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New hydrolyzable polymer could lead to cheaper biomaterials
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phase (silicon oil) at specific flow rates. When the two phases met at a T-junction, droplets formed, which were then collected and dried. The carbon black was found to coalesce into one hemisphere of the CB/ PTFE beads to form the Janus structure. In
Materials Today Volume 18, Number 2 March 2015
addition, Xiang Li and his team could control both the size distribution of the Janus beads and the volume ratio of the CB hemisphere, to a high degree of accuracy, by varying operating parameters. By applying a voltage to the microspheres, their electrical
properties could also be determined. Rotational motion was observed, along with translational motion at elevated voltages, demonstrating the anisotropy of the spheres and the reliability of their technique. Laurie Winkless
New approach makes Ti implant more like bone Metals such as Ti are widely used as implants in orthopedics and dentistry, but better mechanical properties and bioactivity could reduce subsequent failure and rejection. Researchers think they may have hit upon a solution to these issues using densified porous Ti implants loaded with growth factors [H.-D. Jung, et al. Biomaterials 37 (2015) 49–61]. Porous Ti and biological agents that promote cell growth are well-tried approaches for improving the performance of implants, but the researchers from Seoul National University, Korea University, and Stony Brook University have gone a step further. By using freeze casting to fabricate porous Ti, where a metal powder is dispersed in a liquid and cooled until the liquid solidifies leaving behind a metal powder network, the researchers can create scaffolds ductile enough to undergo further densification. From a starting porosity of more than 50%, the researchers end up with a scaffold of just 7% porosity after compression. ‘Our freeze-casting fabrication method allows the densification of porous scaffolds,’ explains Song. ‘This fabrication approach minimizes chemical contamination and structural defects during densification, maintaining the structural integrity of the porous metal without any reduction of mechanical properties.’ Depending on the starting scaffold, the final porosity and mechanical properties can be varied. The new approach enables the mechanical properties to be tuned so
Gradient metallic implant for artificial disc replacement fabricated via freeze-casting densification.
that implants for both filling and load-bearing applications can be fabricated. Next the densified Ti scaffolds are coated with growth factor bone morphogenetic protein-2 (BMP-2), which improves bioactivity and promotes the production of new tissue. ‘Due to the unique pore structures [of our metal implants], the coated bioactive molecules are gradually released from the scaffold, maintaining their efficacy for a prolonged period,’ explains Juha Song of Seoul National University. The slow release of growth factors is a significant advantage as high does of some of these agents, such as BMP-2, can be associated with unpleasant side effects. Moreover, the initial porosity and degree of
densification can be used to control the release rate. The new scaffolds show increased strength combined with low stiffness – almost identical to the mechanical properties to real bone. The use of densified Ti could, the researchers believe, mitigate the problem of stress shielding – where the mismatch in stiffness between natural bone and metal implants causes damage to surrounding tissue – and provide excellent long-term stability for implants. The researchers have even devised graded pore structure implants, where the outer later is porous to allow bone ingrowth and the inner core is dense to provide mechanical stability and prolonged bioactivity. Cordelia Sealy
New hydrolyzable polymer could lead to cheaper biomaterials Scientists have developed a cheap hydrolyzable polymer that can be designed to degrade over time, and which could offer a viable alternative to those used in a range of biomedical applications, such as in the design of drug delivery systems, tissue engineering, surgical sutures and transient 62
Addition of bulky substituents weakens urea bond.
