You have undoubtedly seen the easy-to-use blood-oxygen saturation (SpO2) sensor that clips onto your finger and near-instantly reports results. The development of this dual-LED based unit, which sells for as low as $20, eliminated the need for costly, risky, time-consuming invasive measurements that required drawing of blood, and provides accurate results in real-time. It’s a truly amazing and illustrative example of how modern optoelectronics technologies have made “medicine” much easier, quicker and safer, especially since this approach has no downsides versus the older blood-sample technique (see References 1 through 5).
But there’s one limitation with this fingertip-sensing technique: It can only measure oxygen saturation in the circulatory system just below the skin (the same limit generally applies to the older blood-draw approach as well, of course). Yet doctors may need to monitor that oxygen parameter deep inside the body to assess the health of transplanted organs or tissue to provide an early warning of potential transplant failure, for example.
To solve this sensing problem, researchers at the University of California, Berkeley (UC-B) created a tiny, non-RF wireless implant that can provide real-time measurements of tissue oxygen-saturation levels deep underneath the skin (Fig. 1). The complete device has a volume of just 4.5 mm3 (critical for injectable implants) and is powered by ultrasound energy waves. Despite its array of component types, the entire implant is manufactured with standard medical-implant processes.
1. This general overview of the structure of the wireless, implantable blood-saturation sensor can only broadly indicate the complexity, multidisciplinary nature and overall sophistication of this tiny device, but it provides context and a starting point.
It’s anticipated to be the first entry in a series of miniaturized sensors that could track other key biochemical markers in the body, such as pH or carbon dioxide (CO2). “It’s very difficult to measure things deep inside the body,” said Michel Maharbiz, a professor of electrical engineering and computer sciences at UC Berkeley and project leader. “The device demonstrates how, using ultrasound technology coupled with very clever integrated-circuit design, you can create sophisticated implants that go very deep into tissue to take data from organs.”
Device Design
An external ultrasonic transceiver directs energy from outside of the body toward an implant placed in muscle or deep tissue. The system merges multiple technologies for sensing, energy harvesting, and the data link. The implant has a piezoelectric crystal for energy collection and communication coupled to a luminescence sensor for O2 detection. This wireless link has two roles: It’s an acoustic link to provide power to the sensor via piezoelectric effect-driven harvesting, and it supports bidirectional data transmission.
The implant (Fig. 2) consists of a piezoelectric crystal (750 × 750 × 750 μm3), a microLED, an O2-sensing film, an optical filter, a custom IC (~3.84 mm2) fabricated using a standard TSMC 65-nm low-power CMOS process and a holder that provides mechanical support during wire bonding of the microLED. To assemble the sensor, the crystal is attached and encapsulated, and then the remaining components are assembled on the flex board.
2. Wireless O2-monitoring system overview: (a) Schematic of the system as demonstrated in this paper for local tissue O2 monitoring in sheep. The free-floating wireless O2 implant is surgically placed beneath the biceps femoris muscle, and the wound is closed with sutures. The external ultrasonic transducer, placed on top of the closed surgical site, establishes a wireless acoustic link to the implant. The transducer powers the implanted sensor by delivering acoustic energy through the tissue and then listens for backscatter reflections from the sensor’s piezoelectric crystal, in which O2 data are encoded. The external transducer is driven by a custom mixed-signal system, including decoding and storage of the wireless O2 data received by the external transducer. (b) An expanded view of the wireless sensor platform, including a lead-zirconate-titanate (PZT) piezocrystal and a luminescence sensor, consisting of a µLED, a 3D-printed µLED holder, an O2-sensing film, an optical filter and an IC. (c) The following steps were used for wireless sensor fabrication: (1) a piezocrystal was bonded with conductive silver epoxy, wire-bonded to a flexible printed circuit board (PCB) and encapsulated with parylene-C; (2) other sensor components were assembled on the flex board, and only the regions where the wire bonds are located were encapsulated with ultraviolet (UV)-curable epoxy; (3) the small gap (~50 µm) between the film and the µLED holder was filled by PDMS and (4) sensor parts excluding the piezocrystal were encapsulated with highly O2-permeable black silicone. (d,e) Schematic (d) and photograph of the wireless sensor before black silicone encapsulation and size comparison to a finger (e); scale bar, 5 mm. (f) Top view of the 4.5-mm3 packaged wireless sensor with the piezocrystal at the bottom, the luminescence sensor at the top and a size comparison to a United States dime.
