Research Article www.acsami.org
Stretchable Multichannel Electromyography Sensor Array Covering Large Area for Controlling Home Electronics with Distinguishable Signals from Multiple Muscles Namyun Kim,† Taehoon Lim,† Kwangsun Song,†,‡ Sung Yang,† and Jongho Lee*,†,‡ †
School of Mechanical Engineering and ‡Research Institute of Solar and Sustainable Energy, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea S Supporting Information *
ABSTRACT: Physiological signals provide important information for biomedical applications and, more recently, in the form of wearable electronics for active interactions between bodies and external environments. Multiple physiological sensors are often required to map distinct signals from multiple points over large areas for more diverse applications. In this paper, we present a reusable, multichannel, surface electromyography (EMG) sensor array that covers multiple muscles over relatively large areas, with compliant designs that provide different levels of stiffness for repetitive uses, without backing layers. Mechanical and electrical characteristics along with distinct measurements from different muscles demonstrate the feasibility of the concept. The results should be useful to actively control devices in the environment with one array of wearable sensors, as demonstrated with home electronics. KEYWORDS: stretchable electronics, biosignal, reusable sensors, electromyography, multichannel EMG
1. INTRODUCTION Physiological sensors, combined with other types of functioning wearable electronics, are gaining extensive attention1−4 for many applications, including continuous vital signal monitors,5−9 surgery assistive systems,10,11 implantable electronics,12,13 and electric generators.14,15 Physiological signals such as electroencephalograms (EEG), electromyograms (EMG), electrocardiograms (ECG), or other electrical potentials in living bodies, may serve as essential inputs for active control of prosthetics or for human-machine interface (HMI) systems.16−18 Among these sensors, surface electromyography (sEMG) is widely used, not only to detect neuromuscular diseases and understand muscular movements for medical treatments19−21 but also to provide voluntary muscular signals for active wearable devices.22−24 Recent research reports successful demonstrations of stretchable sEMG sensors for singular signal measurements using ultrathin films5,16 or adhesive substrates,25−27 to make conformal contact to skin and with mechanical properties close to human skin. Other approaches, including net-shaped stretchable pressure or temperature sensors28,29 over large areas enable multichannel data acquisition, which is often desired to measure EMG signals from multiple muscles, with one sensor array. However, it is often challenging to handle or reuse ultrathin, large-area, compliant sensor arrays without additional supporting layers that they restrict movement of interconnects. In this paper, we present reusable, multichannel, large-area EMG sensors covering multiple muscles by employing different levels of stiffness to the interconnects between the electrodes and the frame around the electrodes array. The narrow © 2016 American Chemical Society
compliant interconnects absorb strain around the electrodes while the stiff frame provides mechanical support to maintain the overall shape of compliant sensor arrays, thus, preventing from damaging and tangling the arrays when detaching and handling. This is especially beneficial when reusing large-area sensor arrays that enable multichannel sensing of EMG signals over multiple muscles. Experimental results along with finite element analysis were used to validate the advantages of the design. The demonstration includes sensing EMG signals from multiple muscles and controlling home electronics by voluntarily squeezing of different types of muscles under the large area EMG sensors. The results enabled by the design with different levels of stiffness, provide advancements with capabilities of (1) distinct EMG signal acquisition from multiple muscles and (2) repetitive use of the compliant sensor array that does not include backing layers. Most of previous stretchable EMG sensors are for single-channel or single use or require additional adhesives or coverings for attachments. These results should contribute not only to understand simultaneous muscular movements, but also to interact with multiple active devices differentially.
2. EXPERIMENTAL SECTION Reusable Multichannel EMG Sensor Fabrication. The fabrication of the EMG sensor begins with attaching a polyimide film (PI, thickness 12.5 μm) to a PDMS Received: April 27, 2016 Accepted: July 28, 2016 Published: August 8, 2016 21070
DOI: 10.1021/acsami.6b05025 ACS Appl. Mater. Interfaces 2016, 8, 21070−21076
Research Article
ACS Applied Materials & Interfaces (polydimethylsiloxane, thickness ∼ 30 μm) substrate spun and cured (80 °C for 60 min) on a silicon wafer (diameter 4 in., thickness 380 μm). After depositing Ti (80 nm) and Au (60 nm) with an electron beam evaporator onto the PI film and masking regions of the metal electrodes and interconnects with photolithography (AZ5214E), the exposed metals were removed by wet-chemical etching (Au etchant TFA, Ti etchant TFT, Transene Company). Another layer of polyimide (thickness 3 μm) was spin-coated and cured in vacuum oven at 250 °C for 1 h. A reactive ion etching (RIE) process with flowing O2 (50 s.c.c.m., 100 W for 120 min) and a Ti (20 nm) mask removed unnecessary regions other than electrodes, interconnects, and frames. The RIE process also opens the electrodes for contact with skin and electrical connectors for external amplifiers while keeping the metal lines used for interconnects encapsulated. The EMG sensor was released from the PDMS substrate with the aid of water-soluble tape (3M) and then connected to amplifiers using ACF connectors (Elform) by heating in an oven (150 °C for 5 min with pressure (30 kg/cm2). EMG Measurement and Control System. The reusable multichannel EMG sensor placed on a clean wiper is attached to target skin with the aid of water or conductive gel (Signa Gel, Parker Laboratories). Water or conductive gel are also known to be preventing from a buildup of triboelectric charge.30−33 Gently peeling the clean wiper leaves the EMG sensor on skin. The EMG signals measured using the bipolar method are filtered (low pass filter < 200 Hz, high pass filter > 10 Hz, notch filter removes 60 Hz) and amplified (amplifier gain 2100) with a commercial amplifier (PhysioLab) and collected with a four-channel oscilloscope (DSO-X 2024A, Agilent Technologies). The response of the devices is almost instant and the EMG signals from different muscles are recorded simultaneously in the same time line. Details are in the Supporting Information. The control input signals (2.5 V) from the amplifier are fed to a microprocessor (Arduino Mega 2506 R3, Arduino) that is coded to generate continuous signals (5 V) for on or off. The microprocessor sends out “on” or “off” signals depending on current states. For example, the microprocessor sends out “on” signal while in “off” state and sends “off” signal while in “on” state. The relays (max switching voltage 250 V AC, input voltage 4−32 V DC, Hanyoung Nux) that receive modulated signals from the microprocessor, switch a lamp and fan on and off independently, when squeezing the target muscles.