electronics, and in degradable/compostable packaging materials, coatings and adhesive materials. The researchers, from the University of Illinois at Urbana-Champaign in the US, showed how it was possible to reverse the characteristics of polyurea, a key bonding
Materials Today Volume 18, Number 2 March 2015
material, developing a class of hindered urea bond (HUB) containing polymeric materials – or poly(hindered urea)s (PHUs). As reported in the Journal of the American Chemical Society [Ying, Cheng, J. Am. Chem. Soc. (2014), doi:10.1021/ja5093437], this urea bond is very inert, so the polymer is extremely stable and so can be used in longlasting applications. The new PHUs have significant benefits over other hydrolyzable polymers, especially as they can be created with cheap chemical precursors in ambient conditions using simple and clean chemistry with no catalyst or by-products. This allows for the control of the copolymer recipe for particular needs without complex synthesizing. As researcher Jianjun Cheng said, ‘‘PHUs can
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be completely hydrolyzed within a few days. Since ‘hindrance’ is the cause of the bond destabilization, the hydrolysis kinetics of PHUs can be easily tuned as needed for a specific application.’’ Polyurea typically contain ester and other hydrolyzable bonds in their backbone structures. Here, the team demonstrated the potential of PHUs for the design of water degradable polymeric materials that can be easily synthesized by mixing multifunctional bulky amines and isocyanates. They previously found that urea bonds with bulky substituents can form reversible equilibrium with isocyanate and amine under ambient conditions. As water can react with isocyanate, they figured that it could shift the chemical equilibrium and
degrade the urea bond, leading them to explore the hydrolysis behaviors of hindered polyurea. The findings demonstrate these highly inert materials could become dynamic and degradable with simple structure modification, while for biomaterials it offers a new type of polymers that are an improvement over existing ones in terms of cost, facile synthesis and high kinetic tunability. However, it is important to gain a better understanding of the HUB hydrolysis behaviors, and the researchers hope to investigate changes of hydrolysis kinetics under various environmental conditions, as well as further applications in biomaterials and packaging. Laurie Donaldson
nadium oxide (VO2), which can control infrared transmittance while maintaining transparency to visible light. The resulting material offers improved thermal insulating properties, is photocatalytically-active and doesn’t fog up [Zheng, et al., Nano Energy (2014), doi:10.1016/j.nanoen.2014.09.023]. This performance is the result of the composite’s unique crystal structure – it is effectively a sandwich of two forms of TiO2 (rutile and anatase) and VO2 in its monoclinic phase. In addition, the sandwich structure can be produced using standard thin-film production techniques. The bottom slice of the sandwich consists of TiO2 (rutile), which serves as an antireflection layer. This is followed by the ‘filling’ – a layer of VO2, which controls the amount of solar heat transmitting through the glass in response to temperature changes. The top layer of TiO2 (anatase) provides the photocatalytic properties that make this glass self-cleaning.
The team, led by Ping Jin from the Chinese Academy of Sciences, carried out a series of tests to characterize the final composite thin-film. Optical measurements showed that the 400 400 mm3 sample displayed excellent regulation of infrared light, while remaining transparent at visible wavelengths. UV radiation of the material also resulted in a photo-induced hydrophilicity, which produced in an antifogging surface. By measuring the degradation of stearic acid under UV light, the film was found to be highly photocatalytically-active. The team are confident that their thin film has real applications in the development of a true ‘‘smart window’’. Their multilayer film offers three functions at once – it is antifogging, self-cleaning and energy-saving – but until the robustness of this film has been measured, it may remain in the research lab. Laurie Winkless
A step forward for smart windows? Multilayer windows that are self-cleaning, energy-saving and anti-fogging may be one step closer, thanks to a team of Chinese researchers. Windows are an important factor in a building’s energy efficiency, and with tall, glass-clad structures becoming the norm in our cities, teams of researchers are looking at ways to improve their efficiency, while maintaining their appearance. In the UK alone, 40% of the nation’s total energy bill comes from the way buildings are lit, heated and used, so even small changes in window technology could have a significant effect in reducing total energy consumption. Much of the research on ‘‘smart windows’’ has focused on titanium dioxide (or titania, TiO2) which can be used to produce a self-cleaning surface, thanks to its photocatalytic properties. But Chinese researchers have taken this to a new level, by adding another ‘‘smart’’ ingredient, va-
Wearable self-powered motion sensor A flexible, self-powered piezoelectric sensor for potential use by Alzheimer’s patients has been developed by a team of Korean researchers. Large-scale energy harvesting technologies have played a major role in our energy landscape for over a decade. But in addition, there have been a number of extensive investigations into smaller-scale, ambient technologies, such as piezoelectric nanogenerators. Piezoelectricity can be used
Schematic diagrams of the fabrication process for the piezoelectric hemispheres embedded stretchable composites.
