ± 15 nm and have chip sizes
Figure 2. Layout of the two-channel optoelectronic circuit: channels Ch1 and Ch5 drive the LEDs; Ch2 and Ch4 connect the outputs of the photodiodes to their transimpedance amplifiers. All the chips use a common ground (GND, blue) routed through a separate layer at the bottom of the substrate.
of 310 μm × 310 μm × 250 μm and 650 μm × 650 μm × 180 μm, respectively. The corresponding photodetectors (EPC-660-0.9-1 and EPC880-0.9-1, also Jenoptik AG) show a spectrally-limited sensitivity of λ1 = 660 ± 40 nm and λ2 = 880 ± 58 nm, with peak sensitivities of s1 = 0.45 A/W and s2 = 0.55 A/W and chip sizes of 860 μm × 860 μm × 260 μm and 860 μm × 860 μm × 300 μm, respectively. The nonzero sensitivity in the red wavelength range of the infrared photodetector causes a maximum theoretical crosstalk of 0.72% and the maximum theoretical crosstalk for the infrared channel is below 0.04%. As the circuit layout of Figure 2 shows, the two photodetectors and the two LEDs share a common ground (GND, blue), while the currents for the LEDs can be set individually using the wires labeled Ch1 and Ch5. The photocurrents generated by the two photodetectors are directed to two transimpedance amplifiers via the wires labelled Ch2 and Ch4. A high level of integration is achieved by routing the different channels in parallel as well as through the use of two separate signal and ground layers. For electrically bonding the top surfaces of the optoelectronic chips, MFI technology[13] is used to replace fragile wire bonds with small flexible polymeric cables that route the top contact of the chips to the substrate. The fabrication of the optoelectronic skin is summarized in Figure 3. For generation of the polyimide support for the metallic thin film interconnect, a 4 μm-thick polyimide (U-VARNISH-S-UBE) layer is spin-coated onto a polished
Table 1. Design parameters for various horseshoe interconnect configurations. Radius of curvature, r [μm] 562.5
Metal track width, wtr [μm]
Polyimide width, wpi [μm]
Opening angle, θ [°]
RTV 23 thickness, te [mm]
25
25
30
1
1125
50
50
30
1
2250
100
100
30
1
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Figure 3. Fabrication process of the optoelectronic skin: a) spin-coating of the polyimide; b) lift-off structuring of the Pt-Au-Pt metallization; c) second spin-coating step of the polyimide; d) RIE structuring and opening of the contact pads; e) chip assembly including MFI steps for contacting the topsides of the optoelectronic chips; f) casting in PDMS and epoxy encapsulation of the optoelectronic chips.
silicon handle-wafer at 4500 rpm for 30 s followed by a solvent (n-methyl-2-pyroridon) evaporation step at 120 °C for 3 min and curing for 10 min at 450 °C in nitrogen atmosphere to avoid bubble formation and delamination (Figure 3a). Wiring is defined using a 7 μm-thick layer of negative photoresist AZ nLOF 2070 (MicroChemicals GmbH) spun onto the cured polyimide for 30 s at 3000 rpm and soft-baked at 100 °C for 7 min. After i-line exposure through a foil mask at 4.5 mW cm-2 for 90 s and a 1 min-long post-exposure-bake at 110 °C, development in AZ 726 (MIF) (MicroChemicals GmbH) is performed for 3.5 min. Subsequently, a Pt-Au-Pt layer stack with film thicknesses of tPt = 50 nm, tAu = 400 nm and tPt = 200 nm is evaporated (Figure 3b) and structured by lift-off in acetone. Platinum serves as adhesion promoter to polyimide and as an etch stop for later processes, whereas Au provides high electrical conductivity. Second and third polyimide layers are deposited by subsequent spin-coating steps (Figure 3c). These are structured by reactive ion etching (RIE) using a 28 μm thick layer of AZ 9260 photoresist deposited in a two-step spin-coating process followed by multiple exposure. At the same time, the substrates as a whole are patterned into individual stripes and a single-oxygen RIE step (200 W, 50 sccm, 30 mTorr) is used to open the contact pads and separate the substrates (Figure 3d). After removing the remaining resist in acetone and ultrasonic cleaning, the thus diced substrates can be easily peeled off the handle wafer by tweezers. The optoelectronic chips are then positioned on the substrate using a die-bonder and attached using a conductive epoxy glue based on Araldite 2020 (Huntsman Advanced Materials) onto the contact pads opened using RIE. The top side contact is routed to the polyimide substrate using MFI technique (Figure 3e) in which a small polyimide strip containing a Pt-AuPt wire, forming a loop of 60 μm diameter, is cut out of the contact pad for the backside contact and bent upwards using tweezers and fixed to a gold bump deposited by a ball-wedge bonder. A biocompatible, optical epoxy adhesive (EPO-TEK 302) is used to encapsulate the optoelectonic chips and a final spin-coating process at 600 rpm for 30 s embeds the entire stretchable optoelectronic circuit in a 1 mm thick RTV 23 PDMS layer (Figure 3f). The thus-fabricated optoelectronic skin was characterized with respect to electrical and optical behavior as a function of externally applied strain. The measurement setup employed
