Journal Club Theme of May 2016: Recent Progress in Curvilinear Electronics and Mechanics

Recent Progress in Curvilinear Electronics and Mechanics

Jianliang Xiao

Department of Mechanical Engineering, University of Colorado Boulder

1. Introduction

Stretchable electronics has attracted a lot of attention from both academia and industry due its enormous application in many areas. It brings rubber-like mechanical properties to the otherwise rigid and brittle electronics systems, while retains the high electrical performances of conventional wafer based electronics. Stretchable electronics is usually soft, elastic, and can be freely deformed into complex shapes. Therefore, it can intimately wrap onto 3D complex, arbitrary curvy surfaces, for applications ranging from biointegrated electronics for diagnosis, treatment and human-machine interfaces, to bio-inspired devices that utilizes curvilinear configurations to achieve superior performances. In the past decade, tremendous amount of work has been done on stretchable and flexible electronics, it’s impossible to cover all of them. In this discussion, I’ll specifically focus on recent efforts and achievements in developing curvilinear electronics, and the associated mechanics issues.

2. Bio-integrated Curvilinear Electronics

Because of its soft, elastic nature, curvilinear electronics can be designed to match with the complex, curvy surfaces of human body, to realize bio-integrated electronics for applications such as health monitoring, diagnosis, therapy and drug delivery. Figure 1a is a flexible electronic sensor integrated on the surface of a porcine heart [1]. Because of its ultrathin thickness and extreme mechanical flexibility, the sensor can be strongly bonded on the surface by utilizing capillary force provided by bio fluid even when the heart is beating. This sensor monitors in real time the high density cardiac electrophysiological signals, and therefore can provide important information for diagnosis. Figure 1b shows a 3D multifunctional integumentary membrane wrapped around a rabbit heart [2]. It integrates temperature sensors, pH sensors, ECG sensors, strain gauges and LEDs, and offers capability of measuring different signals simultaneously.  Figure 1c exhibits a flexible mechanical energy harvester based on PZT, integrated on the surface of a bovine heart [3]. During heart beating, it can generate energy that can be stored in a rechargeable battery to support a pacemaker or other devices that need power supply. Figure 1d is a meshed array of extremely flexible electrodes, intimately integrated with the very complex surface features of a feline brain [4]. It can provide accurate, high-density electrocorticography (ECoG) mapping of the brain.

Figure 1. Bio-integrated curvilinear electronics. (a) flexible cardiac sensor mounted on a porcine heart, (b) 3D multifunctional integumentary membrane wrapped around a rabbit heart, (c) flexible mechanical energy harvester integrated on a bovine heart, (d) Flexible mesh array of electrodes on a feline brain, (e) epidermal electronics, (f) wearable drug delivery system, (g) stretchable electronics integrated with a glove, (h) electronic devices mounted on different regions of the ear for EEG monitoring.

The above devices can provide direct and accurate measurement of signals from different organs, but they requite invasive surgery to integrate devices, which is not feasible for daily application. Therefore, electronic devices that can be integrated with human skin are desired. To be suitable for daily use, such devices need to be very stretchable, soft, shape conformal, biocompatible, and breathable, so that these devices can function during daily activity and don’t make people feel uncomfortable. Figure 1e demonstrates epidermal electronics that can be bonded very well onto human skin, even during stretching, compression and torsion [5]. Multifunctional sensors are integrated to provide monitoring of strain, ECG, EMG, and temperature information. Antenna, and wireless power coils, RF coils and diodes are also integrated to provide wireless charging and communication capabilities. Figure 1f is a wearable drug delivery system that can be integrated with human skin [6]. Release of drug in this device can be controlled by stretching applied to the system, to provide anticancer and antibacterial treatment. Figure 1g illustrates stretchable electronics integrated with a glove [7]. Figure 1h shows electronic devices mounted on different regions of the ear, to conveniently monitor EEG signals [8]. Such devices can also be extended to provide brain computer interface.

Mechanics issues associated with these systems include but are not limited to: (1) how to design the electronics to be stretchable, flexible, and shape conformal? (2) how to ensure good contact between the electronics and the surface human skin/organs? (3) how to design the electronics to ensure human comfort when they are integrated with human body? (4) how to ensure reliability of the electronics during complex dynamic loading cycles?

3. Bio-inspired Curvilinear Optoelectronics

Natural imaging systems demonstrate many attractive attributes that cannot be found in commercially available cameras. For example, human eyes form very sharp images with very simple optics. On the contrary, each commercial camera needs a complex set of optical lenses to adjust optical paths of the light rays to achieve comparable sharpness of imaging. Another type of natural imaging system is compound eyes that are commonly found in arthropods. A compound eye is usually composed of hundreds or thousands of individual imaging units, i.e. ommatidia, on a hemispherical surface. Such hierarchical structures of compound eyes, although cause reduced resolutions, can provide very wide field of view angle, low aberration, high sensitivity to motion and infinite depth of field. One common feature in these natural imaging systems, including human eyes and compound eyes, is that the light sensing elements are distributed on curved surfaces. In the human eyes, the sensing elements are located on concave, hemispherical surfaces to accommodate the hemispherical imaging planes formed by the simple optics. In compound eyes, the sensing elements are located on convex, hemispherical surfaces to coordinate with the 3D microlens array to achieve their unique imaging characteristics. Therefore, to achieve the imaging characteristics found in natural eyes in cameras, photodetectors have to be placed on similar curvilinear surfaces.

Figure 2. Bio-inspired curvilinear optoelectronics. (a) Electronic eyeball camera, (b) Tunable electronic eye with zoom, (c) Artificial apposition compound eye camera.

