Stress shielding is caused by the mismatch of stiffness between bone and implant materials,which may give rise to bone resorption and loosening,thereby causing implantation failure.There is a huge gap between Young’s...Stress shielding is caused by the mismatch of stiffness between bone and implant materials,which may give rise to bone resorption and loosening,thereby causing implantation failure.There is a huge gap between Young’s modulus of human bone and low Young’s modulusβTi alloys.A porous structure design can achieve the target of low Young’s modulus,and thus achieve the matching between human bone and implant materials.However,a suitable space holder(SH)that can be applied at high temperatures and sintering pressure has not been reported.In this study,the TiZrNbTa/Ti titanium matrix composite(TMC)with high strength and large ductility was used as scaffold materials and combined the SH technique with the spark plasma sintering(SPS)technique to obtain a porous structure.A novel space holder,i.e.,MgO particles was adopted,which can withstand high-temperature sintering accompanied by a sintering pressure.The porous TiZrNbTa/Ti with 40 vol.%MgO added exhibits a maximum strength of 345.9±10.4 MPa and Young’s modulus of 24.72±0.20 GPa,respectively.It possesses higher strength compared with human bone and matches Young’s modulus of human bone,which exhibits great potential for clinical application.展开更多
One of the biggest challenges in the biocompatibility of implantable metals is the prevention of the stress shielding effect,which is related to the coupling of the bone-metal mechanical properties.This stress shieldi...One of the biggest challenges in the biocompatibility of implantable metals is the prevention of the stress shielding effect,which is related to the coupling of the bone-metal mechanical properties.This stress shielding phenomenon provokes bone resorption and the consequent adverse effects on prosthesis fixation.However,it can be inhibited by adapting the stiffness of the implant material.Since the use of titanium(Ti)porous structures is a great alternative not only to inhibit this effect but also to improve the osteointegration of orthopedic and dental implants,a brief description of the techniques used for their manufacturing and a review of the current commercialized implants produced from porous Ti assemblies are compiled in this work.As powder metallurgy(PM)with space holder(SH)is a powerful technology used to produce porous Ti structures,it is here discussed its potential for the fabrication of medical devices from the perspectives of both design and manufacture.The most important parameters of the technique such as the size and shape of the initial metallic particles,the SH and binder type of materials,the compaction pressure of the green form,and in the sintering stage,the temperature,atmosphere,and time are reviewed according to the bibliography reported.Furthermore,the importance of the porosity and its types together with the influence of the mentioned parameters in the final porosity and,consequently,in the ultimate mechanical properties of the structure are discussed.Finally,a few examples of the PM-SH application for the manufacturing of orthopedic implants are presented.展开更多
To optimize both the mechanical and biological properties of titanium for biomedical implants,a highly flexible powder metallurgy approach is proposed to generate porous scaffolds with graded porosities and pore sizes...To optimize both the mechanical and biological properties of titanium for biomedical implants,a highly flexible powder metallurgy approach is proposed to generate porous scaffolds with graded porosities and pore sizes.Sugar pellets acting as space holders were compacted with titanium powder and then removed by dissolution in water before sintering.The morphology,pore structure,porosity and pore interconnectivity were observed by optical microscopy and SEM.The results show that the porous titanium has porosity levels and pore size gradients consistent with their design with gradual and smooth transitions at the interfaces between regions of differing porosities and/or pore sizes.Meanwhile,the porous titanium has high interconnectivity between pores and highly spherical pore shapes.In this article we show that this powder metallurgy processing technique,employing the novel sugar pellets as space-holders,can generate porous titanium foams with well-controlled graded porosities and pore sizes.This method has excellent potential for producing porous titanium structures for hard tissue engineering applications.展开更多
Titanium-based porous materials can be used in structural applications and medical implants because of their excellent mechanical properties at elevated temperatures, good corrosion resistance and wonderful biocompati...Titanium-based porous materials can be used in structural applications and medical implants because of their excellent mechanical properties at elevated temperatures, good corrosion resistance and wonderful biocompatibility. However, most of the methods used to produce the porous metal can only give limited porosity and uncontrollable pore morphologies. In the present study, a newly developed method of powder metallurgy using the space-holder technique was used to fabricate porous titanium with controllable porosity. The morphological features and mechanical properties of the products were fully investigated. The results show that the porosity is in the range of 55%-75%, and the mean pore size, with an average sphericity of -0.72, is 600 μm. The plateau stresses vary between 10 MPa and 35 MPa. As predicted by the Gibson-Ashby model, the plateau stress decreases with increasing porosity.展开更多
