The new tattoo-like sensor made with Fibroin can read the oxygen levels in the blood

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People get tattoos to remember an event or a person, to make a statement, or simply as an aesthetic embellishment. But imagine a tattoo that could be functional – telling you how much oxygen you are using when exercising, measuring your blood glucose level at any time of day, or monitoring a number of different blood components or exposure to environmental toxins.

Now engineers at Tufts University have taken an important step toward making that happen with the invention of a silk-based material placed under the skin that glows brighter or dimmer under a lamp when exposed to different levels of oxygen in the blood. They reported their findings in Advanced Functional Materials.

Degradable Silk‐Based Subcutaneous Oxygen Sensors - Falcucci - - Advanced  Functional Materials - Wiley Online Library

The novel sensor, which currently is limited to reading oxygen levels, is made up of a gel formed from the protein components of silk, called fibroin. The silk fibroin proteins have unique properties that make them especially compatible as an implantable material.

When they are re-assembled into a gel or film, they can be adjusted to create a structure that lasts under the skin from a few weeks to over a year. When the silk does break down, it is compatible with the body and unlikely to invoke an immune response.

Substances in the blood such as glucose, lactate, electrolytes, and dissolved oxygen offer a window into the body’s health and performance. In health-care settings, they are tracked by drawing blood or being attached to bulky machines. Being able to continuously monitor their levels noninvasively in any setting could be a tremendous advantage when tracking certain conditions.

Diabetics, for instance, have to draw blood to read glucose, often on a daily basis, to decide what to eat or when to take medication. By contrast, the vision mapped out by the Tufts team is to make monitoring much easier, literally by shining a light on a person’s condition.

“Silk provides a remarkable confluence of many great properties,” said David Kaplan, Stern Family Professor of Engineering in the Tufts University School of Engineering, and lead investigator of the study. “We can form it into films, sponges, gels and more. Not only is it biocompatible, but it can hold additives without changing their chemistry, and these additives can have sensing capabilities that detect molecules in their environment. The oxygen sensor is a proof of concept for a range of sensors we could create.”

The small disc of a silk film oxygen sensor glows purple when exposed to UV light and oxygen. A detector can determine the level of oxygen by the brightness and duration of the purple glow. Right side: side-by-side comparison of normal and UV-exposed silk sensor material. (Image: Thomas Falcucci, Tufts University)

The chemistry of the silk proteins makes it easier for them to pick up and hold additives without changing their properties. To create the oxygen sensor, the researchers used an additive called PdBMAP, which glows when exposed to light of a certain wavelength. That glow has an intensity and duration proportional to the level of oxygen in the environment.

The silk gel is permeable to the fluids around it, so the PdBMAP senses the same oxygen levels in the surrounding blood. PdBMAP is also useful because it glows, or phosphoresces, when exposed to light that can penetrate the skin. Other sensor candidates may only respond to wavelengths of light that cannot penetrate the skin.

The researchers rely more on the duration component of phosphorescence to quantify oxygen levels because intensity of the glow can vary with the depth and size of the implant, skin color, and other factors. The duration of the glow decreases as levels of oxygen increase.

Degradable Silk‐Based Subcutaneous Oxygen Sensors - Falcucci - - Advanced  Functional Materials - Wiley Online Library

In experiments, the implanted sensor detected oxygen levels in animal models in real-time, and accurately tracked high, low, and normal levels of oxygen. The importance of being able to track oxygen levels in patients has grown in public awareness with the COVID-19 pandemic, in which patients had to be admitted for hospital treatment when their oxygen levels became critically low.

“We can envision many scenarios in which a tattoo-like sensor under the skin can be useful,” said Tom Falcucci, a graduate student in Kaplan’s lab who developed the tattoo sensor. “That’s usually in situations where someone with a chronic condition needs to be monitored over a long period of time outside of a traditional clinical setting. We could potentially track multiple blood components using a sensor array under the skin.”

The tattoo-like sensors are the latest in a growing portfolio of potential medical products derived from silk protein in Kaplan’s lab, from orthopedic implants to scaffolding for creating new tissue in the heart and bones.


