Bioinspired light-driven soft robots based on liquid crystal polymers.

Nature is a constant source of inspiration for materials scientists, fueling the dream of mimicking life-like motion and tasks in untethered, man-made devices. Liquid crystalline polymers (LCPs) programmed to undergo three-dimensional shape changes in response to light are promising materials for fulfilling this dream. The successful development of autonomous, highly controlled light-driven soft robots calls for an understanding of light-driven actuation, advancements in material function and performance, and progress in engineering principles for transforming actuation into life-like motions, from simple bending to walking, for example. This tutorial review includes an introduction to liquid crystal (LC)-based materials and highlights developments in light-responsive LC polymers, shape programmability and sustained motions to finally achieve bioinspired untethered soft robots able to perform locomotion and tasks.

Monocrystalline silicon photovoltaic luminescent solar concentrator with 4.2% power conversion efficiency.

We report conversion efficiencies of experimental single and dual light guide luminescent solar concentrators. We have built several 5 cm × 5 cm and 10× cm × 10 cm luminescent solar concentrator (LSC) demonstrators consisting of c-Si photovoltaic cells attached to luminescent light guides of Lumogen F Red 305 dye and perylene perinone dye. The highest overall efficiency obtained was 4.2% on a 5 cm × 5 cm stacked dual light guide using both luminescent materials. To our knowledge, this is the highest reported experimentally determined efficiency for c-Si photovoltaic-based LSCs. Furthermore, we also produced a 5 cm × 5 cm LSC specimen based on an inorganic phosphor layer with an overall efficiency of 2.5%.

Electron paramagnetic resonance evidence for a C3' sugar radical in crystalline d(CTCTCGAGAG) X-irradiated at 4 K.

Debije, M. G. and Bernhard, W. A. Electron Paramagnetic Resonance Evidence for a C3' Sugar Radical in Crystalline d(CTCTCGAGAG) X-Irradiated at 4 K. Radiat. Res. 155, 687-692 (2001). A neutral sugar radical formed by the net loss of hydrogen from C3' has been identified in crystalline DNA X-irradiated at 4 K. Crystals of duplex d(CTCTCGAGAG), known to be of B conformation, were studied using electron paramagnetic resonance (EPR) spectroscopy. The C3' radical was identified by using information from dose saturation, power saturation, thermal annealing, and spectrum simulation. The yield of the C3' radical, G(C3'), is 0.03 +/- 0.01 micromol/J, and its concentration does not appear to saturate up to at least 100 kGy. In the region in which total radical concentration increases linearly with dose, the C3' radical makes up about 4.5% of the total radical population trapped in the oligodeoxynucleotide crystal at 4 K. Based on free base release measured in other oligodeoxynucleotides, we suggest that in d(CTCTCGAGAG) the C3' radical is responsible for about one-third of the strand breakage events.

On the efficiency of hole and electron transfer from the hydration layer to DNA: An EPR study of crystalline DNA X-irradiated at 4 K.

The aim of this project was to gain an improved understanding of how the efficiency of hole and electron transfer from the solvation layer to DNA decreases as a function of distance from DNA. The packing of DNA in crystals of known structure makes it possible to calculate the degree of DNA hydration with a precision that is significantly greater than that achievable for amorphous samples. Previous work on oligodeoxynucleotide crystals has demonstrated that the efficiency of free radical trapping by DNA exposed to ionizing radiation at 4 K is relatively insensitive to base sequence, conformation, counterion, or base stacking continuity. Having eliminated these confounding variables, it is now possible to ascertain the degree of radical transfer that occurs from ionized water as a function of DNA hydration (Gamma, in mol water/mol nucleotide). EPR is used to measure the hydroxyl radical concentration in crystals irradiated at 4 K. From a lack of hydroxyl radicals trapped in the inner hydration mantle, we determine that hole transfer to DNA is complete for water molecules located within 8 A. This corresponds to Gamma = 9-11 and indicates that hole transfer is 100% (as efficient as direct ionization of DNA) for water molecules adjacent to DNA. Beyond approximately 8 A (Gamma > 10), hydroxyl radicals are observed; thus deprotonation of the water radical cation is seen to compete with hole transfer to DNA as soon as one water intervenes between the ionized water and DNA. The boundary for 0% hole transfer is projected to occur somewhere between 15 and 20 waters per nucleotide. Electron transfer, on the other hand, is 100% efficient across the entire range studied, 4.2

Direct radiation damage to crystalline DNA: what is the source of unaltered base release?

The radiation chemical yields of unaltered base release have been measured in three crystalline double-stranded DNA oligomers after X irradiation at 4 K. The yields of released bases are between 10 and 20% of the total free radical yields measured at 4 K. Using these numbers, we estimate that the yield of DNA strand breaks due to the direct effect is about 0.1 micromol J(-1). The damage responsible for base release is independent of the base type (C, G, A or T) and is not scavenged by anthracycline drugs intercalated in the DNA. For these reasons, reactions initiated by the hydroxyl radical have been ruled out as the source of base release. Since the intercalated anthracycline scavenges electrons and holes completely but does not inhibit base release, the possibility for damage transfer from the bases to the sugars can also be ruled out. The results are consistent with a model in which primary radical cations formed directly on the sugar-phosphate backbone react by two competing pathways: deprotonation, which localizes the damage on the sugar, and hole tunneling, which transfers the damage to the base stack. Quantitative estimates indicate that these two processes are approximately equally efficient.

Free radical yields in crystalline DNA X-irradiated at 4 K.

The objective of this work is to determine the extent to which various structural factors influence the yield of trapped free radicals, G(tfr), in DNA irradiated at 4 K. G(tfr) was measured in a series of 13 different oligodeoxynucleotides using electron paramagnetic resonance (EPR) spectroscopy. Each sample consisted of crystalline duplex DNA for which the crystal structure was verified to be that reported in the literature. We find that the G(tfr) of these samples is remarkably high, ranging from 0.55 to 0.75 micromol/J. The standard deviation in G(tfr) for a given crystal structure is generally small, typically less than +/-10%. Furthermore, G(tfr) does not correlate with DNA base sequence, conformation, counterion or length of base stacking. Two observations point to the importance of DNA packing: (1) The radical yields in crystalline DNA are greater than those determined previously for DNA films (0.2 to 0.5 micromol/J); and (2) the variability in G(tfr) is less in DNA crystals than in DNA films. We conclude that closely packed DNA maximizes radical trapping by minimizing the interhelical solvent space. Furthermore, the high efficiency of electron and hole trapping at 4 K is not consistent with DNA possessing properties of a metallic conductor. Indeed, it behaves as an insulator, whether it is in A-, B-, or Z-form and whether base stacking is short- (8 bp) or long-range (>1000 bp).