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Quantum tunneling sensors are typically ultra-sensitive devices that have been specifically designed to convert a stimulus into an electronic signal using the wondrous principles of quantum mechanical tunneling. In the early 1990s, William Kaiser developed one of the first micromachined quantum tunneling sensors as part of his work with the NASA Jet Propulsion Laboratory. Since then, there have been scattered attempts at utilizing this phenomenon for the development of a variety of physical and chemical sensors. Although these devices demonstrate unique characteristics, such as high sensitivity, the principle of quantum tunneling often acts as a double-edged sword and is responsible for certain drawbacks of this sensor family. In this review, we briefly explain the underlying working principles of quantum tunneling and how they are used to design miniaturized quantum tunneling sensors. We then proceed to describe an overview of the various attempts at developing such sensors. Next, we discuss their current necessity and recent resurgence. Finally, we describe various advantages and shortcomings of these sensors and end this review with an insight into the potential of this technology and prospects.

Supplying electric power to wearable IoT devices, particularly smart contact lenses (SCLs), is one of the main obstacles to widespread adoption and commercialization. In the present study, we have successfully designed, fabricated, and characterized semi-transparent, self-supported, and flexible single crystalline silicon solar cells using a single-sided micromachining procedure. Optical, mechanical, and electrical simulations, together with the practical measurements, verify the application of our developed solar cells to be mounted on a limited-footprint and flexible SCL. The 15 μm-thick silicon solar cells conformally fit on a dome-shaped contact lens (ROC = 8 mm) without any mechanical and electrical degradation. This homojunction photovoltaic device containing an array of micro-holes exhibits a

We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 VRMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.

Presbyopia, an age-related ocular disorder, is characterized by the loss in the accommodative abilities of the human ocular system and afflicts more than 1.8 billion people world-wide. Conventional methods of correcting presbyopia fragment the field of vision, inherently resulting in significant vision impairment. We demonstrate the development, assembly and evaluation of autofocusing eyeglasses for restoration of accommodation without vision field loss. The adaptive optics eyeglasses consist of two variable-focus piezoelectric liquid lenses, a time-of-flight range sensor and low-power, dual microprocessor control electronics housed within an ergonomic frame. Patient-specific accommodation deficiency models were utilized to demonstrate a high-fidelity accommodative correction. Each accommodation correction calculation was performed in ~67 ms requiring 4.86 mJ of energy. The optical resolution of the system was 10.5 cycles/degree, featuring a restorative accommodative range of 4.3 D. This system can run for up to 19 hours between charge cycles and weighs ~132 g, allowing comfortable restoration of accommodative function, 12 pages, 13 figures

Objective: In this paper, we present a novel, low-profile, scleral-coil based, distance ranging system which is suitable for smart, accommodating contact lenses. Methods: We measure the induced emf between a set of four thin semi-circular coils patterned on flexible Kapton substrates that conform to the eyes’ sclera. This induced emf is a function of eye gaze angles. The system then determines the distance from the eyes to the desired object via the triangulation of these eye gaze angles Results : Experiments on eyeball simulated tissue gels indicate an accurate prediction of object distance in the 0.1-15 D range with a 0.15 D RMS error and object direction in the −15 to 15-degree arc with 0.4-degree RMS error, respectively. The energy required was determined to be as low as 20 μJ per range reading. Conclusion: Experimental data shows that our proposed new method of eye-tracking and distance ranging system can accurately predict eye-gaze angles and object-distance, whilst using only 20 μJ per range reading. Significance: The high-accuracy, low-profile and reduced energy requirements of the proposed eye-tracking technique, make it suitable for applications in the vast field of adaptive optics such as smart contact lenses and other low-power vision corrective applications.

