Xi Liu1  · Zhuoyi Wang1  · Zhi Qiao2  · Yujie Xiao3  · Hui Guo4  · Yousheng Shu1

1 Department of Neurology, Jinshan Hospital, Institute for Translational Brain Research, State Key Laboratory of Brain Function and Disorders, MOE Frontiers Center for Brain Science, Innovative Center for New Drug Development of Immune Inflammatory Diseases, Ministry of Education, Fudan University, Shanghai 200032, China 

2 China Unicom Shanghai Branch, China Unicom, Shanghai 200082, China 

3 College of Elementary Education, Capital Normal University, Beijing 100048, China 

4 Department of Functional Neurosurgery, Shanghai Deji Hospital (the 9th Clinical College, Qingdao University), Shanghai 200331, China

Abstract

Non-invasive infrared (IR) modulation requires IR photons to penetrate multi-layered cranial barriers, including skin, skull bone, and dura mater. However, the wavelength-dependent transmittance profiles and the modulatory effect of IR light on neural signals remain poorly understood. Using synchrotron radiation Fourier-transform infrared (SR-FTIR) microspectroscopy, we systematically mapped transmittance spectra of freshly isolated cranial tissue layers and brain tissues (gray matter and white matter) obtained from adult mice. The viability of brain tissue was maintained through microfluidic and oxygenated artificial cerebrospinal fluid (ACSF) perfusion during spectral acquisition. We identified two IR windows with relatively high transmittance through cranial tissues: a near-infrared (NIR) window (wavelength: 1.5–3.0 μm) and a mid-infrared (MIR) window (3.5–6.0 μm). Notably, the MIR sub-band ranging from 5.2 to 5.7 μm exhibited a relatively higher transmittance through neural tissue, particularly in axon bundles, compared to the surrounding bath solution. Further electrophysiological experiments revealed that MIR with a wavelength of 5.6 μm substantially decreased the amplitude and duration of both somatic and axonal action potentials, while simultaneously facilitating their conduction velocity along both myelinated and unmyelinated axons. Together, these results identify specific IR bands that can penetrate the cranial tissue layers and reveal a wavelength-specific modulation of neural signals, providing a promising non-invasive strategy for IR neuromodulation.

Keywords

Action potential; Light transmittance; Midinfrared light; Neuromodulation; SR-FTIR

[SpringerLink]