Micro/nano-fabrication technology via two-photon polymerization (TPP) nanolithography is a powerful and useful manufacturing tool that is capable of generating two dimensional (2D) to three dimensional (3D) arbitrary micro/nano-structures of various materials with a high spatial resolution. This technology has received tremendous interest in cell and tissue engineering and medical microdevices because of its remarkable fabrication capability for sophisticated structures from macro- to nano-scale, which are difficult to be achieved by traditional methods with limited microarchitecture controllability. To fabricate precisely designed 3D micro/nano-structures for biomedical applications via TPP nanolithography, the use of photoinitiators (PIs) and photoresists needs to be considered comprehensively and systematically. In this review, widely used commercially available PIs are first discussed, followed by elucidating synthesis strategies of water-soluble initiators for biomedical applications. In addition to the conventional photoresists, the distinctive properties of customized stimulus-responsive photoresists are discussed. Finally, current limitations and challenges in the material and fabrication aspects and an outlook for future prospects of TPP for biomedical applications based on different biocompatible photosensitive composites are discussed comprehensively. In all, this review provides a basic understanding of TPP technology and important roles of PIs and photoresists for fabricating high-precision stimulus-responsive micro/nano-structures for a wide range of biomedical applications.
The trapping response of patterned micro-magnets (PMMs) was studied based on the parameters affecting superparamagnetic beads in microfluidic channels. Using replica moulding and electroplating technologies, the PMMs were fabricated on the microchannel bottom, which generated sufficient magnetic forces to bias the moments of magnetic particles in a flowing stream. A simplified physical principle was used to analyse the relative velocity of the magnetic particle in the confined space of a microchannel. The results revealed that the magnetic force contributed to the fluidic flow rate as well as to the hydrodynamic drag force. The relative velocity of magnetic particles was dependent on the frequency under an external magnetic field driven by an alternate current (ac) source. It showed that the magnetic gradient induced hysteresis characteristics of the transmission spectrum, associated with the interaction of superparamagnetic beads and magnetic field.
The characteristics of high-power partially coherent laser beams propagating upwards in the turbulent atmosphere are studied, where the principal features of diffraction, nonlinear self-focusing and turbulence are considered. Based on the “thin window” model, the analytical propagation formulae are derived by using the quadratic approximation of the nonlinear phase shift. It is found that the turbulence effect plays an important role in beam propagation characteristics. But the turbulence and self-focusing effects can be suppressed by increasing the laser elevation. Furthermore, the influence of laser elevation on the turbulence effect is stronger than that on the self-focusing effect, and influence of laser elevation on the self-focusing effect is stronger than that on the diffraction effect. In particular, the optimal focal length and wavelength are proposed to decrease the beam spot size on the target.
The exhaust from engines close to the fuselage can cause extreme turbulence and have a significant impact on the performance of airborne laser systems. In this paper, the combined effect of the jet engine and atmospheric turbulence on propagation of a Gaussian Schell-model (GSM) array beam over long distances is studied in detail. Based on the two-step propagation method, the analytical expressions of the cross-spectral density (CSD) function, mean-squared beam width and spectral degree of spatial coherence (DOC) of GSM array beams are derived. It is shown that the jet engine turbulence has a significant effect on the beam broadening even at a very short distance. As the propagation distance in the jet engine turbulence increases, the beam width is less effected by the spatial coherence. Meanwhile, the resistance of lower spatially coherent beam to the turbulence increases as the distance of the jet engine turbulence or the strength of the atmospheric turbulence increases. It is demonstrated that the effect of jet engine turbulence on the beam spreading can be equivalent to moderate or even severe atmospheric turbulence. Moreover, the DOC is a function with oscillatory phenomenon when the propagation distance is short or the strength of turbulence is low.
The quasi-steady-state (QSS) self-focusing of partially coherent light pulses (PCLPs) in nonlinear media is studied. The analytical formulas of the QSS self-focusing of PCLPs in nonlinear media (e.g., the beam width, spatial coherence width, and focal length) are presented. The effect of spatial coherence on the focal length and focus moving is investigated in detail. In particular, it is found that a PCLP has more advantages to avoid the optical damage of materials than a fully coherent light pulse.
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