The main topic in this note is to discuss the boundedness of pseudo-differential operators and para-product nptfators on ihe Holder and Sobotev space;, respectively It is the preparation of the thoery of Gibbs - Butze...The main topic in this note is to discuss the boundedness of pseudo-differential operators and para-product nptfators on ihe Holder and Sobotev space;, respectively It is the preparation of the thoery of Gibbs - Butzer deffereftital operators and differential equations.展开更多
In the present work,a compressible and lubricating space-holder material commonly known as "acrawax" was used to process Cu foams with various pore sizes and various porosities.The foams were processed witho...In the present work,a compressible and lubricating space-holder material commonly known as "acrawax" was used to process Cu foams with various pore sizes and various porosities.The foams were processed without using binders to avoid contamination of their metal matrices.The lubricant space-holder material was found to facilitate more uniform flow and distribution of metal powder around the surface of the space holder.In addition,the use of acrawax as a space-holder material yielded considerably dense cell walls,which are an essential prerequisite for better material properties.The foams processed with a smaller-sized space holder were found to exhibit better electrical and mechanical properties than those processed with a coarser-sized space holder.The isotropic pore shape,uniform pore distribution throughout the metal matrix,and uniform cell wall thickness were found to enhance the properties pertaining to fine-pore foam samples.The processed foams exhibit properties similar to those of the foams processed through the lost-carbonate sintering process.展开更多
In this paper, we consider functional operators with shift in weighted H?lder spaces. We present the main idea and the scheme of proof of the conditions of invertibility for these operators. As an application, we prop...In this paper, we consider functional operators with shift in weighted H?lder spaces. We present the main idea and the scheme of proof of the conditions of invertibility for these operators. As an application, we propose to use these results for solution of equations with shift which arise in the study of cyclic models for natural systems with renewable resources.展开更多
This paper presents a pilot effort to identify a methodology to more efficiently codify, quantify and illustrate the intrinsic values associated with ecological resources as expressed by stakeholders. Existing methodo...This paper presents a pilot effort to identify a methodology to more efficiently codify, quantify and illustrate the intrinsic values associated with ecological resources as expressed by stakeholders. Existing methodologies examine the value of ecological resources, but are often criticized for their monetary focus. These methods generally produce quasi-market values for non-market resources. The natural and cultural resources associated with a national park are analyzed in terms of the expressed values of active stakeholders to quantitatively produce multiple dimensions of value for each resource relative to all others. The resulting abstract and graphical value-space quantitatively reflects stakeholder participation, reflects non-market intrinsic value, and proactively contributes to environmental management and decision making.展开更多
In this paper we consider operators with endpoint singularities generated by linear fractional Carleman shift in weighted Hölder spaces. Such operators play an important role in the study of algebras generate...In this paper we consider operators with endpoint singularities generated by linear fractional Carleman shift in weighted Hölder spaces. Such operators play an important role in the study of algebras generated by the operators of singular integration and multiplication by function. For the considered operators, we obtained more precise relations between norms of integral operators with local singularities in weighted Lebesgue spaces and norms in weighted Hölder spaces, making use of previously obtained general results. We prove the boundedness of operators with linear fractional singularities.展开更多
基金financially supported by the National Natural Science Foundation of China(No.51871077)the Guangdong Basic and Applied Basic Research Foundation(No.2021A1515012626)+3 种基金the Shenzhen Knowledge Innovation Plan-Fundamental Re-search(Discipline Distribution)(No.JCYJ20180507184623297)the S henzhen Science and Technology Plan-Technology Innovation(No.KQJSCX20180328165656256)the Development and Reform Commission of Shenzhen Municipality-Shenzhen R&D Center for Al-based Hydrogen Hydrolysis Materials(No.ZX20190229)the Startup Foundation from Shenzhen and Startup Foundation from Harbin Institute of Technology(Shenzhen).