The sensor is coated with black silicone to avoid background interference by the luminescence of tissue or blood and has a detection volume of about 0.26 mm3. The crystal size and geometry is a classic engineering-design tradeoff, as it must balance the frequency-dependent acoustic loss in tissue, the power-harvesting capacity and the impact on total implant size.
Optical Measurement
The optical arrangement that senses blood oxygen isn’t the expected LED/photodetector pairing. Instead, it’s a more complicated design that’s more suitable for being implanted in tissue. It uses what is called phase luminometry based on a biocompatible luminometric O2 “optrode,” where the phase shift between the excitation and emission optical energy is measured to determine the luminescence lifetime. The optrode consists of polydimethylsiloxane (PDMS) containing silica particles with surface-adsorbed ruthenium dyes.
When excited by photons at ~465 nm, the ruthenium dyes emit light with a peak intensity at ~621 nm. The photons emitted by the excited ruthenium dyes then undergo “collisional quenching” with O2 molecules, leading to a reduction in luminescence intensity and lifetime. Both the intensity and lifetime depend on O2 concentration.
Why do this instead of using the “simpler,” better-known LED/photosensor arrangement? Either intensity or lifetime can be measured to compute O2. Lifetime is independent of variations in light-source intensity, dye concentration, inner filter effects and other effects, all of which are limitations of intensity-based sensors.
During operation, the emitted light is modulated by a 20-kHz square wave. An optical filter suppresses the excitation light, and a reverse-biased, on-chip photodiode detects the emission. It’s dependent on the luminescence lifetime that, in turn, is related to O2 via the Stern-Volmer equation which characterizes this phenomenon (see Reference 6).
Power and Data
Similarly, the ultrasonic power and data link is a sophisticated arrangement. When in downlink (transmit) mode, it drives the transducer with a 2-MHz carrier wave onto which it encodes digital information in discrete pulses (Fig. 3).
3. Block diagram of the entirely wireless O2-monitoring system: (a) The external transceiver is shown in the top left, the ultrasonic link is in the top middle and the wireless sensor is shown at right. Top left, the external transceiver consists of TX and RX paths, where the TX path encodes downlink data onto a 2-MHz carrier. During TX operation, a level shifter boosts a low-voltage TX signal from a digital controller, and a high-voltage pulser drives an external piezo transducer. The RX path is enabled when the TX path is disabled. The reflected ultrasound backscatter from the sensor’s piezocrystal is captured by the same external piezo transducer and digitized by the RX chain. Top middle, the external piezo coupled to the outside surface of tissue produces ultrasound waves traveling through tissue; these arrive at the sensor after one time-of-flight (ToF). The downlink provides power and a transmit command for the sensor. The uplink consists of amplitude-modulated, backscattered, ultrasound waves that arrive at the external piezo two ToF periods after being sent during TX. Right, the sensor IC architecture; CLK, clock. (b) Timing diagram: The IC rectifies electrical power collected by the sensor’s piezo, and the POR initializes the O2-sensing operation. The on-off keying (OOK) demodulator detects the downlink ultrasound envelope, producing a notch. The first notch starts uplink transmission. Two data packets are transmitted via digital amplitude modulation of the ultrasound backscatter; the first packet contains five MSBs and a one-bit preamble. The uplink data are received by the external piezo when the external transceiver is switched to RX. The uplink transmission stops when the third notch duration is longer than ~64 µs (127 oscillations of a 2-MHz carrier). The IC circuitry is duty-cycled during uplink transmission to reduce energy consumption.
The implant begins harvesting energy following the arrival of an ultrasound pulse; this energy is rectified and stored on the on-chip capacitor. The uplink data transmission begins when the demodulator detects a “falling edge” in the ultrasound data input from the external transceiver. It then generates a “notch” that serves as a reference to time-synchronize the sensor IC and the external transceiver during uplink transmission, and a data packet is transmitted after a notch.
Data packets are encoded in the ultrasound reflections from the sensor’s piezo device and transmitted via backscatter amplitude modulation. This modulation is achieved by changing the electrical load resistance, which is in shunt with the piezo-device impedance. This, in turn, changes the ultrasound reflection coefficient at the piezo boundary and thus the backscatter amplitude.
Obviously, designing and then building such a complicated, multidisciplinary implant is only part of the story, as it must be tested and evaluated both in the lab and in the “field,” including implanting it into live animal tissue (in vivo) to a depth of 10 cm. Full details including photos of how this was done, along with test results, are in their lengthy but highly readable and informative paper “Monitoring deep-tissue oxygenation with a millimeter-scale ultrasonic implant” published in Nature Biotechnology. A comprehensive Supplementary Information posting has additional circuitry diagrams and actual part designations.
