Biomed Microdevices (2015)7: DOI 10.1007/s10544-015-9979-0
Eyeglasses-powered, contact lens-like platform with high power transfer efficiency Young-Joon Kim 1 & Jimin Maeng 2 & Pedro P. Irazoqui 2
# Springer Science+Business Media New York 2015
Abstract We present a contact lens-like platform that is wirelessly powered by an external coil embedded in eyeglasses via magnetic resonance coupling at 13.56 MHz. The platform is composed of a transparent parylene film as a host substrate, an embedded spiral inductor as a power receiving coil, and metal interconnects for additional electronics. A multilayer thin-film parylene packaging process is used to meet the form factor of a contact lens. A 36 μm-thick metal plating technique is employed on a parylene film to enhance the quality factor (Q) of the receiving coil (Q=27.3 at 13.56 MHz). The power transfer method and techniques to compensate for coil misalignment are demonstrated on a pig eye, achieving a power transfer efficiency of 17.5 % at a 20-mm powering distance. The effect of tissue on the coil and the power transfer efficiency is examined. The high power transfer efficiency along with the wearable prototype demonstrated herein make promising progress toward smart contact lens in ocular diagnostics.
Keywords Contact lens . Magnetic resonance coupling . Ocular implants . Power transfer efficiency . Wearable . Wireless power transfer
Young-Joon Kim and Jimin Maeng contributed equally to this work. * Jimin Maeng
[email protected] 1
Center for Implantable Devices, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA
2
Center for Implantable Devices, Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN 47907, USA
1 Introduction Over the past few years, a contact lens-like smart sensor has been receiving spotlight for its minimal-invasiveness and enduser friendliness (Farandos et al. 2015). Studies on contact lens sensors are especially promising since its scenarios can be extended to monitoring symptoms in various diagnostic criteria, including intraocular pressure for glaucoma (Leonardi et al. 2009; Mansouri et al. 2012), glucose level for diabetes (Yao et al. 2011), and lactate level for metabolic diseases (Thomas et al. 2012). Development of these contact lens sensors poses unique challenges. For example, the device has to satisfy the physical parameter constraints of a contact lens, which includes the contact lens diameter (typically 12– 14 mm, (McNamara et al. 1999)) and the base curve radius (BCR, typically 8–10 mm, (Farandos et al. 2015)). The material has to be biocompatible, transparent, and flexible to be eligible for ocular implant (Meng and Sheybani 2014). Also, the device requires a reliable power source embedded within the given device form factors to support the related electronics. Despite substantial miniaturization in recent years, the battery is still considered large compared to other microfabricated components of the sensor (Meng and Sheybani 2014). In biomedical applications, the limited lifetime of batteries and the risk of malfunction or leakage pose additional risks to the end user (Si et al. 2007). To overcome such restrictions for ocular sensing technology, research groups have integrated wireless power transfer technology into their contact lens electronics; however, most groups utilize high frequency electromagnetic (EM) radiation, which may have negative health effects (Leonardi et al. 2009; Pandey et al. 2010). EM radiation also suffers from low power transfer efficiency (PTE) due to the small form factor and poor antenna performance. Some groups employ lower frequency magnetic coupling for ocular device power transfer, but are
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not able to fabricate coils appropriate for a contact lens (Zhao et al. 2012). Others use solar energy as a power source (Chen et al. 2011), but scarce power availability impedes system performance and usability. Therefore, development of an efficient and reliable wireless powering scheme is called for to make the smart contact lens technology more viable. In this study, we propose a parylene-based contact lens platform, wirelessly powered by an external coil embedded in eyeglasses via magnetic resonance coupling (Kurs et al. 2007) at 13.56 MHz. Magnetic resonance coupling is chosen as the powering method since it can provide a substantially higher PTE than a high-frequency EM radiation method. A near-theoretically-maximum PTE can be achieved at an eyeglass-to-eye scenario under variation in distance or orientation. A theoretical background as well as the design method for the magnetic resonance coupling is described. Due to its biocompatibility, transparency, flexibility, and thickness controllability, parylene serves as the host substrate (Ha et al. 2012; Salvatore et al. 2014; Maeng et al. 2015). Once the device components are implemented on the substrate, parylene also serves as an effective sealing material. A multilayer thin-film parylene packaging process and a thick metal plating technique are utilized to implement an embedded high-Q receive coil to achieve a high PTE. The variation in the coil Q under device bending is assessed to confirm the feasibility to be used as a contact lens with a curvature. The effect of coil misalignment and the compensation techniques are provided. Lastly, the effect of EM field on human tissue is examined for safety consideration.
