Bioresorbable materials are those that, over time, can be completely broken down by the human body without leaving behind harmful residue. While commonly utilized in the form of dissolvable stitches, there is great interest in the potential for implanting more complicated, dissolvable medical devices that would need no secondary surgery to remove them once their purpose has been fulfilled. One such application lies in optical fibres for time-domain diffuse optical spectroscopy – an imaging method in which the scattering of pulsed light is used to characterize the composition and microstructure of tissues.

In their new study, physicist Alberto Dalla Mora of the Politecnico di Milano and colleagues have created the first bioresorbable fibres suitable for such diffuse imaging applications. They have tested their fibres' potential in the recovery of optical information, both within a tissue-simulating phantom as well as an ex vivo chicken breast, finding them suitable for use in time-domain diffuse optical spectroscopy and equivalent to a state-of-the-art system with standard glass fibres.

Made using preform drawing to form a connected core and cladding with up to 200 nm in core diameter, the new fibre is composed of calcium phosphate glasses. This class of material offers good biocompatibility without the batch-to-batch variability that is commonly found in biologically-derived materials. If produced large-scale, the researchers anticipate manufacturing costs comparable to, or potentially lower than, that of equivalent silica fibres, which require higher formation temperatures.

While resorbable fibres have previously been developed using other materials – including hydrogels, polymers and silks – the new calcium phosphate glass fibres reduce attenuation between one and eight orders of magnitude, making them the first that are suitable for optical imaging. Furthermore, the new fibre design supports a wider operating window, covering from the ultraviolet to the infrared (250 mm to above 2 µm.)

When placed in a simulated biological fluid, the fibres dissolve completely in around three weeks – and "by adjusting the chemical composition of the glass, we are able to reduce or increase the time for reabsorption while keeping the biocompatibility," Dalla Mora explains.

The fibres could be used in varied applications to monitor physiological processes occurring at depth. These, Dalla Mora says, could include "direct monitoring of the healing process at depth or of the oxygenation of deep tissue implants, the status of the brain after traumatic injuries, etc." A further application could lie in the delivery of photodynamic and photothermal therapies.

With this initial study complete, the researchers are now working to validate the use of their fibres on animal models – after which they will be moving to human trials. The team also believe that they can adapt their fibres to create a single bioresorbable rod combining both optical waveguides with a capillary to deliver either drugs or photosensitizers for cancer therapies.

Hot topic

"Recently, implantable and further bioresorbable devices – including both electronic and optical ones – are gaining more interest, as no secondary surgeries are needed. The field is quite hot," says Hu Tao, a mechanical engineer at the University of Texas at Austin who was not involved in this study. He adds: "The key arguably lies in identifying appropriate applications (such as the bioresorbable optical fibres in this work) and more importantly the controlled degradation behaviours."

"Time-domain diffuse optical spectroscopy is a very promising technique for diagnostics, being able to investigate biological tissues at depth," comments Chiara Vitale-Brovarone, a bioengineer at the Politecnico di Torino who was also not involved in this study. The development of bioresorbable fibres, she adds, "brings diffuse spectroscopy closer to its application in brain monitoring and intravital diagnostics and theranostics."

Vitale-Brovarone concludes: "I believe the marriage of these two technologies - fibres and spectroscopy – stemming from this seed paper will have outcomes that represent a leap forward to the use of time-domain diffuse optical spectroscopy in a clinical environment, also carrying the potential to develop compact and wearable devices for point-of-care diagnostics down to the consumer level."

With recent developments in the miniaturization of diffuse spectrographic instruments, this is a hope echoed by the researchers. In the future, Dalla Mora suggests, "maybe after surgical interventions we will carry small boxes, similar to Holter monitors, connected to bioresorbable optical fibres inserted in our bodies."

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