IMEC breakthrough shrinks wireless bioelectronic communications system into a 1 cm³ system-in-a-cube

IMEC has developed a miniature 1 cm³ 3-dimensional stacked system-in-a-cube (SiC) for wireless bioelectronic communications systems. The low-power 3D SiC, which comprises a radio and digital signal processing (DSP), has broad application in a variety of wireless products, ranging from monitors for human-body information to environmental data. The bioelectronics breakthrough will be first incorporated into a wearable, wireless electroencephalogram (EEG) developed by IMEC and the University Hospital Leuven, Belgium, last year. Using the 3D SiC, patients can wear a comfortable device and maintain maximum mobility during the electroencephalogram, eliminating the hospital stay traditionally required for this procedure.

The system-in-a-cube (SiC) was developed as part of IMEC's Human++ program, which envisions similar SiCs as sensor nodes constituting a body area network (BAN). The BAN will be used to gather vital body information into a central intelligent node, which in turn will communicate wirelessly with a base station. Such BANs require a number of small low-power intelligent wireless nodes with sufficient computing power, wireless capabilities and integrated antenna. IMEC's Human++ program combines experience and expertise in wireless communications, packaging techniques, energy scavenging technologies and low-power design techniques in order to develop devices, which improve the quality of life. Current work focuses on the complete integration of the wearable wireless EEG prototype in the 1 cm³ SiC and is expected to be finalized in October this year.

This first 3D-stack prototype integrates a commercial low-power 8 MIPS (million instructions per second) microcontroller and a 2.4 GHz wireless transceiver, crystals and other necessary passives, as well as a custom-designed matched dipole antenna. Both the microcontroller and wireless transceiver are state-of-the-art in their power consumption.

The high level of integration in a system no larger than 1 cm³ was achieved through Z-axis, or "3D", stacking of separate layers with different functionality. Each layer connects to its neighboring layers through a dual row of fine pitch solder balls. This generic stacking technique allows any kind of module build-up; each layer can have dedicated functionality such as computing, wireless communication, sensing, power scavenging and so on. The bottom layer has a BGA (ball grid array) footprint, allowing standard techniques for module mounting.

Applications range from standalone sensor units to wireless multi-hop beacons. The tasks of the microcontroller unit range from adjusting sensor preamps and digitization to data interpretation, forward error coding and MAC (medium access control) implementations from low to medium complexity. The 12 bit microcontroller ADC (analog to digital converter) provides enough dynamic range for most sensors to be directly interfaced with the module. Not only human body information, such as EEG (brain activity), EMG (muscle activity) or ECG (heart activity) can be wirelessly monitored, but also standard environmental information such as temperature, pressure, and humidity can be transmitted when the appropriate sensor is applied. The unique stacking feature even allows integrating a specific sensor into a single layer, resulting in an application-specific cubic sensor module.

Future developments will mainly focus on further size reduction and the integration of IMEC's low-power processing, wireless and power-scavenging technology. Adding an extra stack layer with solar cells and energy-storage circuitry will offer a complete standalone solution.

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