2 Experimental design and methods 2.1 Concept This report details a highly efficient wireless power transfer (WPT) platform for smart ocular diagnostics embedded into a contact lens (Fig. 1). A contact lens-like device is microfabricated on a parylene substrate and wirelessly powered by an external coil embedded in eyeglasses (Fig. 1a–c). A high-Q thin film coil is integrated in the parylene platform for high-efficiency wireless power transfer. As shown in Fig. 1d, the physical parameters of the device match those of a contact lens. The inner diameter of the contact lens coil is larger than the human pupil to avoid interfering with vision. Since eyeglasses are very natural in everyday life, the external transmit coil can be attached or embedded on eyeglasses. The power is delivered wirelessly from the external coil to the eye-worn parylene platform by 13.56-MHz magnetic resonance coupling (MRC), a means for delivering a near-distance wireless power with high efficiency but with small impact on the tissue. This report describes the 1) design and fabrication of a high-Q eye-wearable coil on a parylene
platform, 2) design, evaluation, and optimization of wireless power transfer into the eye-worn platform, and 3) a safety consideration with an EM field analysis. 2.2 Wireless power transfer theory and design In this work, wireless power transfer via magnetic resonance coupling is implemented by using a resonator-coupled bandpass filter (BPF) design technique in microwave engineering. For a single transmitter to a single receiver MRC design, the wireless power transfer scheme is identical to a resonator-coupled two-pole BPF model. The advantage of using this model is that the PTE can be predicted by calculating the insertion loss of the system. The characteristics of the bandpass filter are obtained by deriving the impedance matrix (1) and its corresponding matrices (2,3) (Cameron et al. 2007). Z ¼ R þ sI þ jM
ð1Þ
The coupling matrix M is described in (2) where subscripts S and L denote source and load, respectively. The unloaded Q (Qi) of the resonator is included in the calculation by (3) where FBW denotes the fractional bandwidth of the passband. The diagonal entries in R and I matrices indicate the loads and resonators in the network, respectively. 3 2 0 M S1 0 0 6 M S1 −M 11 M 12 0 7 7 M ¼6 ð2Þ 4 0 M 12 −M 22 M 2L 5 0 0 M 2L 0 −j Qi FBW 3 2 2 1 0 0 0 0 0 60 0 0 07 60 s 7 6 6 R¼4 ; sI ¼ 4 0 0 0 05 0 0 0 0 0 1 0 0 j w w0 s¼ − FBW w0 w
ð3Þ
M ii ¼
0 0 s 0
3 0 07 7 05 0
ð4Þ
ð5Þ
Once the impedance matrix is derived, the insertion loss is calculated by (6), which can be made identical to the PTE of the system. S 21 ¼ 2Z −1 4;1
ð6Þ
To maximize efficiency, we seek the external coupling values of MS1 and M2L that maximizes /S21/. The coupling values are then effectively converted to lumped components as shown in Fig. 2a (Ha et al. 2014). The electrical parameters of the receive coil on parylene, such as inductance (L) and quality factor, are mainly governed by the physical parameters of a contact lens. The design of the receive coil is described in Section 2.3. On the other hand, the
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Fig. 1 System design. (a) External coil is attached or embedded to eyeglasses. A thin and small coil is embedded on wearable parylene platform and receives power from eyeglasses. (b) Concept demonstration of eyeglasses-powered contact lens. Parylene platform sits on eye and powered by external coil attached to eyeglasses. (c) A
close up of (b). (d) Description of coil size on wearable parylene platform. The inner diameter (di) of the coil is larger than the human pupil (dp, 2– 6 mm) so that vision is not impaired. The outer diameter (do) is smaller than a typical size of a soft contact lens (14 mm)