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a linear stage (PI M-413.32S, Physik Instrumente) to stretch the samples and a digital multimeter to implement four point resistance measurements (Model 27000, Keithley Instruments). The optical characterization of the optoelectronic components used an optical multimeter (ILX Lightwave OMM6810B/ OMH6722B) to measure the total optical power and an optical spectrum analyzer (Ando AQ-6315A) to investigate spectral variations. A laser diode controller (ILX Lightwave LDC-3724B) was used to drive the LEDs under test. The resistance of the stretchable interconnect was first characterized in its unstretched state to elucidate process variations among different samples and to compare the measured resistances with theoretically expected values. The theoretical sheet resistance is RB = 53.3 mΩ, as calculated using using film thicknesses given in the fabrication section (see Figure 3b) and the bulk resistivity ρPt = 1.06 × 10−7 Ωm and ρAu = 2.44 × 10−8 Ωm.[16] The difference between the theoretical (RB) and the measured (R ) sheet resistance averaged over all fabricated samples is (R − RB)/RB = 162 ± 14%. As expected, the evaporated thin-film sheet resistance is higher than the estimated bulk sheet resistance, while the small standard deviation indicates a stable process with a small process variability. In the stretched state, the normalized resistance of the stretchable interconnects becomes a function of the applied strain. The behavior of the normalized resistance of the stretchable interconnects was investigated in two different experiments: the first examined the variation in electrical resistance as the substrate is stretched to catastrophic failure of the interconnect (i.e., rupture of the wiring) and the second evaluated the variation in electrical resistance by repetitively switching between two strain levels, thereby examining the drift in resistance in an accelerated lifetime test. For the first test, the conducting stripes were stretched in discrete steps of Δε = 2.5% until the wires ruptured. It was seen that at around 40% strain the silicone begins to delaminate from the polymide support. Whereas this delamination does not affect the electrical performance, it is a failure mechanism. As strain was increased further, the horseshoe shaped interconnect ruptured at the transition between the stretchable bend and the straight connector. The maximum strain which could be applied before this occurred was almost 100%, at which point the resistance changed by ΔR = 1.14%. Since human skin, on which these substrates are to be placed as sensors, is
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well-known that variations in drive current cause spectral shifts in p-n junctions due to variations in chip temperature, using a constant current source as is done here would result in greater spectral stability. The good stability of the electrical and optical behavior of the components integrated into the flexible foil implies that this circuitry is suitable for use in extracorporal sensing, for example in photoplethysmography. This technique relies on the change in optical transmission or reflection from blood vessels due to the pulse in the cardiovascular system.[17] Using a single wavelength allows the determination of the pulse and the characteristics of the pulse wave; two spatially separated sensors may be used to measure Figure 4. Optical spectrum of the infrared LED (λε,ir = 876 nm for four strain states showing blood pressure. In addition, two or more virtually no spectral shift with applied strain to the substrate, see right inset. The left inset wavelengths may be employed to measure shows optical power Popt and voltage V as function of drive current I. blood composition, including oxygenation and hemoglobin concentration.