Self-organizing microtubule networks in cellulose science
by Jeffrey M. Catchmark
The plant cell engages in a unique set of integrated nanoscale processes relevant to some of the most important challenges which face our society: sustainable energy production, storage, utilization and materials manufacturing. Inside a plant cell, the process of photosynthesis stores energy contained in sun light through the assembly of glucose (sugar) molecules from water and carbon dioxide. The plant cell uses glucose as a fuel through the process of cellular respiration. Amazingly, plant cells as well as bacteria and other organisms contain unique nanomanufacturing proteins which also use glucose to assemble nanofibers known as cellulose, the most abundant renewable material resource on the planet. The production and orientation of these fibers is orchestrated by the plant cell on the nanoscale through a hierarchical assembly process producing a unique natural composite structural material across a length scale extending over 10 orders of magnitude. On the most fundamental level, this organization is being orchestrated by microtubules and biological molecular motors. Biological motors are proteins and literally nanoscale motors measuring ~4nm x 4nm x 7nm with a tail for binding to cargo which extends as long as ~80nm. These motors are powered by the hydrolysis of adenosine triphosphate (ATP) and ‘walk' on polymers of tubulin known as microtubules. These microtubules measure 25nm in diameter and up to ~25 microns in length. Biological molecular motors like kinesin play a role in the formation of microtubule networks which guide the motion of cellulose synthesizing proteins orienting the cellulose fibrils. These cellulose fibrils are combined with hemicelluloses and lignin to create the plant cell wall and on a larger scale, wood.
Researchers are now exploring new methods of assembling cellulose fibers into composite materials using the plant cell as a model system. Over the past several years, biological proteins such as biomotors and microtubules are being manipulated outside biological cells. Researchers have found that engineered biomotor complexes can organize intricate microtubule networks outside of the cell exhibiting a wide range of geometries. These networks exhibit order extending from the nanoscale to the millimeter scale. Our group at Penn State University is beginning to use these dynamic microtubule networks for aligning and weaving nanoscale cellulose fibrils into new materials using biological molecular motors. The first step toward this goal is the coupling of biological motor proteins to cellulose. Cellulose particles were successfully functionalized with NHS-dPEG™12 Biotin. Biotinylated kinesin were linked to the cellulose particles using neutravidin. To determine if the cellulose particles were functionalized with kinesin biomotors, a solution containing microtubules and kinesin functionalized cellulose was prepared and the microtubule motility dynamics studied. The movie shown depicts microtubule movement on the cellulose particle, clearly demonstrating that the cellulose has been functionalized with kinesin biomotors. Current work focuses on using nanoscale cellulose whiskers and actual ordered microtubule networks for creating engineered cellulosic materials.
Nanoscale engineering of cellulosic material may provide many advantages for forest products. The nanoscale organization of cellulose fibers may reduce the amount of fiber needed for wood composites and paper while improving material properties. Improvement in material properties could include lighter weight, equivalent or better strength or even better insulating properties associated with an increase in trapped air within the nanoporous material. Moreover, new applications of cellulose fiber may be realized. For example, cellulose is being examined as a tissue scaffold material for biological applications. Cellulose materials containing engineered nanoscale order may provide improved biocompatibility. Another example could be improved dielectric properties for electronics on paper. Paper which exhibits better insulating properties and improved surface morphology could enable high frequency electronic sensors or radio frequency identification (RFID) devices to be printed directly onto its surface. As nanotechnology permeates this field, many new materials, processes and applications will undoubtedly be developed.
Jeffrey M. Catchmark, Ph.D.
Engineering Science and Mechanics
College of Engineering
Agricultural and Biological Engineering and
School of Forest Resources
College of Agricultural Sciences
The Pennsylvania State University