UCL photoacoustic imaging maps microplastics in living tissue

A project at University College London has developed a new approach to the detection of microplastics within living tissues and ecological systems, a key question in modern environmental health.

Current methods of spotting and quantifying the presence of microplastics in biological samples are typically invasive and autopsy-based, noted the project, limiting the scope of sampling and restricting sample collection. Optical microscopy can be applied in certain cases, for example with small transparent organisms, but does not translate easily to larger optically opaque animals.

As published in Advanced Science, the UCL breakthrough involves the use of photoacoustic imaging, the versatile modality whereby pulses of laser light directed into tissue are absorbed by specific target molecules and generate tiny high frequency sound waves. In the case of microplastics, detecting these sound waves with ultrasound detectors can then create a detailed map showing where microplastics are located within the body.

This study marks the first time that the native optical absorption properties of microplastics have been exploited to generate photoacoustic signals, commented the project.

"Everyone  on earth is exposed to microplastics," said Stephen Patrick from the UCL Centre for Advanced Biomedical Imaging. "They are found everywhere: in our food, drink, clothing and home furnishings. There is growing concern over their effects on human health, which until now has been difficult to study inside living tissue. Most existing methods rely on biopsies or analysis of tissue after dissection, which limits what researchers can observe over time."

Alongside its imaging technique, the project also developed a new methodology for modelling environmental microplastics in lab trials. Most published lab studies use commercial polystyrene microspheres, when polystyrene is not the most abundant contaminant in the environment or the human body. 

UCL therefore developed a new technique to synthesize microplastics from more representative consumer plastic sources, using a sandpaper-based method to mimic environmental degradation. 

Linking exposure to long-term health effects

In trials, mice were given controlled amounts of microplastics by injection - around half a milligram per experiment, so the researchers could precisely track how the particles moved through living tissue over time. As with humans, the animals were also likely to already have low background levels of microplastics from food and drinking water.

"To test the ability of photoacoustic imaging to detect microplastics over extended periods in vivo, imaging was performed before and after subcutaneous injection of 0.5 mg of black polypropylene particles using wavelengths between 600 and 680 nanometers," wrote the team. 

"The tissue surrounding the injected region was removed after two months and cryo-sectioned for histology. The injected mass of particles was found in the sub-muscular region on stained sections, in line with their appearance on the photoacoustic images."

By showing that microplastics can be visualised inside living tissue without altering or destroying it, this work lays important groundwork for future studies, commented UCL, and a photoacoustic approach could also give valuable information about other plastics present in the living body.

Surgical implants such as hernia meshes could be a particular focus, due to their frequent mechanical failure, side effects, and need for replacement. UCL is following up its microplastics study with further research that aims to improve patient outcomes and the safety of these implanted devices.

"Since the photoacoustic signal is directly related to the amount of microplastic, our method could overcome the limitations of existing indirect methods of estimating microplastic accumulation," said researcher Olumide Ogunlade. "We anticipate it will ultimately help researchers link everyday exposure to microplastics with long‑term health effects, in a way that better reflects what happens in real life."