文摘Stress shielding is caused by the mismatch of stiffness between bone and implant materials,which may give rise to bone resorption and loosening,thereby causing implantation failure.There is a huge gap between Young’s modulus of human bone and low Young’s modulusβTi alloys.A porous structure design can achieve the target of low Young’s modulus,and thus achieve the matching between human bone and implant materials.However,a suitable space holder(SH)that can be applied at high temperatures and sintering pressure has not been reported.In this study,the TiZrNbTa/Ti titanium matrix composite(TMC)with high strength and large ductility was used as scaffold materials and combined the SH technique with the spark plasma sintering(SPS)technique to obtain a porous structure.A novel space holder,i.e.,MgO particles was adopted,which can withstand high-temperature sintering accompanied by a sintering pressure.The porous TiZrNbTa/Ti with 40 vol.%MgO added exhibits a maximum strength of 345.9±10.4 MPa and Young’s modulus of 24.72±0.20 GPa,respectively.It possesses higher strength compared with human bone and matches Young’s modulus of human bone,which exhibits great potential for clinical application.
基金the Ministry of Science and Innovation of Spain for financial support(Nos.RTI2018098075-B-C21 and RTI2018-098075-B-C22)the EU through the European Regional Development Funds(No.MINECO-FEDER,EU)+1 种基金Generalitat de Catalunya(No.2017SGR-1165)the KTT Excellence Program,funded by the European Union through the European Regional Development Fund(EDF),the Government of Catalonia and the UPC。
文摘One of the biggest challenges in the biocompatibility of implantable metals is the prevention of the stress shielding effect,which is related to the coupling of the bone-metal mechanical properties.This stress shielding phenomenon provokes bone resorption and the consequent adverse effects on prosthesis fixation.However,it can be inhibited by adapting the stiffness of the implant material.Since the use of titanium(Ti)porous structures is a great alternative not only to inhibit this effect but also to improve the osteointegration of orthopedic and dental implants,a brief description of the techniques used for their manufacturing and a review of the current commercialized implants produced from porous Ti assemblies are compiled in this work.As powder metallurgy(PM)with space holder(SH)is a powerful technology used to produce porous Ti structures,it is here discussed its potential for the fabrication of medical devices from the perspectives of both design and manufacture.The most important parameters of the technique such as the size and shape of the initial metallic particles,the SH and binder type of materials,the compaction pressure of the green form,and in the sintering stage,the temperature,atmosphere,and time are reviewed according to the bibliography reported.Furthermore,the importance of the porosity and its types together with the influence of the mentioned parameters in the final porosity and,consequently,in the ultimate mechanical properties of the structure are discussed.Finally,a few examples of the PM-SH application for the manufacturing of orthopedic implants are presented.
基金the support of the ARC Research Hub for Advanced Manufacturing of Medical Devices(IH150100024)M.J.Bermingham acknowledges the support of the ARC Discovery Early Career Researcher Awards(DE160100260).
