Mechanical isolation of cellulose nanofibers and their utilisation in novel nanocomposites for medical applications Aji P. Mathew and Kristiina Oksman Niska Division of Manufacturing and Design of Wood and Bionanocomposites Luleå University of Technology(LTU), Sweden Wood & Biofiber Plastic Composites Conference and Cellulose Nanocomposites Symposium, Madison, May 11-13, 2009
Outline Background and Aim Processing Isolation of nanofibers from wood Cellulose nanofiber based composites Characterisation Results and discussions Conclusions
EU-Project: TEM-PLANT The aim of the project is to develop products with superior biomechanical features using hierarchical structures existing in nature (wood, collagen) to be used as scaffolds for bone and ligaments substitution. Fratzl P. Current Opinion in Colloid & Interface Science 2003;8(1):32-39.
Aim of this Study Produce wood based cellulose nanofibers Develop an efficient process route Understand the fibrillation of hardwood and softwood Develop fibrous nanocomposites based on cellulose nanofibers which resembles natural ligament/tendon www.emeraldinsight.com
Isolation of Wood Nanofibers Raw Materials Bleached hardwood fibers, dried pellets (Rayonier, USA, 97% cellulose) Bleached softwood fibers, dried sheets (Domsjö Fabriker AB, Sweden, 94% cellulose) Isolation process Different mechanical treatments Testing Microscopy Mechanical testing of prepared fibre networks
Mechanical Isolation Hardwood and softwood cellulose fibers Soaking in water and mixing Repeated until steady state of isolation Repeated until steady state of fibrillation Sieving step for hardwood cellulose Cellulose fiber suspension Refining Cellulose fiber suspension High pressure Homogenisation Cellulose nanofibers Methodology
Isolation of Cellulose Nanofibers Hardwood and softwood cellulose fibers Soaking in water and mixing Softwood Hardwood Repeated till steady state of fibrillation Repeated till steady state of isolation Sieving step for hardwood cellulose Cellulose fiber suspension Refining Cellulose fiber suspension High pressure Homogenisation Cellulose nanofibers Fiber size 50 G 50 G 5 H 100 H
Fiber Size Softwood 15 25 µm diameter Hardwood < 50 nm diameter
Mechanical Properties of Nanofiber Networks The mechanical property is an indication of extent of nanofiber isolation Hardwood is not affected much by ultrafine grinding but isolated by homogenisation Softwood fiber is isolated during grinding and stabilized after 5 homogenization steps
Results from the Cellulose Isolation Study Cellulose nanofibers can be prepared from hard- and softwood species Optimum process conditions were 50 grinding & 100 homogenization steps for hardwood cellulose 50 grinding & 5 homogenization steps for softwood Energy consumption is higher for hardwood Softwood cellulose fibers are the preferable raw material for composites
Cellulose Nanofiber Based Composites Target: Composites with strength of 28-40 MPa and strain of 20-30% at body temperature (37 C) and high moisture conditions. 1 1) Santis de R, et al. Composite Science and Technology 2004;64:861 871. Natural rubber-cellulose nanofiber composites
Characterization Scanning Electron Microscopy, JEOL Tensile Testing, Shimadzu Autograph AGX At room temp. At 37 C, 95% RH Stress Relaxation, DMA 800, TA Instruments At 37 C,0.5% strain
Natural Rubber-Cellulose Nanofiber Composites Cellulose Nanofibers NR latex solution casting, drying NR Cellulose composites (NR/CNF75)
Morphology NR NR NR-CNF25 NR/CNF75 NR-CNF50 NR-CNF75 Ductile fracture for natural rubber Layered structure visible for composite
Mechanical properties Sample Strength (MPa) Strain (%) E-Mod (MPa) NR # 1.2±0.3 580.8±50.3 2.0 ± 0.3 * NR/CNF75 77.3±5.0 4.5±1.3 3900 ± 300 NR/CNF75 (wet,37 C) 36.1±4.5 15.4±5.8 12.4 ± 4.4 # Tested at a strain rate of 500mm/min,*Modulus at 50% elongation Nanofibers increased the strength and modulus In simulated body conditions, the mehanical properties are lower, but with in the required performance range.
Stress relaxation Increase in stress occured after each step probably due to fibrils orientation during loading
Conclusions Cellulose nanofibers can be isolated from hardwood and softwood Softwood fibrillation is more energy efficient and economical Cellulose nanofibers have a great potential in biomedical applications The mechanical performance of the developed composites closely resemble natural ligaments and tendons Studies on biocompatibility, water sorption and prototype development are under progress
Acknowledgements European Commission for the financial support (NMP4-CT-2006-033277 TEM-PLANT) Our students Wolfgang Stelte (PhD) and Niclas Björngrim (MSc) for nanofiber processing Dr Benoit Duchemin for great discussions
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