Blood glucose monitoring device
A continuous glucose monitor (CGM) is a device used for monitoring blood glucose on a continual basis instead of monitoring glucose levels periodically by drawing a drop of blood from a finger. This is known as continuous glucose monitoring. CGMs are used by people who treat their diabetes with insulin, for example people with type 1 diabetes, type 2 diabetes, or other types of diabetes, such as gestational diabetes.
A continuous glucose monitor has three parts:
Currently approved CGMs use an enzymatic technology which reacts with glucose molecules in the body's interstitial fluid to generate an electric current that is proportional to glucose concentration. Data about glucose concentration is then relayed from a transmitter attached to the sensor to a receiver and display that shows the data to the user. 
Some CGM devices must be calibrated periodically with traditional blood glucose measurements, but others do not require calibration by the user.
Benefits.
Continuous glucose monitoring is gaining in popularity for a variety of reasons.
Limitations.
Continuous glucose monitoring has some important limitations:
Flash glucose monitoring.
The original Freestyle Libre monitor introduced by Abbott Diabetes Care in 2015 was described as doing "flash glucose monitoring," with a disposable 14-day sensor probe under the skin (as with other CGM sensors), but factory-calibrated without requiring calibration against a fingerstick glucose test. The sensor measures the glucose level of interstitial fluids (as a proxy for blood sugar levels) continuously; up to eight hours of these readings, averaged over each 15-minute period, are stored in the sensor unit, unlike most other CGM systems, which use a wireless link (typically Bluetooth) to an external device for each reading. Data stored in the sensor are transmitted on demand to a "reader" held within a centimeter or two of the sensor unit, employing near-field communication (NFC) technology. As only eight hours worth of data can be stored, downloads must not be spaced more than eight hours apart.
Differences in US insurance coverage favoring "flash glucose monitoring" over "continuous glucose monitoring" were an advantage to early adoption of Abbott's less expensive system. In the UK, flash glucose monitors and sensors are available to many patients without charge on the National Health Service (NHS).
The later Freestyle Libre 2 version of Abbott's device uses different, incompatible, sensors. It can be programmed to transmit a low blood sugar (hypoglycemia) or high sugar warning via Bluetooth to a nearby device and, as of 2023, transmits glucose readings via Bluetooth on a 60-second basis effectively making a CGM and not a flash glucose monitor. The following Freestyle Libre 3 is smaller, and transmits its readings via Bluetooth, as other meters do; it is not described as flash monitoring.
History.
United States.
The first CGM system was approved by the FDA in 1999. Continued development has extended the length of time sensors can be worn, options for receiving and reading data, and settings for alerting users to high and low glucose levels.
The first iteration of the Medtronic MiniMed took glucose readings every ten seconds with average readings reported every five minutes. Sensors could be worn for up to 72 hours.
A second system, developed by Dexcom, was approved in 2006. The sensor was approved for use for up to 72 hours, and the receiver needed to be within five feet for transmission of data.
In 2008, the third model was approved, Abbott Laboratories' Freestyle Navigator. Sensors could be worn for up to five days.
In 2012, Dexcom released a new device that allowed for the sensor to be worn for seven days and had a transmission distance of 20 feet. Dexcom later introduced an app allowing data from the sensor to be transmitted to an iPhone. This system was approved for pediatric use in 2015.
In September 2017, the FDA approved the first CGM that does not require calibration with fingerstick measurement, the FreeStyle Libre. The Libre is considered a "flash monitoring" system (FGM), and thus not a true ("real-time") CGM system. This device could be worn for up to ten days, but required 12 hours to start readings. and was followed by an updated device that could be worn for up to 14 days, and needed only one hour to start a new sensor. The FreeStyle Libre 2 was approved in Europe in October 2018, and enabled configuration of alerts when glucose is out of range.
In June 2018, the FDA approved the Eversense CGM system (manufactured by Senseonics Inc) for use in people 18 years of age and older with diabetes. This is the first FDA-approved CGM to include a fully implantable sensor to detect glucose, which can be worn for up to 90 days. The Eversense XL, a 180-day version of the system, was approved in Europe in October 2017.
China.
China develops and produces CGM systems. The first CGM system to be approved for the European Union is manufactured by Medtrum Technologies. The sensor's intended use is up to 14 days and measures glucose levels every 2 minutes via a smartphone application. Medtrum was founded in 2008 and is based in Shanghai, China.
At the end of 2017, Medtrum introduced the TouchCare A6 CGM (later A7 or Slim in some countries) which measures glucose levels in the interstitial fluid up to 14 days. The TouchCare system comes with mobile applications, including a remote view application. The TouchCare system has glucose alerts and requires calibration every 24 hours.
At the end of 2021 the Medtrum Nano was announced, a very slim device not requiring calibration, approved for up to 14 days use, with customizable glucose alerts.
Medtrum makes both CGM and insulin pumps, both controlled by a single smartphone application which enables the user to monitor glucose levels and trigger insulin delivery in a closed-loop system.
United Kingdom.
UK NICE guidelines introduced for the NHS in March 2022 in England and Wales advise that all Type 1 diabetic patients should be offered either flash glucose monitoring or CGM. People with Type 2 diabetes should be offered flash glucose monitoring or CGM if they use insulin twice daily or more, are otherwise advised to finger-prick eight times a day, have recurrent or severe hypoglycemia, have impaired hypoglycemia awareness, or cannot monitor their own blood sugar levels but they or a caretaker could use a scanning device. Details differ in Scotland and Northern Ireland.
