Mid-infrared photoacoustic polarization uncovers fiber alignment in heart tissue

A research team at POSTECH has developed a new imaging technique that can analyze the structural health of tissues, such as the heart and tendons, without any staining. The method quantitatively measures the alignment and organization of protein fibers, offering a novel approach for diagnosing fibrosis, evaluating engineered tissues, and advancing regenerative medicine.

 

The research was conducted by Professor Chulhong Kim (Department of Electrical Engineering, Department of Convergence IT Engineering, Department of Mechanical Engineering, Department of Medical Science and Engineering, Graduate School of Artificial Intelligence) and Professor Jinah Jang (Department of Mechanical Engineering, Department of Convergence IT Engineering, Department of Medical Science and Engineering), along with doctoral candidate Eunwoo Park (Department of Convergence IT Engineering) and Dr. Dong Gyu Hwang (the Center for 3D Organ Printing and Stem Cells). The findings were published in the international optics journal, Light: Science & Applications.

 

Healthy biological tissues such as cardiac muscle rely on highly aligned protein fibers to maintain mechanical strength and function—similar to how tightly twisted strands strengthen a rope. However, in conditions such as myocardial infarction, fibrosis, or cancer, this alignment deteriorates, leading to structural disorganization and tissue malfunction. Detecting such microscopic changes is essential, but traditional histological and immunofluorescent staining methods are labor-intensive, antibody-dependent, and prone to inconsistent, limiting objective assessment.

 

To overcome these limitations, the POSTECH team developed a mid-infrared dichroism-sensitive photoacoustic microscopy (MIR-DS-PAM), a label-free imaging technique that reveals both chemical composition and structural anisotropy in tissue. When tissue is illuminated with mid-infrared light, proteins absorb specific wavelengths according to their molecular bonds. By adding polarization control to this process, MIR-DS-PAM detects vectorial absorption linked to fiber alignment, enabling quantitative analysis of microstructural organization.

 

The team demonstrated the technique using engineered heart tissues. As the tissue matured, MIR-DS-PAM detected increasing protein accumulation and progressive alignment in extracellular matrix proteins, particularly collagen fibers. Furthermore, in fibrosis models, the system clearly distinguished healthy tissue with organized fibers from diseased tissue with disrupted architecture, achieving a strong correlation with fluorescence microscopy while eliminating the need for dyes or labeling.

 

Professor Chulhong Kim remarked, “MIR-DS-PAM provides reliable and quantitative structural information in a label-free manner, comparable to fluorescence microscopy.” Professor Jinah Jang added, “This technique will greatly accelerate research in engineered tissues and disease modeling as it allows comprehensive tissue evaluation.”

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