Figure 1. Schematic and optical images of the reusable multichannel surface EMG sensor. (a) Exploded schematic illustration of the multichannel EMG sensor in a tilted view. The metal electrodes and interconnects (Au/Ti) are encapsulated within PI films with openings at the top of the electrodes. No supporting backing layer is put under the whole structure. (b) Optical image of the EMG sensor from the top. The interconnects are relatively narrower than the frame. (c and d) Reusable multichannel EMG sensor attached to cover a large area of the upper arm. (e) Optical image of the EMG sensor pulled away from the skin with one electrode attached. The interconnects around the electrodes provide higher compliance. (f) Image of the multichannel EMG sensor with the frame suspended sideways. (g) Multichannel EMG sensors with and without the frame, before, and after repetitive use. The frame prevents the multichannel electrode arrays from tangling.
3. RESULTS AND DISCUSSION Figure 1a shows an exploded schematic illustration of the reusable multichannel surface EMG sensor. The fabrication process starts with depositing and patterning the metal layers (Ti/Au) on a polyimide (PI) film, followed by encapsulation with another layer of PI film. Reactive ion etching (RIE) opens the electrode metals and defines the lateral shape of the EMG sensor. More details are given in the Experimental Section. Figure 1b shows an optical image of the fabricated EMG sensor web (total size 71 mm × 68 mm) that consists of the electrodes (diameter 5.2 mm, center-to-center distance 10 mm), interconnects (width 0.55 mm), frame (width 1.2 mm), and external connect (4 mm × 16 mm). The metal layers (Au) on the electrode (diameter 4 mm) and external connect (4 mm × 16 mm, 25 pads, pad width 250 μm, pad spacing 500 μm) are exposed for sensing EMG signals and for connecting to external amplifiers, respectively, while electrical traces (width 50 μm,
spacing 100 μm) in the interconnects are encapsulated with a layer of PI film. The size of electrodes (diameter 5.2 mm) is determined to have a signal-to-noise ratio of 2.5 or higher, considering the low cost amplifier, and the interelectrode distance (10 mm) is chosen to be a half of the distance (∼20 mm) between dorsal interossei muscles on a hand. Figure 1c−d show optical images of the multichannel EMG sensor attached on a large area of the upper arm with the aid of water. There is no backing layer covering the EMG sensor, providing less restriction in movement of the interconnects when stretching or compressing the sensor array. Gentle movements (strain < 30%) of skins by voluntary squeezing, stretching, or compressing muscles on a forearm, leg, hand, or face do not detach the electrodes from the skin because the compliant interconnects move freely. The wider frame provides 21071
DOI: 10.1021/acsami.6b05025 ACS Appl. Mater. Interfaces 2016, 8, 21070−21076
Research Article
ACS Applied Materials & Interfaces
Figure 3. Reuse process and electrical performances. (a) Optical images of the reuse process. (i) Prepare the multichannel surface EMG sensor on a piece of clean wiper that serves as a carrier substrate, then spray water to attach EMG sensor on carrier substrate. (ii) Attach the sensor on skin with the aid of water or conductive gel; then peel off the carrier substrate. (iii) Measure EMG signals. (iv) Peel the sensor from skin by pulling the frame. (v) Clean the sensor by immersion in IPA and water successively, then dry. (b) EMG signals at the first and fiftieth uses and (c) signal-to-noise ratio (SNR, ∼2.88) of the measurements up to 50 reuse cycles when the sensor array is attached with water. (d) EMG signals and (e) SNR (∼10.93) when the sensor is attached with conductive gel. There was no significant degradation in the signals and SNR.
Figure 2. Mechanical and electrical characteristics of the EMG sensor: (a) Experimental setups for force and displacement curves of the isolated interconnect and frame. The sample is attached to the force sensor (GS0-10, Transducer Techniques) and linear stage with a micrometer (resolution 0.01 μm). The stage provides displacement and the sensor reads force at each displacement. (b) Experimental results of the force−displacement characteristics: the force increases linearly for lower displacement (