physical dimension of the external transmit coil has fewer restrictions. We use a 45-mm-diameter AWG-14 wire for the external transmit coil, which fits in a typical circular shaped eyeglasses. The electrical parameters of the transmit coil are measured in advance as shown in Fig. 2b. Based on the parameter values obtained, a theoretical calculation predicts an insertion loss of −5.05 dB (equivalent to a PTE of 31.3 %) assuming a 20-mm powering distance (i.e., eyeglasses-to-eye distance) (Fig. 2d). The human eye is frequently in motion and the location of the external eyeglasses can change, which will vary the coupling coefficient and degrade PTE. As an effort to achieve maximum efficiency throughout the variation in coupling strength, the capacitance at the transmit side is implemented as a programmable capacitor bank (Fig. 2c) by using relay switches (TQ2-L-5 V, Panasonic). The discrepancy of the capacitance is less than 12 % from the target capacitance condition in the current scenario. Although it is best to implement a similar adaptive tuning technique in the receive side as well, the physical dimension of a contact lens poses a challenge. Therefore, the efficiency compensation network is only implemented in the transmit side while the receive side has a fixed condition. Even though
the compensation calculation shows mum efficiency powering distance
happens only in the transmit side, over 90 % of the theoretical maxican be recovered throughout the of 5–30 mm (Fig. 2e).
2.3 Coil design The receive coil embedded on the parylene platform should be carefully designed to maximize the PTE, while meeting the size requirements of a contact lens. We design our coil as an octagonal spiral inductor (Fig. 1d) to achieve the required L and Q values assessed in advance. First, the outer diameter (do in Fig. 1d) and the inner diameter (di in Fig. 1d) of the coil is determined as 10 mm and 6 mm, respectively, accounting for the size of a typical commercial soft contact lens (14 mm) and the diameter of a human pupil (2–6 mm for different luminance levels (Winn et al. 1994)). Then, the number of turns (N), coil width (W), and coil space (S) are determined as N= 3, W= 0.5 mm, S=0.25 mm to obtain a reasonable inductance value (>100 nH) required for our powering design. Given these parameters, the way to maximize Q is to increase the metal thickness to minimize the series
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Fig. 2 a Circuit diagram of the wireless power transfer between eyeglasses and parylene platform. b Measured electrical parameter of the external coil on eyeglasses. c The compensation network for distance variation. d Calculated frequency response of the WPT system.
e Comparison of the calculated maximum achievable PTE and the PTE obtained by tuning the capacitance in transmit (TX) side. f Simulated Q for receive coil embedded in parylene over different metal thickness (in air)
resistance of the coil. Simulation results show that the Q improves steadily as the metal thickness increases but beyond a thickness of 30 μm, the increment becomes very small while fabrication becomes difficult (Fig. 2f). Here, we employ a 36 μm-thick Cu electroplating. On a side note, it is confirmed by simulation that with an octagonal-shape spiral design, Q increases by 10 % while L decreases by 10 % compared to a rectangular design.
a parylene host substrate (5–30 μm, typically) and their multilayer interconnect through micro via-holes. In this work, a thick (36 μm) metal electroplating is uniquely combined to the parylene packaging process to achieve a high-Q coil that enhances the power receiving capability. The final package is released from a rigid carrier such that a flexible electronics is achieved on a freestanding, transparent, and biocompatible parylene film. This fabrication strategy allows the device to meet the physical requirements of a contact lens in terms of size, thickness, transparency, biocompatibility, and bendability. Figure 3 illustrates the contact lens platform fabrication process. First, a sacrificial photoresist (PR) layer is spincoated and cured on a standard silicon wafer. On top of the PR layer, a 30-μm-thick parylene film is coated using a roomtemperature parylene deposition system (PDS 2010, Specialty
2.4 Fabrication The fabrication of the proposed contact lens platform is based on a thin-film multilayer parylene packaging technology (Maeng et al. 2011), which allows conformal deposition and patterning of successive thin (
