Materials scientists at Friedrich Schiller University Jena have developed a mechanoluminescent material that not only allows them to generate local heat input using ultrasound, but also provides feedback on the local temperature.
Excite light emission and measure temperature with ultrasound [1]
If mechanoluminescent materials are subjected to external mechanical stress, they emit visible or invisible light. This type of excitation can be caused by buckling or gentle pressure, for example, but also completely contact-free via ultrasound. In this way, the effect can be triggered remotely and light can be brought to places that are normally in the dark - for example in the human body.
If the ultrasound treatment is to be used at the same time to generate heat locally, it is important to closely monitor the resulting temperatures in such a sensitive environment.
Scientists from Jena have now succeeded in developing such a mechanoluminescent material. Using ultrasound, it can generate a local heat input and provide feedback on the temperature on site [2].
During their work, the Jena scientists often focus on the mechanical properties of inorganic materials, in particular how mechanical processes can be observed optically. Mechanically induced light emission can provide many details about the response of a material to mechanical stress. However, in order to expand the field of applications, it is sometimes necessary to obtain additional information about the temperature prevailing locally during the load - especially when the excitation is carried out using ultrasound. Initially, the scientists were simply interested in sensor materials in the form of extremely fine particles which - when placed in an environment to be investigated - can both act on their environment through external ultrasonic excitation and also feed back information about this effect.
To this end, the Jena scientists have combined an oxysulphide semiconductor with the rare earth erbium oxide. The semiconducting structure then absorbs the mechanical excitation through ultrasound - the erbium oxide provides the light emission. The temperature can then be read from the spectrum of the emitted light using optical thermometry. This provides full control over the temperature development in the material, which can also be influenced by the ultrasound. An increase in temperature can be stimulated from the outside, measured by the light emission and a complete control circuit established.
The remote-controlled light emission combined with temperature control opens up completely new areas of application for such mechanoluminescent materials, for example in medicine. One possible field of application could be photodynamic therapy, in which light is used to control photophysical processes that can support the organism in healing. Using multi-responsive mechanoluminescent materials in the form of extremely fine particles, light and heat could not only be generated at a desired location, but also controlled in a targeted manner. As biological tissue is transparent to the emitted infrared light, a desired temperature can be set and controlled from the outside during treatment.
Other applications in which light and heat are to be directed to dark places are more obvious. For example, photosynthesis or other light-driven reactions could be specifically triggered, observed and controlled. Likewise - back to the beginning - the material can be used as a sensor for generating or observing material changes or as an invisible, coded marker on material surfaces.
Ultrasound to support local anesthesia in clinics [3]
In local anesthesia, an anesthetic is injected under the skin or directly into the tissue. A tube with a metal needle inside is inserted into the region where the medication is to be administered. To make this process even more precise and minimally invasive, the Institute for Applied Biopolymer Research (ibp) at Hof University of Applied Sciences is researching a micro-tube that is visible in ultrasound and is intended to make the work of anaesthetists in hospitals much easier. The plastic tube should be clearly visible in ultrasound thanks to innovative microstructures.
The challenge with local anesthesia is that the plastic tube is not visible using ultrasound, which makes it difficult to position the tube precisely once the metal needle has been removed. Currently, this shortcoming is still compensated for by administering larger quantities of anesthetics or using more expensive X-ray procedures with contrast agents, which can lead to side effects.
The aim of the project is to develop a new plastic tube that is clearly visible in ultrasound thanks to innovative microstructures. To this end, the microstructure in the polymer, on the surface and at the tip of the tube is to be modified. This will significantly improve ultrasound visibility, which will make it much easier for the anaesthetist to position the tube and the low-friction guidance of the tube will protect the tissue.
The first step of the project is to specify the requirements profile and application-specific specifications for biocompatibility, i.e. the compatibility between natural human tissue and the material. This will be followed by a test setup and initial in-vitro tests to assess ultrasound visibility. Followed by further research results, the functional samples produced will finally be examined under real conditions of use, including final tests to examine the shelf life and durability of the microstructures.
