- Email: [email protected]
Injectable nanofibrous microspheres help new tissue growth
News and opinions ticle creams, and suggest that the approach could prove effective for allergic contact dermatitis associated with other metals like palladium and cadmium. Huw Summers, chair of nanotechnology for health at Swansea University, agrees that results indicate that this could be a successful way of reducing skin allergies. ‘‘Nanoparticle sequestration of metal ions has been demonstrated before,’’ he notes. ‘‘[But] here the authors
225 take the approach into a biomedical setting and adapt it for medicinal use.’’ E-mail address: [email protected] 1748-0132/$ — see front matter doi:10.1016/j.nantod.2011.04.003
Injectable nanofibrous microspheres help new tissue growth Cordelia Sealy Predesigned scaffolds can be used to regrow replacement tissue or even organs for transplantation, but irregularly shaped wounds or defects are more difficult to repair. An injectable cell carrier would be ideal—–and now researchers from the University of Michigan have come up with just such a system [X. Liu, et al., Nature Materials (2011), doi:10.1038/NMAT2999]. Peter X. Ma and his team have developed nanofibrous hollow microspheres, synthesized from star-shaped poly(Llactic acid) (SS-PLLA), which can carry cells and be injected into wound sites. The SS-PLLA polymers self-assemble into the microspheres, without the need for any templates. The nanofibers have an average diameter of 160 ± 67 nm, which is on the same scale as collagen fibers. Furthermore, the open and porous microspheres into which the nanofibers self-assemble encourages cell seeding, proliferation and tissue regeneration (Fig. 1). The researchers tested the hollow nanofibrous microspheres as an injectable scaffold carrying chondrocytes for cartilage regeneration in vitro, in nude mice and for osteochondral defect repair in rabbits. Compared to solid microspheres and nanofibrous microspheres without a hollow interior, the hollow nanofibrous microspheres appear to facilitate more and better quality cartilage regeneration. According to Ma, as much as three or four times as much tissue is formed when using the hollow nanofibrous microspheres compared with the control group. In addition, the hollow nanofibrous microspheres largely degrade after eight weeks, leaving only cartilage-specific matrix material in the wound, unlike the other microspheres, which persist in the tissue. The hollow nanofibrous microspheres produce cartilage very similar in biomechanical properties to natural cartilage, say the researchers, and which fully filled the osteochondral wound in the rabbit model, smoothly integrating with the host cartilage. The researchers believe the hollow nanofibrous microspheres are worthy of further investigation as chondrocyte carriers in clinical applications, as well as carriers of other
cells for tissue regeneration. Ma says that the team will now test the system in larger animal systems and eventually on human patients. The use of a hierarchical self-assembled material architecture is novel and very interesting, says Markus J. Buehler of Massachusetts Institute of Technology. ‘‘What I find interesting is that the authors do not simply adapt the material itself but actually thought about optimizing the structure of the molecules and how they assembly into a sphere-like structure at different levels,’’ he told Nano Today. The advantage of taking a hierarchical approach to tissue engineering is that it provides a more controlled way to regenerate tissues, spatially and biologically, in terms of the exact type of tissue grown, he adds. Buehler believes that the approach has great potential for many future
Figure 1 sphere.
High resolution photo of a hollow nanofibrous micro-
226 scientific and clinical applications. Dan Luo of Cornell University agrees that the work demonstrates the importance of controlling the architecture of biomaterials from the micro- to the nanoscale in order to regulate cell behavior in vivo. ‘‘[These] novel cell carriers show promise in animal studies for knee repair in particular, but also shed lights on
News and opinions how to realistically mimic interstitial environment for tissue repair and tissue engineering in general,’’ he says. E-mail address: [email protected] 1748-0132/$ — see front matter doi:10.1016/j.nantod.2011.04.002
Nanocrystal doping follows the dots Cordelia Sealy The doping of bulk semiconductors has enabled a wealth of microelectronic and optoelectronic devices. But while semiconductor nanocrystals could herald a new generation of devices produced through cheap and easy wet-chemical approaches, doping has remained elusive. Now, however, researchers from Hebrew and Tel Aviv Universities in Israel claim to have developed a simple, room-temperature chemical method for doping semiconductor nanocrystal quantum dots [D. Mocatta et al., Science 332 (2011) 77]. The new method simply relies on mixing a solution of the dopant metal salt dissolved in toluene and a surfactant with preformed quantum dots. Using this solidstate diffusion method, the researchers believe that Cu and Ag metal impurities are introduced into InAs quantum dots, enabling control of the band gap and the Fermi energy to yield both n- and p-doped nanocrystals.
The doping of quantum dots presents a unique scenario—– introducing even a single impurity atom into a 4 nm nanocrystal of around 1000 atoms results in a doping level of 7 × 1019 cm−3 , well into the heavily doped regime of bulk semiconductors. But the results of such heavy doping in nanocrystals are very different to the effects in bulk semiconductors. Instead of producing metallic (or ‘degenerate’) behavior, the doping of nanocrystals appears to introduce impurity states that significantly change the density of states (DOS) because of quantum confinement effects. Multiple impurities in the confined nanocrystals also appear to enhance disorder effects, altering the electronic structure via a process known as the Urbach tail mechanism. The researchers report that introducing Ag atoms produces a red shift in the absorption and emission spectra
Figure 1 Lewis chemical structure diagrams of an InAs lattice containing metallic impurities. Bonding electrons are represented by either black lines or paired dots, of the same color as the atom to which they belong. A plus sign indicates the lack of an electron in a bonding orbital. (A) Cu impurity in an interstitial site in the InAs lattice, which donates valence electrons to induce n-type doping. (B) Substitutional Ag occupying an In site in the InAs lattice, which introduces two electron acceptor sites resulting in a deficiency of valence electrons and p-type doping. Credit: P. Huey/Science ©2011 AAAS.