文摘To optimize both the mechanical and biological properties of titanium for biomedical implants,a highly flexible powder metallurgy approach is proposed to generate porous scaffolds with graded porosities and pore sizes.Sugar pellets acting as space holders were compacted with titanium powder and then removed by dissolution in water before sintering.The morphology,pore structure,porosity and pore interconnectivity were observed by optical microscopy and SEM.The results show that the porous titanium has porosity levels and pore size gradients consistent with their design with gradual and smooth transitions at the interfaces between regions of differing porosities and/or pore sizes.Meanwhile,the porous titanium has high interconnectivity between pores and highly spherical pore shapes.In this article we show that this powder metallurgy processing technique,employing the novel sugar pellets as space-holders,can generate porous titanium foams with well-controlled graded porosities and pore sizes.This method has excellent potential for producing porous titanium structures for hard tissue engineering applications.
基金supported by the National Natural Science Foundation of China (No. 50504020)the Chongqing Natural Science Foundation, China (No. 2008BB4051)
文摘Titanium-based porous materials can be used in structural applications and medical implants because of their excellent mechanical properties at elevated temperatures, good corrosion resistance and wonderful biocompatibility. However, most of the methods used to produce the porous metal can only give limited porosity and uncontrollable pore morphologies. In the present study, a newly developed method of powder metallurgy using the space-holder technique was used to fabricate porous titanium with controllable porosity. The morphological features and mechanical properties of the products were fully investigated. The results show that the porosity is in the range of 55%-75%, and the mean pore size, with an average sphericity of -0.72, is 600 μm. The plateau stresses vary between 10 MPa and 35 MPa. As predicted by the Gibson-Ashby model, the plateau stress decreases with increasing porosity.
文摘The main topic in this note is to discuss the boundedness of pseudo-differential operators and para-product nptfators on ihe Holder and Sobotev space;, respectively It is the preparation of the thoery of Gibbs - Butzer deffereftital operators and differential equations.
文摘In the present work,a compressible and lubricating space-holder material commonly known as "acrawax" was used to process Cu foams with various pore sizes and various porosities.The foams were processed without using binders to avoid contamination of their metal matrices.The lubricant space-holder material was found to facilitate more uniform flow and distribution of metal powder around the surface of the space holder.In addition,the use of acrawax as a space-holder material yielded considerably dense cell walls,which are an essential prerequisite for better material properties.The foams processed with a smaller-sized space holder were found to exhibit better electrical and mechanical properties than those processed with a coarser-sized space holder.The isotropic pore shape,uniform pore distribution throughout the metal matrix,and uniform cell wall thickness were found to enhance the properties pertaining to fine-pore foam samples.The processed foams exhibit properties similar to those of the foams processed through the lost-carbonate sintering process.
文摘In this paper, we consider functional operators with shift in weighted H?lder spaces. We present the main idea and the scheme of proof of the conditions of invertibility for these operators. As an application, we propose to use these results for solution of equations with shift which arise in the study of cyclic models for natural systems with renewable resources.
文摘This paper presents a pilot effort to identify a methodology to more efficiently codify, quantify and illustrate the intrinsic values associated with ecological resources as expressed by stakeholders. Existing methodologies examine the value of ecological resources, but are often criticized for their monetary focus. These methods generally produce quasi-market values for non-market resources. The natural and cultural resources associated with a national park are analyzed in terms of the expressed values of active stakeholders to quantitatively produce multiple dimensions of value for each resource relative to all others. The resulting abstract and graphical value-space quantitatively reflects stakeholder participation, reflects non-market intrinsic value, and proactively contributes to environmental management and decision making.
文摘In this paper we consider operators with endpoint singularities generated by linear fractional Carleman shift in weighted Hölder spaces. Such operators play an important role in the study of algebras generated by the operators of singular integration and multiplication by function. For the considered operators, we obtained more precise relations between norms of integral operators with local singularities in weighted Lebesgue spaces and norms in weighted Hölder spaces, making use of previously obtained general results. We prove the boundedness of operators with linear fractional singularities.
基金Project(51174243)supported by the National Natural Science Foundation of ChinaProject(106112015CDJZR135502)supported by the Fundamental Research Funds for the Central Universities,China