Smart, adaptive contact lenses (SCLs) are amongst the most anticipated, next-generation, standalone medical devices. SCLs require the integration of thin microelectronic components, tunable lenses, and micro-power sources onto a common non-planar substrate. Here, we report a miniaturized, sliding metal-air electrochemical micro-battery driven by natural eye blinking motion that can be integrated with an SCL platform as a source of electrical energy. The metal-air battery (3⋅8 mm2) consists of a Mg anode and a Pt cathode. The electrolyte of the battery is the eye-tear liquid and is introduced to the battery structure during the natural eye-blinking cycle, which activates the battery. The open-circuit voltage across the eye-tear activated metal-air battery (ETMAB) was measured to be 2.2 V and the maximum specific capacity of 3561 mA h g−1 was obtained at a discharge current density of 5 mA·cm−2. Impedance matching analysis exhibits the maximum generated power density of 1.3 mW·cm−2 at the load of 740 Ω.

Using high-performance LC (E7) filled microfabricated refractive Fresnel chambers, we experimentally demonstrate a thin low-profile adaptive optical system with high analog tunability (2.1 D) that can be integrated with an adaptive contact-lens system.

This paper reports a novel method for detecting ethylene by using KMn04/polyimide composite thin-films as the analyte-sensitive layer in a capacitive MEMS ethylene sensor which requires zero-DC power for reliable operation. The technique utilizes a MEMS parallel-plate capacitor with the thin-film as a dielectric layer. Exposure of ethylene to the KMnO 4 molecules embedded within the polyimide matrix results in oxidation of ethylene into CO 2 and H 2 0, which induces swelling of the thin-film and also changes its dielectric-constant. This leads to a change in device capacitance. Preliminary results indicate a nearly reversible linear decrease of sensor capacitance of ~1 - 14 pF when exposed to ethylene concentrations ranging from 130 - 1000 ppm, yielding a normalized capacitance sensitivity of ~3.74×10−6/ppm. Selectivity tests revealed a favorable response towards ethylene compared to gases such as hexanal, indole, pentane, acetone, and IPA.

Polyimide and PDMS are the most common materials used in the packaging of flexible electronics. Silicon dioxide being photolithographically patternable, can be used to selectively bond polyimide (PI) to PDMS to realize flexible microsystems integrating fluidic and electronic components. In this work, we characterize the plasma bond strength of polyimide (Kapton) to PDMS with thin-film SiO 2 as an intermediate bonding layer. Peel strengths of samples prepared with different types of deposition methods – sputtering, plasma-enhanced chemical vapor deposition (PECVD), and atomic layer deposition (ALD) were evaluated. The measured forces (normalized to the width of the bonded area), required to separate the PDMS and Kapton films, either due to adhesive or cohesive failures, were 174-860 N/m, 79-179 N/m, and 7-12 N/m for samples bonded with sputtered, PECVD and ALD oxide, respectively. The strength of the sputtered oxide bond is comparable to the strength of the best chemical adhesive method available.

This paper reports the development of a high-sensitivity gas sensor and the demonstration of selectively detecting green leaf volatiles (GLV, here hexanal) released from damaged plant leaves. The developed sensor is a conductivity sensor that utilized a 5.2-nm gap as the key detection site between electrodes. The gap was coated with with customized molecular probes toward binding to hexanal. The fabricated sensor demonstrated the detection of a GLV, hexanal, from the collected gas samples from damaged plant leaves, sorghum. Gas samples were collected into a tedlar bag from a glass jar containing damaged sorghum leaves. When exposed to hexanal concentrations from 77.7 to 5181.3 ppm, it produced output signal changes in resistance by 1.64~4.45 times. The sensor response time was measured as 41.6 min at a hexanal concentration of 77.7 ppm.

We present the fabrication and experimental results of a flexible electrostatic energy harvester chip suitable for a smart contact lens application. The harvester consists of a variable area liquid capacitor formed between a hydrophobic film on the flex substrate and the rolling meniscus of a non-wetting eye tear drop. The harvester was tested under tear sliding motion emulating eyelid blinking. The 2×10 mm2 harvester generated 1.36 µJ of energy per blink and average power of 0.265 µW when pre-charged at 6 V and driven at typical blinking velocity conditions.

Power generation for the Internet of Things (IoT), particularly wearable electronics, is a significant challenge and a subject of great interest in the field of photovoltaics research. Here, we have designed and fabricated single crystalline silicon solar cells using a single-sided micromachining process. Preliminary results indicate that the solar cell is flexible and ~50% transparent. Furthermore, the open-circuit voltage and the short-circuit current density of the fabricated device under indoor light with the power density of 2 mW.cm-2 are 0.4 V and 110 µA.cm-2, respectively. The fabricated solar cells have been specifically designed to conformally fit on the commercial contact lenses.