to convert mechanical energy to electricity – by applying a stress to a piezo-material, a voltage is produced inside the crystal, driving electron flow. But this process is also reversible, meaning that piezoelectric nanogenerators can act as motion sensors, sensitive to even the smallest displacements. A paper published in Nano Energy [Chun, et al., Nano Energy (2015), doi:10.1016/j. nanoen.2014.10.010] reports on the development of a new, stretchable piezoelectric 63
phase (silicon oil) at specific flow rates. When the two phases met at a T-junction, droplets formed, which were then collected and dried. The carbon black was found to coalesce into one hemisphere of the CB/ PTFE beads to form the Janus structure. In
Materials Today Volume 18, Number 2 March 2015
addition, Xiang Li and his team could control both the size distribution of the Janus beads and the volume ratio of the CB hemisphere, to a high degree of accuracy, by varying operating parameters. By applying a voltage to the microspheres, their electrical
properties could also be determined. Rotational motion was observed, along with translational motion at elevated voltages, demonstrating the anisotropy of the spheres and the reliability of their technique. Laurie Winkless
New approach makes Ti implant more like bone Metals such as Ti are widely used as implants in orthopedics and dentistry, but better mechanical properties and bioactivity could reduce subsequent failure and rejection. Researchers think they may have hit upon a solution to these issues using densified porous Ti implants loaded with growth factors [H.-D. Jung, et al. Biomaterials 37 (2015) 49–61]. Porous Ti and biological agents that promote cell growth are well-tried approaches for improving the performance of implants, but the researchers from Seoul National University, Korea University, and Stony Brook University have gone a step further. By using freeze casting to fabricate porous Ti, where a metal powder is dispersed in a liquid and cooled until the liquid solidifies leaving behind a metal powder network, the researchers can create scaffolds ductile enough to undergo further densification. From a starting porosity of more than 50%, the researchers end up with a scaffold of just 7% porosity after compression. ‘Our freeze-casting fabrication method allows the densification of porous scaffolds,’ explains Song. ‘This fabrication approach minimizes chemical contamination and structural defects during densification, maintaining the structural integrity of the porous metal without any reduction of mechanical properties.’ Depending on the starting scaffold, the final porosity and mechanical properties can be varied. The new approach enables the mechanical properties to be tuned so
Gradient metallic implant for artificial disc replacement fabricated via freeze-casting densification.
that implants for both filling and load-bearing applications can be fabricated. Next the densified Ti scaffolds are coated with growth factor bone morphogenetic protein-2 (BMP-2), which improves bioactivity and promotes the production of new tissue. ‘Due to the unique pore structures [of our metal implants], the coated bioactive molecules are gradually released from the scaffold, maintaining their efficacy for a prolonged period,’ explains Juha Song of Seoul National University. The slow release of growth factors is a significant advantage as high does of some of these agents, such as BMP-2, can be associated with unpleasant side effects. Moreover, the initial porosity and degree of
densification can be used to control the release rate. The new scaffolds show increased strength combined with low stiffness – almost identical to the mechanical properties to real bone. The use of densified Ti could, the researchers believe, mitigate the problem of stress shielding – where the mismatch in stiffness between natural bone and metal implants causes damage to surrounding tissue – and provide excellent long-term stability for implants. The researchers have even devised graded pore structure implants, where the outer later is porous to allow bone ingrowth and the inner core is dense to provide mechanical stability and prolonged bioactivity. Cordelia Sealy
New hydrolyzable polymer could lead to cheaper biomaterials Scientists have developed a cheap hydrolyzable polymer that can be designed to degrade over time, and which could offer a viable alternative to those used in a range of biomedical applications, such as in the design of drug delivery systems, tissue engineering, surgical sutures and transient 62
Addition of bulky substituents weakens urea bond.