225 take the approach into a biomedical setting and adapt it for medicinal use.’’ E-mail address: [email protected] 1748-0132/$ — see front matter doi:10.1016/j.nantod.2011.04.003
Injectable nanofibrous microspheres help new tissue growth Cordelia Sealy Predesigned scaffolds can be used to regrow replacement tissue or even organs for transplantation, but irregularly shaped wounds or defects are more difficult to repair. An injectable cell carrier would be ideal—–and now researchers from the University of Michigan have come up with just such a system [X. Liu, et al., Nature Materials (2011), doi:10.1038/NMAT2999]. Peter X. Ma and his team have developed nanofibrous hollow microspheres, synthesized from star-shaped poly(Llactic acid) (SS-PLLA), which can carry cells and be injected into wound sites. The SS-PLLA polymers self-assemble into the microspheres, without the need for any templates. The nanofibers have an average diameter of 160 ± 67 nm, which is on the same scale as collagen fibers. Furthermore, the open and porous microspheres into which the nanofibers self-assemble encourages cell seeding, proliferation and tissue regeneration (Fig. 1). The researchers tested the hollow nanofibrous microspheres as an injectable scaffold carrying chondrocytes for cartilage regeneration in vitro, in nude mice and for osteochondral defect repair in rabbits. Compared to solid microspheres and nanofibrous microspheres without a hollow interior, the hollow nanofibrous microspheres appear to facilitate more and better quality cartilage regeneration. According to Ma, as much as three or four times as much tissue is formed when using the hollow nanofibrous microspheres compared with the control group. In addition, the hollow nanofibrous microspheres largely degrade after eight weeks, leaving only cartilage-specific matrix material in the wound, unlike the other microspheres, which persist in the tissue. The hollow nanofibrous microspheres produce cartilage very similar in biomechanical properties to natural cartilage, say the researchers, and which fully filled the osteochondral wound in the rabbit model, smoothly integrating with the host cartilage. The researchers believe the hollow nanofibrous microspheres are worthy of further investigation as chondrocyte carriers in clinical applications, as well as carriers of other
cells for tissue regeneration. Ma says that the team will now test the system in larger animal systems and eventually on human patients. The use of a hierarchical self-assembled material architecture is novel and very interesting, says Markus J. Buehler of Massachusetts Institute of Technology. ‘‘What I find interesting is that the authors do not simply adapt the material itself but actually thought about optimizing the structure of the molecules and how they assembly into a sphere-like structure at different levels,’’ he told Nano Today. The advantage of taking a hierarchical approach to tissue engineering is that it provides a more controlled way to regenerate tissues, spatially and biologically, in terms of the exact type of tissue grown, he adds. Buehler believes that the approach has great potential for many future
Figure 1 sphere.
High resolution photo of a hollow nanofibrous micro-
226 scientific and clinical applications. Dan Luo of Cornell University agrees that the work demonstrates the importance of controlling the architecture of biomaterials from the micro- to the nanoscale in order to regulate cell behavior in vivo. ‘‘[These] novel cell carriers show promise in animal studies for knee repair in particular, but also shed lights on
News and opinions how to realistically mimic interstitial environment for tissue repair and tissue engineering in general,’’ he says. E-mail address: [email protected] 1748-0132/$ — see front matter doi:10.1016/j.nantod.2011.04.002
Nanocrystal doping follows the dots Cordelia Sealy The doping of bulk semiconductors has enabled a wealth of microelectronic and optoelectronic devices. But while semiconductor nanocrystals could herald a new generation of devices produced through cheap and easy wet-chemical approaches, doping has remained elusive. Now, however, researchers from Hebrew and Tel Aviv Universities in Israel claim to have developed a simple, room-temperature chemical method for doping semiconductor nanocrystal quantum dots [D. Mocatta et al., Science 332 (2011) 77]. The new method simply relies on mixing a solution of the dopant metal salt dissolved in toluene and a surfactant with preformed quantum dots. Using this solidstate diffusion method, the researchers believe that Cu and Ag metal impurities are introduced into InAs quantum dots, enabling control of the band gap and the Fermi energy to yield both n- and p-doped nanocrystals.
The doping of quantum dots presents a unique scenario—– introducing even a single impurity atom into a 4 nm nanocrystal of around 1000 atoms results in a doping level of 7 × 1019 cm−3 , well into the heavily doped regime of bulk semiconductors. But the results of such heavy doping in nanocrystals are very different to the effects in bulk semiconductors. Instead of producing metallic (or ‘degenerate’) behavior, the doping of nanocrystals appears to introduce impurity states that significantly change the density of states (DOS) because of quantum confinement effects. Multiple impurities in the confined nanocrystals also appear to enhance disorder effects, altering the electronic structure via a process known as the Urbach tail mechanism. The researchers report that introducing Ag atoms produces a red shift in the absorption and emission spectra
Figure 1 Lewis chemical structure diagrams of an InAs lattice containing metallic impurities. Bonding electrons are represented by either black lines or paired dots, of the same color as the atom to which they belong. A plus sign indicates the lack of an electron in a bonding orbital. (A) Cu impurity in an interstitial site in the InAs lattice, which donates valence electrons to induce n-type doping. (B) Substitutional Ag occupying an In site in the InAs lattice, which introduces two electron acceptor sites resulting in a deficiency of valence electrons and p-type doping. Credit: P. Huey/Science ©2011 AAAS.