| RESEARCH HIGHLIGHTS |


channels them into the output beam. This simultaneously ensures that the output can be adjusted to a specific wavelength while generating ultrafast pulsed light. They also used bidirectional pumping — injecting energy into the gain medium from both ends of the fiber — to ensure a high optical power over as wide a range of wavelengths as possible. The gain occurs when thulium ions


are excited to higher-energy states; they then release more photons when they return to lower-energy states. “This is the state-of-the-art, widely tunable


all-fiber laser with pulsed output at this wavelength,” says Yu. “We have shown that every parameter, from the pumping scheme to the use of nonlinear polarization evolution, is critical to the final output.”


Yu’s team believe that their simple, inexpen-


sive and compact laser could one day be used in combination with high-power amplifiers to generate other forms of laser, including extreme ultraviolet and soft X-ray beams.


1. Yan. Z, Sun, B., Li, X., Luo, J., Shum, P. P. et al. Widely-tunable Tm-doped mode-locked all-fiber laser. Scientific Reports 6, 27245 (2016).


Drug delivery:


POLYMER SCAFFOLDS BUILD A BETTER PILL TO SWALLOW


NANOPARTICLE DRUGS CAN MAKE IT EASIER FOR MEDICATIONS TO REACH THEIR TARGETS


Nanoscale, cross-linked polymer scaffolds (pic-


tured) can help deliver a surprisingly high amount of drugs with poor water solubility to aqueous targets.


The huge doses of drugs required to combat cancer could be reduced thanks to the work of A*STAR researchers, and the drugs themselves may become more effective. The researchers have developed a polymeric ‘scaffold’ that helps drugs that often have trouble entering the bloodstream, such as anti-cancer agents, form highly stable nanoparticles with improved bioavailability1. Many medications that target tumor


cells are made from water-repelling hydrocarbon molecules, which require extra processing or high doses rates to enter aqueous biological environments. A safer alternative is to ‘nanosize’ pharmaceuticals into 10 to 1,000 nanometer particles using either mechanical grinding or special crystallization techniques. These extra-small medications easily slip into water and are effective against tumors, but it is hard to prevent them from agglomerating into


16 A*STAR RESEARCH


larger precipitates with less potency. Ulrike Wais and Alexander Jackson from


the A*STAR Institute of Chemical and Engineering Sciences and Haifei Zhang at the University of Liverpool have developed a way to lessen agglomeration problems by using poly(ethylene glycol) and acrylamide (PEG-PNIPAM) — biocompatible polymers that are highly water soluble and can stabilize water-repelling molecules because they have similar surfactant-like hydrocarbon chains. The team synthesized PEG-PNIPAM


into ‘hyperbranched’ spheres that are reinforced with short carbon cross-linking molecules. They then mixed the spheres with test drug compounds such as ibuprofen and blended them together to create an emulsion between the water-repelling and water-attracting components.


“THE DRUGS AND POLYMER SPHERES HAD INTEGRATED INTO A POROUS, SCAF- FOLD-LIKE STRUCTURE.”


The next step required a way to freeze-dry


the emulsion so it could be pulverized into nanoparticles, but this involved solving a tricky processing problem. “If phase separation occurs before the sample is completely frozen, drug crystals form that are neither nanosized nor stabilized against agglomeration by the scaffold,” explains Wais. The researchers prevented phase separation


during freeze-drying by ensuring the emulsification was extremely uniform before spraying it as tiny droplets into a pool of liquid nitrogen. Dynamic light scattering and


ISSUE 5 | OCTOBER – DECEMBER 2016


Reprinted from Ref. 1, Copyright 2016, with permission from Elsevier.


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