This article appeared in Electronic Design.
References
1. EE World, “Blood oxygen meters, Part 1: Background and principles”
2. EE World, “Blood oxygen meters, Part 2: IC implementations”
3. American Thoracic Society, “Pulse Oximetry”
4. American College of Physicians, “Vital signs are vital: The history of pulse oximetry”
5. US National Library of Medicine, National Institutes of Health, “Takuo Aoyagi—a Tribute to the Brain Behind Pulse Oximetry”
6. Wikipedia, “Stern-Volmer relationship”
At a glance:
- The pandemic fueled demand for other types of mobile medical equipment, particularly mobile nursing stations and medical carts carrying supplies and medications.
- It is possible for industrial designers to achieve market-specific, competitive designs while implementing a standardization approach for key elements and mechanisms.
- The best way for medical device manufacturers to take full advantage of standardization is by establishing early working relationships with mechanism suppliers.
At the onset of the COVID pandemic, manufacturers of critical care medical equipment were faced with a demand far greater than they had ever prepared for, specifically for ventilators and other life-support equipment.
The impact was global, broad-based and impacted their ability to meet demand in two critical ways. First, massive increases in demand forced major re-working of manufacturing schedules and processes, just as COVID-related restrictions forced many plants and engineering staffs to either shut down or strictly limit the number of personnel working in facilities.
The restrictions also forced manufacturers and equipment assemblers to modify production floors, erect barriers, and implement social distancing and protective equipment practices to keep the virus from spreading within their plants.
Second, the sudden shutdown of “just-in-time” global supply chains, built around highly predictable demand scenarios for a whole range of common components like mobile devices, electronic components, touchscreens and secure locking mechanisms, prevented many medical device manufacturers from completing orders due to temporary loss of global components supplies.
While the COVID pandemic was unprecedented, it has presented medical device manufacturers with an opportunity to examine their approach to equipment design, manufacturing and component supply chains. The vulnerabilities in this complex system of suppliers for various parts point to the need to adopt a more secure approach to medical equipment design and manufacturing.
The industry can draw lessons from other manufacturing segments, such as automotive, that use standardization as a key foundation for their system design and manufacturing philosophy. This strategy allows vital mechanisms to be standardized across multiple platforms to create a streamlined manufacturing process from design to procurement.
For many commonly used mechanisms—latches and locking mechanisms, hinges and touchscreen monitor mounting devices—standardization across multiple models and configurations can enhance supply chain security and help ensure that urgently needed medical device equipment such as respirators can be delivered when they’re needed the most.
COVID pushed medical device manufacturers to examine their approach to equipment design, manufacturing and component supply chains.Southco
Disruption of Medical Equipment Demand
The sudden changes in demand for medical equipment driven by COVID were uneven and highly disruptive in a number of ways. The most obvious demand increases were for ventilators and other breathing assistance devices. However, the pandemic also ramped up demand for other types of mobile medical equipment, particularly mobile nursing stations and medical carts carrying supplies and medications.
Additional equipment was needed as hospital wards, medical centers and large-scale public facilities like convention centers were converted into COVID intensive care wards. For example, there was a sudden increase in demand for air purification and mobile/portable air handling equipment of all sizes and capacities to support temporary tent structures. There were even shortages of shipping containers to securely move all this equipment.
In addition, because so much of the medical establishment had to focus on treating the pandemic patients, demand for nonessential medical equipment, such as portable MRI machines, colonoscopy equipment and dental surgery systems, flatlined as elective treatments were postponed or suspended.
As with any unprecedented and sudden shift in demand for goods, this strain waterfalled throughout the supply chain as component suppliers rushed to increase production and delivery of vital components. On the procurement side, manufacturers had to coordinate across dozens of vendors to ensure the expedited delivery of components.
This disruption was compounded by major global restrictions on air travel, including air freight, to help prevent the spread of the virus. In response, many suppliers shifted to ocean transport, forcing increases in costs as well as changes in lead times from components suppliers, from four to six weeks to 14 to 16 weeks.
It also led to shortages of raw materials such as steel and high-quality plastics and components widely used in many mechanisms that are typically produced in high volumes in one part of the world—cast aluminum parts made in the Asia-Pacific region, for example.
So even the simplest of products, such as compression latches for securely closing shipping containers or air handling equipment, or springs and other components needed to manufacture adjustable monitor mounts for mobile nursing stations, became difficult for some mechanism manufacturers to supply, delaying the completion and delivery of vitally needed medical equipment.