Figure 2a shows a fully functional hemispherical electronic eyeball camera that mimics the structure and function of human eye [9]. This layout of photodetectors was first fabricated in planar geometry and then transformed into hemispherical shape to resemble the geometry of retina in human eyes. Good imaging capability has been demonstrated with a simple plano-convex lens. Figure 2b exhibits a tunable hemispherical eyeball camera system that combines the advantages of human eyes and commercial cameras [10]. In this system, the curvatures of both the optical lens and the hemispherical imaging plane can be adjusted coordinately to realize zoom, a capability can be found in commercial cameras but not in human eyes. Figure 2c is an artificial apposition compound eye camera, mimicking the compound eyes of insects [11]. Arrays of elastomeric microlenses and stretchable photodetectors were separately fabricated and integrated in their planar geometries, which were then transformed into a hemispherical shape to realize the artificial compound eye camera. This digital camera exhibits an extremely wide field of view angle (160 degrees) and infinite depth of filed.   

Mechanics issues in these systems include: (1) how to design the optoelectronic system to sustain large deformation during shape transformation? (2) how to predict and track the positions of photodetectors to ensure accurate postprocessing of images? (3) how to predict the deformation of the system to guide operation and adjustment of different elements to comply with optics? (4) how to ensure reliability during long term operation?

4. Discussion

Curvilinear electronics has shown incredible advancement in the past decade, and many interesting applications have been demonstrated. Mechanics of different systems has also been studied [12-16]. Some more recent studies have been trying to combine curvilinear electronics with soft active materials to realize smart electronics that can respond to environment, or can change shapes in a programmable manner [17-18]. Despite all these exciting achievements in curvilinear electronics, one challenge to be resolved before mass production and commercialization is how to manufacture stretchable electronics and systems in an economical way. This work demonstrated an interesting way to achieve this goal by mechanically cutting laminated thin films into stretchable layouts [19].

References:

[1] J. Viventi et al., A Conformal, Bio-interfaced Class of Silicon Electronics for Mapping Cardiac Electrophysiology, Science Translational Medicine 2, 24ra22 (2010).

[2] L. Xu et al., 3D multifunctional integumentary membranes for spatiotemporal cardiac measurements and stimulation across the entire epicardium, Nature Communications 5, 3329 (2014)

[3] C. Dagdeviren et al., Conformal piezoelectric energy harvesting and storage from motions of the heart, lung, and diaphragm, Proc. Natl. Acad. Sci. USA 111, 1927 (2014)

[4] D.-H. Kim et al., Dissolvable Films of Silk Fibroin for Ultrathin Conformal Bio-Integrated Electronics, Nature Materials 9, 511-517 (2010)

[5] D.-H. Kim et al., Epidermal Electronics, Science 333, 838 (2011)

[6] J. Di et al., Stretch-Triggered Drug Delivery from Wearable Elastomer Films Containing Therapeutic Depots, ACS Nano 9, 9407-9415 (2015)

[7] H.C. Ko et al. Curvilinear electronics formed using silicon membrane circuits and elastomeric transfer elements. Small 5, 2703 (2009).

[8] J.J.S. Norton et al., Soft, curved electrode systems capable of integration on the auricle as a persistent brain–computer interface, Proc. Natl. Acad. Sci. USA 112, 3920-3925 (2015)

[9] H. C. Ko et al., A Hemispherical Electronic Eye Camera Based on Compressible Silicon Optoelectronics. Nature 454, 748-753 (2008).

[10] I. Jung, J. Xiao et al., Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability, Proc. Natl. Acad. Sci. USA 108, 1788-1793 (2011)

[11] Y. M. Song, Y. Xie, V. Malyarchuk, J. Xiao et al., Digital Cameras With Designs Inspired By the Arthropod Eye, Nature 497, 95–99 (2013)

[12] S. Wang, J. Xiao, I. Jung, J. Song, H. C. Ko, M. P. Stoykovich, Y. Huang, K.-C. Hwang and J. A. Rogers, Mechanics of Hemispherical Electronics, Appl. Phys. Lett. 95, 181912 (2009).

[13] S. Wang, J. Xiao, J. Song, H. C. Ko, K.-C. Hwang, Y. Huang, and J. A. Rogers, Mechanics of curvilinear electronics, Soft Matter 6, 5757–5763 (2010)

[14] C. Lü, M. Li, J. Xiao, I. Jung, J. Wu, Y. Huang, K.-C. Hwang, and J.A. Rogers, Mechanics of tunable hemispherical electronic eye camera systems that combine rigid device elements with soft elastomers, Journal of Applied Mechanics-Transactions of the ASME 80, 061022 (2013)

[15] Z. Li, and J. Xiao, Mechanics and Optics of Stretchable Elastomeric Microlens Array for Artificial Compound Eye Camera, J. Appl. Phys. 117, 014904 (2015)

[16] Z. Li, and J. Xiao, Strain tunable optics of elastomeric microlens array, Extreme Mechanics Letters 4, 118-123 (2015) 

[17] C. Yu et al., Electronically Programmable, Reversible Shape Change in Two- and Three-Dimensional Hydrogel Structures, Adv. Mater. 25, 1541-1546 (2013)

[18] S. Lin et al., Stretchable Hydrogel Electronics and Devices, Adv. Mater., DOI: 10.1002/adma.201504152 (2015)

[19] S. Yang et al.,  “Cut-and-Paste” Manufacture of Multiparametric Epidermal Sensor Systems, Adv. Mater. DOI: 10.1002/adma.201502386 (2015)