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Better photocells from bigger Buckyballs
RESEARCH NEWS
Smarter implants NANOMEDICINE
One of the major obstacles preventing the development of implantable biosensors, artificial kidneys, and other “active” medical devices requiring a controlled implant-tissue interface has been the reduction in device function after implantation. The major difficulty in creating implantable active medical devices that function for long periods of time is poor control over materials-tissue interaction. Several materials have previously been considered for reducing protein adsorption, promoting integration of the medical device with the surrounding tissues, and maintaining transport of biological molecules between the device and the surrounding tissues over an extended period of time. The surfaces of these materials must be sufficiently thin and porous in order to allow the devices to rapidly respond to variations in biological molecule concentration. Unfortunately, hydrogels, phospholipids, surfactants, and flow-based systems have not generally been successful in maintaining a selectively permeable in vivo
SEM of anodized aluminum oxide membrane coated with platinum using atomic layer deposition in order to prevent aluminum release.
tissue-material interface. On the other hand, nanoporous ceramics such as anodized aluminum oxide are similar in structure to natural filters within the human body (e.g., glomerular basement membrane in the kidney). Unlike polymers, ceramics generally demonstrate corrosion resistance, wear resistance, and biologically inert behavior for long periods of
time. Roger Narayan at the University of North Carolina (UNC) at Chapel Hill in collaboration with Jeffrey Elam and Michael Pellin at Argonne National Laboratory [Narayan, et al., Biomed. Mater. (2008) 3, 034107] have used atomic or pulsed layer deposition to modify the surfaces of these nanoporous membranes. The atomic layer deposition process is particularly attractive as it allows one to reduce the pore diameter while retaining a narrow pore size distribution. The UNC-Argonne group has developed several nanoporous ceramic membranes for use in active medical devices. For example, membranes have been functionalized with diamond-like carbon or poly(ethylene) glycol in order to minimize protein adsorption. The group is also looking into the possibility of integrating smart capabilities into functionalized nanoporous ceramic membranes, such as the incorporation of biomimetic technologies to further control nanopore diffusion characteristics. Roger Narayan
Better photocells from bigger Buckyballs CARBON
Much research activity is presently devoted to organic photovoltaic devices (OPV), in particular ones comprising polymers as donors and a variety of C60 fullerenes with organic molecules attached as acceptors. Now, a group of scientists collaborating from several research institutions, namely the Georgetown University, Washington DC, Luna Innovations Inc., Virginia, the FriedrichAlexander-Universität, Erlangen, Germany, the National Renewable Energy Laboratory, Colorado, and the University of Santa Barbara have developed a novel fullerene species for this application [Ross, et al., Nature Materials (2009), doi:10.1038/NMAT2379]. “We believe that our discovery is a significant contribution to the improvement in conversion efficiencies of organic solar cells,” says Martin Drees, corresponding author. In contrast to the acceptor materials utilized to date, Drees and his colleagues used fullerenes large enough to incarcerate trimetallic nitrides (therefore called trimetallic nitride endohedral fullerenes, or TNEFs) and filled them with Lu3N. The main advantage over the presently used empty C60 molecules and their derivatives is the higher open circuit voltage. Drees and his group found values of about
10
A pristine C70 molecule encapsulating a Lu3N ion
890 mV (in comparison to 630 mV for present state-of-the-art C60 devices), in fact the highest reported for any fullerene OPV. The reason for the low voltage output of the C60 devices is the orbital mismatch of the donor polymer and the fullerene acceptors, a situation which the researchers could significantly improve by incorporating Lu3N-ions in the bigger fullerenes. Since the solubility of pristine TNEFs is not nearly as high as solution processing of OPV
APRIL 2009 | VOLUME 12 | NUMBER 4
devices would require, Drees and his group created several much more soluble variations by exohedral functionalization, i.e., attaching organic molecules to the outside, as has been done previously with the smaller molecules. “The processability of our materials is similar to the current C60 acceptor,” explains Drees. “Combined with the improved open circuit voltage that our materials offer they are an ideal replacement since they can be easily integrated into already existing manufacturing processes.” This new material looks promising, since neither the material itself nor its production are more intricate than current state-of-the-art acceptor materials. So far, these compounds were studied only in combination with a well-tried, widely used polymer. In order to obtain higher efficiencies, Drees and his group are going to investigate combinations of TNEFs and new polymers with broader absorption spectra. “Our modeling shows that efficiencies of more than ten percent should be achievable with some already published donor polymer systems,” hopes Drees. Michel Fleck
Smarter implants NANOMEDICINE
One of the major obstacles preventing the development of implantable biosensors, artificial kidneys, and other “active” medical devices requiring a controlled implant-tissue interface has been the reduction in device function after implantation. The major difficulty in creating implantable active medical devices that function for long periods of time is poor control over materials-tissue interaction. Several materials have previously been considered for reducing protein adsorption, promoting integration of the medical device with the surrounding tissues, and maintaining transport of biological molecules between the device and the surrounding tissues over an extended period of time. The surfaces of these materials must be sufficiently thin and porous in order to allow the devices to rapidly respond to variations in biological molecule concentration. Unfortunately, hydrogels, phospholipids, surfactants, and flow-based systems have not generally been successful in maintaining a selectively permeable in vivo
SEM of anodized aluminum oxide membrane coated with platinum using atomic layer deposition in order to prevent aluminum release.
