Silk patterns made by direct femtosecond laser writing

Abstract

Silk patterns in a film of amorphous water-soluble fibroin are created by tailored exposure to femtosecond-laser pulses (1030 nm/230 fs) without the use of photo-initiators. This shows that amorphous silk can be used as a negative tone photo-resist. It is also shown that water insoluble crystalline silk films can be precisely ablated from a glass substrate achieving the patterns of crystalline silk gratings on a glass substrate. Bio-compatible/degradable silk can be laser structured to achieve conformational transformations as demonstrated by infrared spectroscopy.

FIG. 1.

Optical profilometer surface mapping of an amorphous silk fibroin film after the laser exposure of 10 μm period grating pattern followed by the water development: top (a) and cross-sectional (b) views. Laser exposure conditions: pulse energy E_p = 20 nJ, overlap of N = 10⁵ pulses/mm at repetition rate of 100 kHz, NA = 0.26. Inset in (b) shows a typical AFM 3D surface profile; E is orientation of the linear polarization.

FIG. 2.

(a) FTIR absorbance spectra of silk fibroin films: (1) crystallized after methanol-ethanol treatment, (2) amorphous water-soluble, (3) after laser irradiation, and (4) after laser irradiation and development in water. Color of the wavenumber values that mark spectral features denote which secondary structure element they are associated with. (b) Dependence of the intensity of amide-I band after laser irradiation and water development from the irradiation fluence. Laser exposure conditions: base irradiation fluence F_th = 81 mJ/cm², pulse density 100 pulses/μm (20 pulses overlapping) at a repetition rate of 100 kHz, using a NA = 0.5 lens. FTIR spectra recorded from 500 × 500 μm silk patterns on a CaF₂ substrate on a Vertex70 Bruker FTIR microscope in the transmission mode.

FIG. 3.

AFM image of a laser crystallized silk and a cross sectional height profile. Laser exposure conditions: pulse energy E_p = 20 nJ, pulse density 100 pulses/μm (20 pulses overlapping) at a repetition rate of 100 kHz, NA = 0.26 lens. Conditions are for the onset of glass ablation.

FIG. 4.

Topology of the crystalline silk film laser ablated (a) and subsequently water washed (b). Laser exposure conditions: pulse energy E_p = 22 nJ, pulse density 100 pulses/μm (20 pulses overlapping) at a repetition rate of 100 kHz, NA = 0.26 lens.