Closed-loop system.
The CGM is a key element in the development of a "closed-loop" system for the treatment of type I diabetes. A closed-loop system monitors blood glucose by CGM and sends data to an insulin pump for calculated delivery of insulin without user intervention. A number of insulin pumps currently offer an "auto mode" however this is not yet a fully closed loop system. There are several implementations, including the artificial pancreas system and the open source OpenAPS.
Emerging CGM technologies.
The continuous glucose monitoring space remains subject to extensive research and development in building lower cost, more accurate and more easy-to-use sensing solutions, some of which aim to be noninvasive. A noninvasive CGM has been defined as a medical device that can measure glucose levels in the body without puncturing the skin, drawing blood or causing any pain.
As of August 2023, besides Dexcom and Abbott Diabetes, no other manufacturer has attained a significant market share worldwide. There have been regulatory approvals of noninvasive sensing systems in Europe, though market adoption has been low, not affecting the Abbott-Dexcom dominance.
Emerging invasive CGM technologies.
Multiple invasive CGM solutions have been under development since the early 2000s.
Senseonics has commercialized its 180-day Eversense XL sensing systems in both the U.S. and European markets. In June 2023, it announced what it deemed favorable safety and accuracy data for its 365-day sensor, suggesting it may be commercializable in the future. In June 2024, Senseonics announced intent to launch sales of the 365-day sensor in 2025. 
A solution built by U.S. firm GlySens, aimed to remove the need for an external reader by creating a sensor that could be implanted under the skin, that directly transmitted glucose values to an external app. As of August 2023, this undertaking has stalled and the system has not been approved anywhere and the company is defunct.
Another invasive CGM technology under development by Profusa Inc, based in Emeryville, California, builds on sensing research projects previously undertaken by the company under DARPA grants. This technology is composed of a hydrogel microsensor that is placed under the skin subcutaneously in a non-surgical procedure. In a 2020 literature review several biomedical engineers supported Profusa’s claims that the non-surgical insertion procedure differentiates it favorably from Senseonics’ Eversense system, as the latter requires a surgical procedure to insert and remove the sensor. The Profusa sensor allegedly also does not need to be removed because it overcomes the foreign body response. A reader is placed on the skin on top of where the sensor is, with the sensor transmitting a light signal to it. The sensor is claimed to last for three to six months. The is information then passed on to a smartphone where it can be tracked through an app. As of August 2023, this sensor has not attained regulatory approval in any jurisdiction, though a similar Profusa system measuring oxygen levels under the skin, has CE certification in Europe. Profusa has filed to go public via SPAC transaction.
A similar approach was under development by another California-based company called Metronom Health. This company has not released news releases, nor has any news covered any progress in terms of its research and development.
Yet another invasive approach is being developed by Belgium-based Indigo Diabetes. Indigo states that it is developing a CGM called a "continuous multi-metabolite monitoring system (CMM)". It is designed to provide people living with diabetes access to information on their glucose and other metabolite levels at any given time. It has yet to attain regulatory approval. The company completed a clinical trial in April 2024. 
Emerging noninvasive CGM technologies.
The ease of use many CGM users expect would be provided by a safe and accurate noninvasive device has led to significant innovation and research. 
Noninvasive approaches can be divided into interstitial fluid-based, radio frequency-based or breath-based. Interstitial fluid-analyzing sensors either use a device to analyze fluid on the skin or under the skin by sending infrared lasers to detect glucose levels in fluid. Radio frequency devices go through the skin and may derive glucose level information from blood directly.
Apple has reportedly been working on a noninvasive CGM that it seeks to integrate into its Apple Watch. In March 2023 it was reported to have established proof-of-concept of a noninvasive CGM. Another company working on noninvasive CGM is Masimo, which sued Apple for patent infringement in this area in 2020. Masimo has also filed new patents through its subsidiary Cercacor (pending as of September 2023) covering a joint continuous glucose monitoring and pump-closed loop delivery system.
Samsung announced that it would be incorporating glucose monitoring with its smartwatch with a targeted release year of 2025. As of October 2023 the last update was in December 2022. It is not clear whether the watch will integrate readings from an external CGM such as Dexcom's or Abbott's, or work standalone. The company in 2020 published literature regarding a non-invasive method it had developed with MIT scientists to engage in continuous glucose monitoring using spectroscopy. The company has filed patents related to this technology.
SugarBeat, built by Nemaura Medical, is a wireless non-invasive blood glucose monitoring system using a disposable skin patch. The patch connects to a rechargeable transmitter which detects blood sugar and transfers the data to a mobile app every five minutes. The patch can be used for 24 hours. Electronic currents are used to draw interstitial fluid to the surface to analyse the glucose level. SugarBeat has achieved regulatory approval in Saudi Arabia and Europe, though market penetration rates remain very low. The company declared US$503,906 in revenue for the fiscal year ending March 2022, which compares to Dexcom's more than $3 billion. As of  2023[ [update]] it had submitted a US FDA premarket approval application for sugarBEAT.
Another noninvasive system is built by US company Movano Health. It uses a small ring placed on the arm. Movano said in 2021 that it was building the smallest ever custom radio frequency (RF)-enabled sensor designed for simultaneous blood pressure and glucose monitoring. Movano is listed as MOVE on NASDAQ. By August 2023 Movano had shifted to building sensor rings for other parameters, such as heart rate, blood oxygen levels, respiration rate, skin temperature