[17–19] To demgenerally subject to strains smaller than 10%, the technology is onstrate this functionality, the stretchable optoelectronic skin suitable for conforming to the movement of the host tissue in incorporating two LEDs and the corresponding photodetectors medical applications. on an approximately 4 cm × 2 cm polyimide foil embedded in In the second experiment, the long-term resistance/strain RTV 23 (see Figure 2), was applied externally to the epidermis behavior of the stretchable interconnect was investigated. Since for a photoplethysmographic measurement. The photoplethysstrains up to 10% are of interest for the applications considered mograms were recorded by driving the LEDs with a laser diode here, long-term stability was measured by operating the LEDs controller (ILX Lightwave LDC-3724B), measuring the photoand varying the strain cyclically 105 times between 10% and currents through transimpedance amplifiers (Texas Instru25%. It was seen that the value of the resistance averaged over ments OPA140) and finally performing analog to digital conall cycles remained virtually constant at R10% = 153.22 ± 0.01 Ω version with a data acquisition card (National Instruments NI and R25% = 153.31 ± 0.01 Ω. Since both resistances differ only USB-6251 BNC). The recorded photoplethysmograms in the slightly and the resistance values themselves showed only a red and infrared spectral ranges, shown in Figure 5, clearly small variation over all 100 000 cycles, we believe that these show the relevant features of the pulse, including heart rate, stretchable interconnects are suitable for driving optoelecsystole, diastole, and the pulse wave reflection. tronics over longer periods of time. The advantage of this system over standard methods, of The optomechanical behavior was evaluated by measurement which the finger clip is the most popular, is quite clearly in its of the output spectrum of the LEDs. Previous work has shown size and ability to conform to the shape and movement of the that the spectral characteristics of LEDs in a SMD package attached to a flexible substrate can shift significantly when subject to a strain of ε = 67%.[11] Measurements of this characteristic on components using the new technology presented here were undertaken in the red and infrared wavelength ranges in the unstrained and in three different strained states, 10%, 50% and 75%. In contrast to earlier work, spectral measurements on both LEDs do not show a measurable spectral shift at different strain states, as is clearly seen in Figure 4. The observed spectral peaks averaged over all four strains were λε,red = 688.44 ± Figure 5. Photoplethysmograms in the red and the infrared spectral ranges, recorded using 0.29 nm and λε,ir = 876.25 ± 0.23 nm. One possible explanation for the greatly the stretchable sensor circuit attached conformally to the epidermis. a) The photoplethysmoenhanced stability seen here may be that graphic sensor shown wrapped conformally around a finger. b) Photoplethysmogram recorded in the red spectral range; the red dots show the unfiltered photoplethysmogram, while the black the technology presented results in comcurve results from low-pass filtering at 3 Hz. c) The filtered and unfiltered photoplethysmoplete support of the optoelectronic chip with grams measured in the infrared spectral range. Both photoplethysmograms (b and c) clearly polyimide, thereby reducing stress on the show the characteristics of a photoplethysmogram, the heart rate, the systole, the diastole, and semiconductor chip. In addition, since it is the pulse wave reflection.
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skin. Optoelectronic skin allows continuous measurement of parameters using a sensor whose mechanical properties make it no less uncomfortable than a plaster, such that long-term, continuous medical diagnostics with no impediment to the patient are possible. In conclusion, we have shown for the first time that chiplevel optoelectronics can be assembled onto flexible and stretchable foils with no reduction in performance and interconnected with stretchable wiring of high robustness. The electrical and spectral stability of the components under repeated strain is sufficiently high to allow medically relevant measurements, and the mechanical properties are such that this circuitry may be conformally applied to arbitrary surfaces, even those subject to movement and stretching.
Acknowledgements The authors thank Dr. Juan Ordonez and Dr. Martin Schuettler of the Laboratory of Biomedical Microtechnology for their help with microfabrication. Received: September 4, 2013 Revised: September 25, 2013 Published online: November 27, 2013
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