title: Strength reliability and in vitro degradation of three-dimensional powder printed strontium-substituted magnesium phosphate scaffolds.
abstract: Strontium ions (Sr(2+)) are known to prevent osteoporosis and also encourage bone formation. Such twin requirements have motivated researchers to develop Sr-substituted biomaterials for orthopaedic applications. The present study demonstrates a new concept of developing Sr-substituted Mg3(PO4)2 - based biodegradable scaffolds. In particular, this work reports the fabrication, mechanical properties with an emphasis on strength reliability as well as in vitro degradation of highly biodegradable strontium-incorporated magnesium phosphate cements. These implantable scaffolds were fabricated using three-dimensional powder printing, followed by high temperature sintering and/or chemical conversion, a technique adaptable to develop patient-specific implants. A moderate combination of strength properties of 36.7MPa (compression), 24.2MPa (bending) and 10.7MPa (tension) were measured. A reasonably modest Weibull modulus of up to 8.8 was recorded after uniaxial compression or diametral tensile tests on 3D printed scaffolds. A comparison among scaffolds with varying compositions or among sintered or chemically hardened scaffolds reveals that the strength reliability is not compromised in Sr-substituted scaffolds compared to baseline Mg3(PO4)2. The micro-computed tomography analysis reveals the presence of highly interconnected porous architecture in three-dimension with lognormal pore size distribution having median in the range of 17.74-26.29μm for the investigated scaffolds. The results of extensive in vitro ion release study revealed passive degradation with a reduced Mg(2+) release and slow but sustained release of Sr(2+) from strontium-substituted magnesium phosphate scaffolds. Taken together, the present study unequivocally illustrates that the newly designed Sr-substituted magnesium phosphate scaffolds with good strength reliability could be used for biomedical applications requiring consistent Sr(2+)- release, while the scaffold degrades in physiological medium.
  
  
  CONCLUSIONS
  The study investigates the additive manufacturing of scaffolds based on different strontium-substituted magnesium phosphate bone cements by means of three-dimensional powder printing technique (3DPP). Magnesium phosphates were chosen due to their higher biodegradability compared to calcium phosphates, which is due to both a higher solubility as well as the absence of phase changes (to low soluble hydroxyapatite) in vivo. Since strontium ions are known to promote bone formation by stimulating osteoblast growth, we aimed to establish such a highly degradable magnesium phosphate ceramic with an enhanced bioactivity for new bone ingrowth. After post-processing, mechanical strengths of up to 36.7MPa (compression), 24.2MPa (bending) and 10.7MPa (tension) could be achieved. Simultaneously, the failure reliability of those bioceramic implant materials, measured by Weibull modulus calculations, were in the range of 4.3-8.8. Passive dissolution studies in vitro proved an ion release of Mg(2+) and PO4(3-) as well as Sr(2+), which is fundamental for in vivo degradation and a bone growth promoting effect. In our opinion, this work broadens the range of bioceramic bone replacement materials suitable for additive manufacturing processing. The high biodegradability of MPC ceramics together with the anticipated promoting effect on osseointegration opens up the way for a patient-specific treatment with the prospect of a fast and complete healing of bone fractures.
authors.affiliation: Department for Functional Materials in Medicine and Dentistry, University Hospital Würzburg, Pleicherwall 2, 97070 Würzburg, Germany.
authors.forename: Susanne
authors.initials: S
authors.lastname: Meininger
authors.affiliation: Laboratory for Biomaterials, Materials Research Centre, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Sourav
authors.initials: S
authors.lastname: Mandal
authors.affiliation: Laboratory for Biomaterials, Materials Research Centre, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Alok
authors.initials: A
authors.lastname: Kumar
authors.affiliation: Department for Functional Materials in Medicine and Dentistry, University Hospital Würzburg, Pleicherwall 2, 97070 Würzburg, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Groll
authors.affiliation: Laboratory for Biomaterials, Materials Research Centre, Indian Institute of Science, Bangalore 560012, India; Centre for Biosystems Science and Engineering, Indian Institute of Science, Bangalore 560012, India. Electronic address: bikram@mrc.iisc.ernet.in.
authors.forename: Bikramjit
authors.initials: B
authors.lastname: Basu
authors.affiliation: Department for Functional Materials in Medicine and Dentistry, University Hospital Würzburg, Pleicherwall 2, 97070 Würzburg, Germany. Electronic address: uwe.gbureck@fmz.uni-wuerzburg.de.
authors.forename: Uwe
authors.initials: U
authors.lastname: Gbureck
mesh_terms: D000327:Adsorption; D001672:Biocompatible Materials / Q000737:chemistry; D018786:Bone Substitutes / Q000737:chemistry*; D019245:Compressive Strength; D006801:Humans; D007477:Ions; D017616:Magnesium Compounds / Q000737:chemistry*; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009985:Orthopedics / Q000379:methods; D010710:Phosphates / Q000737:chemistry*; D011208:Powders; D011312:Pressure; D066330:Printing, Three-Dimensional; D015203:Reproducibility of Results; D013314:Stress, Mechanical; D013324:Strontium / Q000737:chemistry*; D013696:Temperature; D013718:Tensile Strength; D054457:Tissue Scaffolds / Q000737:chemistry*; D014961:X-Ray Diffraction; D055114:X-Ray Microtomography
chemical_list: D001672:Biocompatible Materials; D018786:Bone Substitutes; D007477:Ions; D017616:Magnesium Compounds; D010710:Phosphates; D011208:Powders; C030781:magnesium phosphate; D013324:Strontium
keywords: Magnesium phosphate; Scaffold; Strontium; μ-CT