Working Mechanism of Unintrusive Optical Sensing Devices

Unintrusive biomedical measures are often conducted optically, with a light source of a certain wavelength (λ) being revealed to the area of the skin where the evaluation is desired [63]. The sensing device detects reflected and absorbed light, as well as refracted light, and then characterizes and quantifies the biological data (identical sensations as employed by spectrophotometer).

When transmitting an optical signal through the skin, the λ is the most important component, since it controls how far the light can penetrate. Depending on the required penetration depth and substantial absorption peak for the relevant sensing application, the λ of these light sources can range from UV to deep IR.

The detectors range from broadband photodiodes (PDs) to avalanche photodetectors and photomultiplier tubes. Several illustrations of related passive devices for light capture, λ selection, and light steering are integrated optics, diffraction gratings, narrowband optical filters, and bulk lenses.

The skin may also be employed as a window to see how the hidden organs are doing physiologically. The use of functional near-IR spectroscopy (fNIRS) to study oxygenation variations in the human brain is one such approach [64]. The current high-temporal-resolution multichannel systems simultaneously perform numerous measurements and display the findings in the form of a map or picture across a specified cortical area, employing three separate NIRS methods and complicated data analysis tools.

The implementation of multichannel wearable/wireless devices that allow fNIRS measurements even during regular everyday activities represents the promise that exists for fNIRS more than for any other neuroimaging modality, as shown in Figure 4.

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Figure 4
Multichannel fNIRS instrument.

Through its optical interfaces, the skin can sometimes provide a passive conduit for physiological data collecting from hidden vascular systems and organs. As a result, when constructing wearable optical sensing devices, it is critical to consider the skin’s optical characteristics [65]. The human skin’s absorption, transmission, and scattering may be studied by separating the skin into three layers with different optical characteristics:

  • (1)The stratum corneum, which is extremely keratinized owing to the presence of dead squamous cells.
  • (2)The hidden epidermis, which comprises skin pigmentation (mostly melanin) that absorbs shorter λ, such as UV, and visible (VIS) light to some extent [66].
  • (3)The dermis, which is extremely vascularized and can be described through VIS light and contains carotene, blood hemoglobin, and bilirubin [67]. Because of its thickness relative to the layers above, the dermis attenuates most of the VIS light, as seen in Figure 5.
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Figure 5
At different sites in the skin layers, incident light displays reflection, absorption, and scattering effects. In terms of λ, light penetrates into the skin [68].

Device geometries are determined by the application and evaluation sites on the skin. The light source is mounted opposite the detector in most hard-wired arrangements, as well as traditional wireless devices. This setup guarantees that the detected light interacts with the target tissue across a long optical channel length, resulting in high signal attenuation for pulsatile change extraction [69].

This geometry has the problem of being limited to relevant parts of the anatomy, such as the finger or ear lobe, and it does not provide a simple method for system downsizing [70]. Backscattered reflection approaches allow the light source and detector to be placed near one another in the same plane. As a result, evaluations may be taken via interfaces to practically any part of the body, with minimal downsizing and wireless operation.

Measurements in the reflectance mode, on the other hand, are prone to motion artefacts [71]. In this case, parasitic noise is created by tiny variations in the relative placement of the optical modules to the probing volume. In this context, digital and analog filtering techniques can be useful [72], and systematic compensatory methods that use accelerometers as motion sensors can offer considerable benefits, albeit at the cost of increased device complexity.

As a result, traditional gear for PPG reflection mode measurements is often big and heavy, particularly when wireless operation and power supply are involved. There are additional difficulties in harmonizing the total power utilization and total size of the system with the measurement’s signal to noise ratio, where the device current for the light source and the distance between the source and the detector are essential characteristics [73].

reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8838083/


More information: Thomas Falcucci et al, Degradable Silk‐Based Subcutaneous Oxygen Sensors, Advanced Functional Materials (2022). DOI: 10.1002/adfm.202202020

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