electronics, and in degradable/compostable packaging materials, coatings and adhesive materials. The researchers, from the University of Illinois at Urbana-Champaign in the US, showed how it was possible to reverse the characteristics of polyurea, a key bonding
Materials Today Volume 18, Number 2 March 2015
material, developing a class of hindered urea bond (HUB) containing polymeric materials – or poly(hindered urea)s (PHUs). As reported in the Journal of the American Chemical Society [Ying, Cheng, J. Am. Chem. Soc. (2014), doi:10.1021/ja5093437], this urea bond is very inert, so the polymer is extremely stable and so can be used in longlasting applications. The new PHUs have significant benefits over other hydrolyzable polymers, especially as they can be created with cheap chemical precursors in ambient conditions using simple and clean chemistry with no catalyst or by-products. This allows for the control of the copolymer recipe for particular needs without complex synthesizing. As researcher Jianjun Cheng said, ‘‘PHUs can
NEWS
be completely hydrolyzed within a few days. Since ‘hindrance’ is the cause of the bond destabilization, the hydrolysis kinetics of PHUs can be easily tuned as needed for a specific application.’’ Polyurea typically contain ester and other hydrolyzable bonds in their backbone structures. Here, the team demonstrated the potential of PHUs for the design of water degradable polymeric materials that can be easily synthesized by mixing multifunctional bulky amines and isocyanates. They previously found that urea bonds with bulky substituents can form reversible equilibrium with isocyanate and amine under ambient conditions. As water can react with isocyanate, they figured that it could shift the chemical equilibrium and
degrade the urea bond, leading them to explore the hydrolysis behaviors of hindered polyurea. The findings demonstrate these highly inert materials could become dynamic and degradable with simple structure modification, while for biomaterials it offers a new type of polymers that are an improvement over existing ones in terms of cost, facile synthesis and high kinetic tunability. However, it is important to gain a better understanding of the HUB hydrolysis behaviors, and the researchers hope to investigate changes of hydrolysis kinetics under various environmental conditions, as well as further applications in biomaterials and packaging. Laurie Donaldson
nadium oxide (VO2), which can control infrared transmittance while maintaining transparency to visible light. The resulting material offers improved thermal insulating properties, is photocatalytically-active and doesn’t fog up [Zheng, et al., Nano Energy (2014), doi:10.1016/j.nanoen.2014.09.023]. This performance is the result of the composite’s unique crystal structure – it is effectively a sandwich of two forms of TiO2 (rutile and anatase) and VO2 in its monoclinic phase. In addition, the sandwich structure can be produced using standard thin-film production techniques. The bottom slice of the sandwich consists of TiO2 (rutile), which serves as an antireflection layer. This is followed by the ‘filling’ – a layer of VO2, which controls the amount of solar heat transmitting through the glass in response to temperature changes. The top layer of TiO2 (anatase) provides the photocatalytic properties that make this glass self-cleaning.
The team, led by Ping Jin from the Chinese Academy of Sciences, carried out a series of tests to characterize the final composite thin-film. Optical measurements showed that the 400 400 mm3 sample displayed excellent regulation of infrared light, while remaining transparent at visible wavelengths. UV radiation of the material also resulted in a photo-induced hydrophilicity, which produced in an antifogging surface. By measuring the degradation of stearic acid under UV light, the film was found to be highly photocatalytically-active. The team are confident that their thin film has real applications in the development of a true ‘‘smart window’’. Their multilayer film offers three functions at once – it is antifogging, self-cleaning and energy-saving – but until the robustness of this film has been measured, it may remain in the research lab. Laurie Winkless
A step forward for smart windows? Multilayer windows that are self-cleaning, energy-saving and anti-fogging may be one step closer, thanks to a team of Chinese researchers. Windows are an important factor in a building’s energy efficiency, and with tall, glass-clad structures becoming the norm in our cities, teams of researchers are looking at ways to improve their efficiency, while maintaining their appearance. In the UK alone, 40% of the nation’s total energy bill comes from the way buildings are lit, heated and used, so even small changes in window technology could have a significant effect in reducing total energy consumption. Much of the research on ‘‘smart windows’’ has focused on titanium dioxide (or titania, TiO2) which can be used to produce a self-cleaning surface, thanks to its photocatalytic properties. But Chinese researchers have taken this to a new level, by adding another ‘‘smart’’ ingredient, va-
Wearable self-powered motion sensor A flexible, self-powered piezoelectric sensor for potential use by Alzheimer’s patients has been developed by a team of Korean researchers. Large-scale energy harvesting technologies have played a major role in our energy landscape for over a decade. But in addition, there have been a number of extensive investigations into smaller-scale, ambient technologies, such as piezoelectric nanogenerators. Piezoelectricity can be used
Schematic diagrams of the fabrication process for the piezoelectric hemispheres embedded stretchable composites.
to convert mechanical energy to electricity – by applying a stress to a piezo-material, a voltage is produced inside the crystal, driving electron flow. But this process is also reversible, meaning that piezoelectric nanogenerators can act as motion sensors, sensitive to even the smallest displacements. A paper published in Nano Energy [Chun, et al., Nano Energy (2015), doi:10.1016/j. nanoen.2014.10.010] reports on the development of a new, stretchable piezoelectric 63