Biofouling and Foreign Body Response (FBR) Mitigation

Biofouling and Foreign Body Response (FBR) Mitigation in Continuous Glucose Monitors

The foreign body response (FBR) is the body's attempt to wall off anything it doesn't recognize, and it limits how long and how accurately a CGM sensor works [1]. As of October 2026, marketed sensors deal with it in one of two ways. Skin-worn sensors such as the Dexcom G7 15 Day and the FreeStyle Libre 3 Plus are replaced after up to 15 days [2][3]. The implanted Eversense 365 is cleared for up to 1 year, and it gets there with a steroid-releasing collar, a glucose indicator modified to resist inflammation, and regular fingerstick calibration [4][5]. This article covers that long-wear side; our page on FBR mechanisms in CGMs explains the biology.

What the reaction does over days and months

Two human studies show the problem at different time scales. In a Spanish study, 12 people with obesity had the tissue around two worn sensors biopsied: one sensor inserted 7 days earlier, one 24 hours earlier [1]. Neutrophils, the first white cells to arrive, dominated at 24 hours. By 7 days macrophages, immune cells that engulf foreign material, had taken over, and less accurate sensors had about three times as many around the insertion hole (32.6 against 10.6 cells per 0.01 mm²) [1].

The longer view comes from explants, devices removed and examined after use. In a 2004 University of Wisconsin study, enzyme sensors were implanted under the skin of five people with type 1 diabetes, then surgically removed so the tissue around them could be examined [6]. Only one tracked glucose usefully for long: 6.2 months, with recalibration every 1 to 4 weeks. Three had electronic packaging failures. The fifth never tracked glucose, because it never formed a lasting connection with the tissue around it [6].

How Eversense handles the foreign body response

A steroid that mostly stays put

The original Eversense, approved by the FDA on June 21, 2018 for up to 90 days and since replaced by later versions, has a silicone collar holding 1.75 mg of dexamethasone acetate, an anti-inflammatory steroid that elutes locally to reduce inflammation around the sensor [7]. In a 90-day rat study, neither the sensor nor a heart pacing lead that releases the same drug caused adverse tissue reactions [7].

Here's what surprised us: most of the steroid never leaves. Sensors removed at 90 days in the clinical studies still held about 80% to 90% of their dexamethasone, so roughly 0.18 to 0.35 mg was released [7]. Blood levels stayed low. In people with two sensors, 18 of 27 had brief detectable levels in the first 8 days, peaking at 114 pg/mL, and all fell below the 50 pg/mL detection limit by day 8 [7].

The Eversense 365 keeps the same 1.75 mg collar, which the FDA's summary says is there "to minimize local foreign body reaction" [4]. The system is contraindicated in people for whom dexamethasone or dexamethasone acetate may be contraindicated, and it wasn't tested in people using systemic glucocorticoids [4]. Whether either applies to you is a question for your care team.

Protecting the chemistry from inflammation

Calming the tissue isn't enough, because inflammation also attacks the sensor. Eversense reads glucose with a fluorescent indicator that binds it through boronic acid groups. During development, in-body testing showed reactive oxygen species, especially hydrogen peroxide from the inflammatory response, oxidizing those groups and weakening glucose binding [5]. A coating of platinum nanoparticles on the hydrogel stretched the original sensor's useful life to up to 3 months [5]; our article on implantable fluorescence sensors covers that chemistry.

The next fix was a decoy. In the 180-day PROMISE study, 43 participants wore a second sensor whose hydrogel included 4-vinylphenylboronic acid, a "sacrificial" boronic acid that draws the oxidative attack and protects the boronic acid that binds glucose [5]. Of the standard sensors, 65% survived to 180 days, against 90% of the modified ones, and MARD (mean absolute relative difference, the average gap between sensor and lab readings) was 9.1% against 8.5% [5]. In February 2022 the FDA approved that modified design as the Eversense E3 sensor for up to 180 days, describing its hydrogel as changed "to extend in vivo functional life" [8]. The Eversense 365 has since superseded the E3.

What a year of wear looks like

In the study behind the 365's clearance, 110 sensors were evaluated and the survival rate at day 365 was 90% [4]. Accuracy didn't fade: MARD was 9.7% in days 1 to 30 and 8.8% in days 331 to 365, and no 30-day interval went above 9.9% [4]. Fingersticks are part of how it holds up: after a 24-hour warm-up, the system needs 4 calibrations within 36 hours, one a day through day 13, then one a week [4]. The PROMISE authors wrote that a months-long sensor requires calibration, which lets the algorithm track sensitivity lost to oxidation [5].

How 10- to 15-day sensors compare

Skin-worn sensors take the opposite approach and are replaced long before a year. The Dexcom G7 15 Day is labeled for up to 15 days plus a 12-hour grace period in adults 18 and older [2]; Abbott's FreeStyle Libre 3 Plus and 2 Plus sensors are labeled for up to 15 days [3].

Survival is where the curve bends. At day 10, 90.0% of G7 15 Day sensors were still running, but only 73.9% reached day 15, and 19.1% were ended early by the sensor's own self-diagnostics [2]. When the FDA's reanalysis set aside sensors that needed staff help or extra adhesive at insertion, or were pulled at a scheduled visit, survival through day 15 was 78.4% [2]. The review doesn't say how many early endings came from the tissue, so we don't read the drop as an FBR rate, and these studies weren't designed as a head-to-head comparison with Eversense.

What registry and long-term implant studies show

Most real-world data on repeated implants come from Eversense. A post-market registry in Europe and South Africa enrolled 3,023 people who had 5,417 sensors inserted between June 2016 and August 2018, about 1,260 patient-years of follow-up [9]. It recorded no device-related serious adverse events; the most common related events were infection at the sensor site (0.96%) and a sensor that couldn't be removed on the first attempt (0.76%) [9]. Full intended life was reached by 91% of 90-day sensors and 75% of 180-day sensors [9]. Two of the five authors were Senseonics employees [9].

Repeated insertions raise one more question. The FDA's 2018 summary lists scar tissue building up at the insertion site as a potential risk, though it judged that risk not expected to occur based on European post-market data and the trials [7].

Another approach designs around the capsule instead of fighting it. A University of California San Diego team built an enzyme sensor using differential oxygen detection, which the authors say makes it less sensitive to encapsulation; it ran continuously in two pigs for 222 and 520 days [10]. A first-generation prototype stayed implanted in people with diabetes for up to 180 days, with calibration drifting an average of -2.6% per week [11]. The authors held equity in GlySens, the company developing the device [10][11].

What is still unknown

In the records and studies we read, the year-long answer is a combination rather than a single fix: a steroid collar, oxidation-resistant chemistry and weekly calibration [4][5]. The 365's summary doesn't report how much dexamethasone a sensor still holds after a year, and the only elution figure we found covers 90 days [4][7]. Only 173 registry participants had used the system for at least a year when it was reported, so evidence on many years of repeated implants is still thin [9].