We are reporting the fabrication and test of ultrathin, flexible semi-transparent single-crystal solar cells using a silicon undercut microfabrication techniques suitable for powering up exposed biomedical microsystems. The resulting cells are 15 micrometers thick. The solar cells provide a short circuit current density of 0.54 mA cm-2 and the open circuit voltage of 340 mV under low indoor lighting (13 mW.cm-2) conditions achieving an average light transmission of ~25%.

Biosensors are essential tools which have been traditionally used to monitor environmental pollution, detect the presence of toxic elements and biohazardous bacteria or virus in organic matter and biomolecules for clinical diagnostics. In the last couple of decades, the scientific community has witnessed their widespread application in the fields of military, health care, industrial process control, environmental monitoring, food-quality control, and microbiology. Biosensor technology has greatly evolved from the in vitro studies based on the biosensing ability of organic beings to the highly sophisticated world of nanofabrication enabled miniaturized biosensors. The incorporation of nanotechnology in the vast field of biosensing has led to the development of novel sensors and sensing mechanisms, as well as an increase in the sensitivity and performance of the existing biosensors. Additionally, the nanoscale dimension further assists the development of sensors for rapid and simple detection in vivo as well as the ability to probe single-biomolecules and obtain critical information for their detection and analysis. However, the major drawbacks of this include, but are not limited to potential toxicities associated with the unavoidable release of nanoparticles into the environment, miniaturization induced unreliability, lack of automation, and difficulty of integrating the nanostructured-based biosensors as well as unreliable transduction signals from these devices. Although the field of biosensors is vast, we intend to explore various nanotechnology enabled biosensors as part of this review article and provide a brief description of their fundamental working principles and potential applications., Comment: 35 pages, 7 figures

By stacking multiple thin and flexible LC based lenses, we experimentally demonstrate a low-power adaptive optical system with enhanced focusing tunability that is suitable for integration with a smart contact lens system.

In this article, we present an eye-tracking system for distance ranging, developed for varifocal contact lens applications. The system comprises of a set of four ~150 µm-thick, semi-circular coils, which are patterned on flexible Kapton substrates and can easily conform to the sclera. The object-distance from the eyeballs is determined via triangulation of the eye gaze angles, which is calculated from the induced electromagnetic voltage between the coils. Preliminary experiments with the coil-system, placed on eyeball simulated polymeric spheres resulted in accurate prediction of object distance in the range of 0.1-15 D (Diopter) with an RMS error of 0.1 D and object direction in the range of -15 to 15-degree arc with 0.4 degree RMS error, respectively. Diopter is the reciprocal of the object distance (in meters). The energy required to perform a single range reading was ~ 20 µJ.

Structured AbstractPurposeWe describe new phenomenological illumination-dependent static models of the accommodation deficiency for patients with presbyopia. Such models are suitable for vision restoration with adaptive-optics accommodating eyeglasses and contact lenses.MethodsData from fifteen participants over the age of 45 and diagnosed with presbyopia was collected. Participants were asked to wear a pair of mechanically-tunable eyeglasses and clearly identify the optotypes corresponding to the LogMAR 0.0 line on Early Treatment of Diabetic Retinopathy Study (ETDRS) charts, by suitably adjusting the optical powers of the lenses on these tunable eyeglasses for each measurement. Seven ETDRS charts, placed at distances from the patients varying from 4 m through 30 cm, were used under three chart illumination levels (75 lx, 500 lx and 800 lx). The optical power of the lenses in the patient-adjusted tunable eyeglasses was subsequently measured using a Shack-Hartman wavefront sensor for each chart, which provided the accommodation deficiency data of the participants.ResultsThe measured accommodation deficiency data from 15 presbyopes was curve-fitted to a model for each patient. The calculated root-mean-square error values for the fitted models ranged between 0.09 D – 0.67 D over a 3.08D accommodation stimulus range.ConclusionsThe data shows that while accommodation deficiency in humans is a function of the stimulus, it is also strongly dependent on the object illumination and age of the patients. The models adequately describe the relation between static accommodation deficiency, accommodation stimulus and object illumination.