Traveling ultrasonic waves as a drive [4]
Microscopically small nanomachines that move like submarines with their own propulsion - for example in the human body, where they transport active substances and release them in a targeted manner: What sounds like science fiction has become an increasingly fast-growing field of research over the past 20 years. However, most of the particles developed to date only work in the laboratory. Propulsion, for example, is a hurdle: some particles have to be supplied with energy by light, others use chemical propulsion that releases toxic substances. Neither of these is suitable for use in the body. One solution to the problem could be acoustically driven particles. Johannes Voß and Prof. Dr. Raphael Wittkowski from the Institute of Theoretical Physics and Center for Soft Nanoscience at the University of Münster have now clarified key questions that have so far stood in the way of the application of acoustic propulsion (Fig. 1) [5].
Fig. 1: Acoustic drive for nanomachines depends on their orientation. Münster physicists simulate for the first time the propulsion of freely orientable nanoparticles by traveling ultrasonic waves / Study in "ACS Nano"
Ultrasound is used in acoustically driven nanomachines because it is harmless for applications in the body. In many of the existing publications on the use of ultrasound to drive nanomachines, the particles in the experiments have almost always been exposed to a standing ultrasonic wave. Although this makes the experiments much simpler, it is not very meaningful with regard to possible applications. This is because traveling ultrasonic waves would be used there. Standing waves are generated when waves traveling in opposite directions are superimposed. This is rarely practicable in real applications.
Furthermore, research to date has not taken into account the fact that the particles can move in any direction in applications, thus ignoring the question of whether the drive depends on the orientation of the particles. Instead, it only focused on those particles that are aligned perpendicular to the ultrasonic wave. The Münster research team has now investigated the effects of orientation for the first time using complex computer simulations. The result of the investigations is that the propulsion of the nanoparticles depends on their orientation. At the same time, the acoustic propulsion mechanism works so well with traveling ultrasound waves for all orientations of the particles, i.e. not just exactly perpendicular to the ultrasound wave, that these particles can actually be used for biomedical applications. As a further aspect, the WWU physicists have investigated the propulsion of the particles when they are exposed to ultrasound coming from all directions ("isotropic ultrasound"). This type of ultrasound is also relevant for some potential applications.
The results have shown how the particles will behave in applications and that the propulsion has the right properties to actually be able to use the particles in these applications. Important properties of acoustically driven nanoparticles were revealed that had not previously been investigated, but which need to be understood in order to make the step from basic research to the planned applications of the particles possible.
The physicists at the University of Münster investigated cone-shaped particles, as these can move quickly even at low ultrasound intensity, i.e. they have efficient propulsion and can also be easily produced in large numbers. The particles are just under a micrometer in size, i.e. just under a thousand nanometres. The particles could therefore move through the bloodstream without clogging the fine blood vessels. The size of the particles can be selected according to the requirements of the intended application; their propulsion mechanism also works with smaller and larger particles. The particles were simulated in water in the studies, but the drive is also suitable for other liquids and tissues. With the help of computer simulations, the team investigated systems and their properties that could not be studied in the many previous experimental studies.
Literature
[1] Otto Schott Institute for Materials Research at Friedrich Schiller University Jena
[2] Y. Ding; B. So; J. Cao; L. Wondraczek: Ultrasound-induced mechanoluminescence and optical thermometry toward stimulus-responsive materials with simultaneous trigger response and read-out functions, Advanced Science, DOI: 10.1002/advs.202201631
[3] Institute for Applied Biopolymer Research (ibp) at Hof University of Applied Sciences
[4] Westfälische Wilhelms-Universität Münster
[5] J. Voß; R. Wittkowski: Orientation-dependent propulsion of triangular nano- and microparticles by a traveling ultrasound wave, 2022, ACS Nano, DOI: 10.1021/acsnano.1c02302
