Abstract
Magnetic particle imaging (MPI) is a tomographic imaging technique which determines the spatial distribution of magnetic nanoparticles (MNPs). Multi-contrast MPI provides the ability to detect environmental conditions of MNPs, such as temperature or viscosity. One parameter that has not been investigated but shows high potential for medical diagnosis is the pH value, as it is an indicator of inflamed or tumorous tissue. In this work, we present an approach to resolve the pH value using multi-contrast MPI. Our proof-of-concept is based on a stimuli-responsive, magnetic hydrogel that exhibits reversible swelling in response to a pH change. The pH contrast is generated indirectly via the pH-responsive hydrogel swelling modulating the signal of embedded MNPs. Magnetic particle spectrometry measurements show that the hydrogels’ magnetic response correlates with the pH value, which could provide a new way of contactless pH monitoring. Finally, the feasibility of resolving different pH values in a multi-contrast MPI image is demonstrated.
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Data availability
All the data needed to evaluate the findings in this article are present in the article and/or in the Supplementary Information. Additional data related to this paper can be requested from the corresponding authors via email.
Code availability
The code used to perform the MPS measurements is publicly available on GitHub. You can access the code repository via the following link: https://github.com/MagneticParticleImaging/MPIMeasurements.jl. The code used for multi-contrast image reconstruction can also be accessed via the following link: https://github.com/MagneticParticleImaging/MPIReco.jl. The acquired MPI dataset and detailed reconstruction parameters are available upon reasonable request.
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Acknowledgements
Research funding: The main work has been done at the Fraunhofer IMTE, which is supported by the EU (EFRE) and the State Schleswig-Holstein, Germany (Projects: IMTE - Grant: LPW-E/1.1.1/1536, 124 20 002 and IMTE 2 - Grant: LPW21-L/2.2/262, 125 24 009). For finalization, the work was additionally funded by the DFG project “harmoMPI” (project no. 540326102) at PTB.
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B.K., J.A., and A.B. contributed to the conceptualization of the working principle. B.K. developed the hydrogel patch fabrication and prepared the samples. B.K. and J.A. performed magnetic characterization measurements of the samples. B.K., J.A., and M.G. performed evaluation of magnetic characterization. B.K., J.A., and A.B. contributed to experiment planning and execution. A.B. and M.G. supervised the project. B.K., J.A., and A.B. contributed to writing the paper. All authors reviewed the manuscript.
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Communications Engineering thanks Satoshi Ota and the other anonymous reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: [Jing Zhong] and [Rosamund Daw].
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Kluwe, B., Ackers, J., Graeser, M. et al. Multi-contrast magnetic particle imaging for tomographic pH monitoring using stimuli-responsive hydrogels. Commun Eng (2026). https://doi.org/10.1038/s44172-026-00586-8
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DOI: https://doi.org/10.1038/s44172-026-00586-8