We present the design, fabrication, and response of a low-power, polymer-based VOC sensor based on the self-leveling of mechanically leveraged structures. The device utilizes folded polymer-coated microcantilevers to achieve passive temperature compensation without the need for additional compensating sensors or electronics. We demonstrate that a self-leveling vapor sensor provides the same gas response as a simple microcantilever geometry, showing ~20% change in device capacitance when subjected to 35-85 %RH change while showing nearly-zero baseline drift due to changes in ambient temperature when the temperature is increased from 23-72{\deg}C which is ~52-fold better than a simple microcantilever geometry. The response of the VOC sensor was measured using three polymers (Polyimide, Polyurethane, and PDMS) against five different analytes (Ethanol, Acetone, Benzene, Hexane, and Water) and an SVM-based model was used to show target specificity. The sensor also showed an absorption response time ({\tau}90) of ~138s. We propose that the self-leveling vapor sensor geometry is a significant improvement to a simple microcantilever vapor sensor as it offers the same performance but shows near-complete elimination of temperature-induced baseline drift., Comment: 15 pages, 13 figures. Manuscript accepted for publication in Sensors and Actuators B: Chemical

This paper reports the design and implementation of a wake-up gas sensor node, including both a low-power sensor and a wake-up electrical circuit that was particularly capable of utilizing the “on/off” characteristic of a previously reported nano-gap sensor to produce the needed output electrical power for alerting operations (e.g. LED lighting) under power budgets. The designed circuit, as predicted by a SPICE model, consumed 3.7mW of power for its “on” operation at an applied voltage of 1V, while only consuming ∼970nW during its “off” operation. The measured power for both operations enabled an average power consumption of $155.1\mu\mathrm{W}$ and depicted the wake-up functionality of the integrated gas sensor node for longterm battery-supported deployment. The integrated sensor node successfully demonstrated the lighting of an LED as an alarm signal, when exposed to a threshold concentration of 550 ppm of 1,5-diaminopentane gas.

This paper reports the successful demonstration of modulating the threshold point in gas concentrations at which a gas sensor turns “on/of” for wake-up operation by adjusting the tunneling areas of a nano-gap in the previously reported ultra-low-power gas sensor. The nano-gap area of the sensor determines the total binding sites and thus the total amounts of “on” tunneling-current at a given concentration. Since the total current amounts trigger wake-up, their increase per given concentration could enable a lower minimum detectable wake-up concentration. Experimental results confirmed that the increase in the nano-gap area from 6.5 to $62.5\ \mu \mathrm{m}^{2}$ by 861.5% enabled the reduction in the required wake-up gas concentrations from 250.0 to 50.0 ppm by 80.0%, showing an exponential decay trend. Such a reduction in the wakeup threshold allows the previously-reported sensor to achieve the required wake-up range of 200.0 ppm for early detection of diamine gases for health safety, while indicating the design feasibility to enable various wake-up concentrations for adjustable operation.

This paper reports an inexpensive method for stiction suppression in MEMS devices using the sublimation of camphor. Suspended microcantilever structures which would otherwise collapse to the substrate due to stiction, were successfully released using this method. The technique uses deposition of camphor nanoparticles acting as temporary supports under suspended microstructures using an immersion process. The sample is then dried, and the camphor sublimates under heating, preventing device failure due to capillary forces. This new method does not require any specialized equipment or additional processing steps; therefore, it is simpler and more economical than alternate conventional release methods.

Liquid-filled tunable-focus lenses have been demonstrated to be suitable for autofocus eyewear applications. Traditionally, these lenses are constructed using an elastomeric polymer chamber filled with a high-index liquid. In this work, we investigate the effect of elastomeric creep on the deformation and eventual degradation of these tunable lenses. We use numerical analysis of a deformable circular disk representative of the lens and provide rigorous experimental results testing the creep property of a number of elastomers. Finally, we provide a comparative study of different elastomeric materials and select the best one for this application.