Early engagement with trusted mechanism suppliers can help medical equipment OEMs identify which existing, standard products they already have in their portfolio that can be most easily and cost-effectively adapted to meet the requirement of new mechanisms.Southco
Standardization Offers Supply Disruption Solutions
Implementing standardization for many mechanisms on medical equipment begins with smart decisions and planning at the design stage. Many medical equipment manufacturers sell their equipment into different markets. Since there can be significant competition for products like ventilators and mobile nursing stations, manufacturers will invest time and resources to custom-design their equipment to have distinctive features and appeal to enhance the end-user experience.
It is possible for industrial designers to achieve market-specific, competitive designs while implementing a standardization approach for key elements and mechanisms. Leading suppliers of these types of mechanisms often have well-established component portfolios that designers can “mine” to create the solutions they need.
A good example is monitor mounts for touchscreens on mobile medical equipment like ventilators, carts and nursing stations. These are equipped with mounts that are easily adjustable, but have enough resistance so they don’t shift when personnel use the touchscreen. Some are mounted directly onto the mobile equipment frame while others may be on extended swing-arms for greater flexibility.
Medical device manufacturers could custom-design these mechanisms from the ground up with specific, virtually one-of-a-kind tooling for all the components. The risk is that supply chain disruptions can prevent all the needed components from coming together and being assembled into a finished mechanism.
Suppliers with large monitor mount portfolios can prevent this scenario while still supporting the manufacturer’s goals of creating a distinct and competitive design for their equipment. Working with the manufacturer’s industrial designers, they can pull from their existing portfolios of standard mounts to provide the functional “skeleton” of the device with the exterior housing being customized.
Southco’s AV Monitor Mount series contains integrated constant torque positioning technology that can be fine-tuned to the needs of different applications, while delivering consistent and easy one-handed positioning of a variety of monitor weights and sizes.Southco
Economies of Scale and Supply Chain Security
There are several key advantages to working with established mechanism suppliers when medical equipment manufacturers move to a more standardized approach for supplying their needs.
Leading suppliers of standard mechanisms like hinges, latches, locking devices and monitor mounts typically produce hundreds of thousands—even millions—of parts every year for a variety of industries and applications.
That has enabled them to fine-tune their portfolios to serve a broad range of needs and equipment types. A depth of product portfolio also provides economies of scale, since the standard mechanisms being manufactured at much higher rates for multiple customers drive down the cost per unit versus custom-manufacturing smaller production runs.
By building from an established portfolio, a medical device manufacturer can be more confident that the mechanism supplier has sufficient production capacity to adapt to any sudden surges in demand for components. This is especially true for companies with a global/local manufacturing footprint.
For example, Southco is a global supplier of access mechanisms and positioning technology widely used in medical equipment applications. The company has multiple production plants in key locations across the globe. Its global manufacturing platform can respond to changing situations without imperiling customer orders, lead times or product quality.
Quality control is crucial to successful standardization, through highly uniform manufacturing and quality processes, no matter what region or facility is performing the work. Each plant has the same equipment; the same tooling; and the same plant layouts, production flows and procedures, whether they are located in the U.S., Europe, India or China.
This global/local approach can be extremely valuable to medical device manufacturers seeking to compete in multiple global markets. It also provides the critical supply chain security that is a key benefit of standardization—especially when major disruptive events like COVID-19 occur.
If a North American ventilator manufacturer has a strong relationship with a global mechanism supplier, and the supply chain flow from Europe or China is disrupted, the ventilator manufacturer will still be able to meet the surging demand for their products as long as their mechanism supplier can provide the same monitor mounts from their local production base.
The best way for medical device manufacturers to take full advantage of standardization is by establishing early working relationships with mechanism suppliers as the design of new or updated medical equipment is begun.
Early engagement with leading mechanism suppliers with large product portfolios and engineering staffs will enable the supplier to fully understand the functional and design requirements for positioning, access and security equipment on the new system. They can then assess what existing, standard products they have in their portfolio that can be most easily and cost-effectively adapted to meet the new mechanism’s requirements.
It can also ensure that, once production commences, if there are any disruptive events like COVID that causes a dramatic surge in demand of medical equipment, the right devices can be supplied with as little disruption as possible to meet critical healthcare needs.
Robert Shelley is general manager for Southco’s Diversified Technologies business. He has more than 25 years of experience working in various roles supporting the company throughout North America. In his current role, he provides strategic direction for Southco’s healthcare, industrial equipment, HVAC, electronics and telecom business.
June 04, 2021 at 01:40AM
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Deep-Implantable Blood-Oxygen Sensor Blends Multiple Sophisticated Technologies - Machine Design
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