Two-photon lithography (TPL), a high-resolution 3D printing technique, is capable of producing nanoscale features smaller than 1/100 the width of a human hair. The technique could enable X-ray computed tomography (CT) to analyze stress or defects noninvasively in embedded 3D-printed medical devices or implants.
Two-photon lithography typically requires a thin glass slide, a lens, and an immersion oil to help the laser light focus to a fine point where curing and printing occurs. It differs from other 3D printing methods in resolution because it can produce features smaller than the laser light spot, a scale no other printing process can match. The technique bypasses the usual diffraction limit of other methods because the photoresist material that cures and hardens to create structures — previously a trade secret — simultaneously absorbs two photons instead of one.
The technique requires resist materials optimized for two-photon lithography, and forming 3D microstructures with features less than 150 nanometers. Previous techniques built structures from the ground up, limiting the height of objects because the distance between the glass slide and lens is usually 200 microns or less. By putting the resist material directly on the lens and focusing the laser through the resist, objects multiple millimeters in height can be printed. By tuning and increasing the amount of X-rays the photopolymer resists were able to absorb, attenuation was improved by more than 10 times over the photoresists commonly used for the technique.
Because the laser light refracts as it passes through the photoresist material, the linchpin to solving the puzzle was “index matching” — discovering how to match the refractive index of the resist material to the immersion medium of the lens so the laser could pass through unimpeded. Index matching opens the possibility of printing larger parts with features as small as 100 nanometers.
By tuning the material’s X-ray absorption, X-ray computed tomography can be used as a diagnostic tool to image the inside of parts without cutting them open, or to investigate 3D-printed objects embedded inside the body, such as stents, joint replacements, or bone scaffolds. These techniques also could be used for optical and mechanical metamaterials, and 3D-printed electrochemical batteries.