tissue-material interface. On the other hand, nanoporous ceramics such as anodized aluminum oxide are similar in structure to natural filters within the human body (e.g., glomerular basement membrane in the kidney). Unlike polymers, ceramics generally demonstrate corrosion resistance, wear resistance, and biologically inert behavior for long periods of
time. Roger Narayan at the University of North Carolina (UNC) at Chapel Hill in collaboration with Jeffrey Elam and Michael Pellin at Argonne National Laboratory [Narayan, et al., Biomed. Mater. (2008) 3, 034107] have used atomic or pulsed layer deposition to modify the surfaces of these nanoporous membranes. The atomic layer deposition process is particularly attractive as it allows one to reduce the pore diameter while retaining a narrow pore size distribution. The UNC-Argonne group has developed several nanoporous ceramic membranes for use in active medical devices. For example, membranes have been functionalized with diamond-like carbon or poly(ethylene) glycol in order to minimize protein adsorption. The group is also looking into the possibility of integrating smart capabilities into functionalized nanoporous ceramic membranes, such as the incorporation of biomimetic technologies to further control nanopore diffusion characteristics. Roger Narayan
Better photocells from bigger Buckyballs CARBON
Much research activity is presently devoted to organic photovoltaic devices (OPV), in particular ones comprising polymers as donors and a variety of C60 fullerenes with organic molecules attached as acceptors. Now, a group of scientists collaborating from several research institutions, namely the Georgetown University, Washington DC, Luna Innovations Inc., Virginia, the FriedrichAlexander-Universität, Erlangen, Germany, the National Renewable Energy Laboratory, Colorado, and the University of Santa Barbara have developed a novel fullerene species for this application [Ross, et al., Nature Materials (2009), doi:10.1038/NMAT2379]. “We believe that our discovery is a significant contribution to the improvement in conversion efficiencies of organic solar cells,” says Martin Drees, corresponding author. In contrast to the acceptor materials utilized to date, Drees and his colleagues used fullerenes large enough to incarcerate trimetallic nitrides (therefore called trimetallic nitride endohedral fullerenes, or TNEFs) and filled them with Lu3N. The main advantage over the presently used empty C60 molecules and their derivatives is the higher open circuit voltage. Drees and his group found values of about
10
A pristine C70 molecule encapsulating a Lu3N ion
890 mV (in comparison to 630 mV for present state-of-the-art C60 devices), in fact the highest reported for any fullerene OPV. The reason for the low voltage output of the C60 devices is the orbital mismatch of the donor polymer and the fullerene acceptors, a situation which the researchers could significantly improve by incorporating Lu3N-ions in the bigger fullerenes. Since the solubility of pristine TNEFs is not nearly as high as solution processing of OPV
APRIL 2009 | VOLUME 12 | NUMBER 4
devices would require, Drees and his group created several much more soluble variations by exohedral functionalization, i.e., attaching organic molecules to the outside, as has been done previously with the smaller molecules. “The processability of our materials is similar to the current C60 acceptor,” explains Drees. “Combined with the improved open circuit voltage that our materials offer they are an ideal replacement since they can be easily integrated into already existing manufacturing processes.” This new material looks promising, since neither the material itself nor its production are more intricate than current state-of-the-art acceptor materials. So far, these compounds were studied only in combination with a well-tried, widely used polymer. In order to obtain higher efficiencies, Drees and his group are going to investigate combinations of TNEFs and new polymers with broader absorption spectra. “Our modeling shows that efficiencies of more than ten percent should be achievable with some already published donor polymer systems,” hopes Drees. Michel Fleck