Sanjeeva Murthy is a Materials Scientist working as an Associate Research Professor at the New Jersey Center for Biomaterials (NJCBM), Rutgers University.
He is a specialist in biomaterials, polymers and biological structures. Dr. Joachim Kohn founded the NJCBM in 1991 with an objective to improve public health and patient care through the development and commercialization of future generations of biomaterials.
This laboratory is currently involved in the synthesis and characterization of new polymers for regenerative medicine, and making them into devices, biomedical implants, scaffolds for tissue repair and replacement, cardiovascular stents, surgical meshes, ocular drug delivery systems and bone regeneration scaffolds. Some of the devices developed at this laboratory that are in clinical use include antibacterial pace-maker pouches, drug-eluting hernia patches and degradable stents.
Developments in the pipeline include nerve conduits and bone fixation devices. In all these applications, processing polymers into different forms, points, films, fibers and pins is a common denominator. In these fabrication steps, the processing parameters need to be optimized to mitigate degradation during processing. Capillary rheometry is a key tool for them to determine the processing parameters for extrusion and 3D printing of their materials.
During the development of fabrication methods for NJCBM’s polymers, the team often faces questions on how to address the issues related to the fabrication of degradable polymers. Processing degradable polymers poses special problems. Duration of processing, presence of volatiles, shear rate and effect of temperature all contribute to polymer degradation. NJCBM realized the importance of a capillary rheometer after learning the hard way by trying to process their polymers by trial and error on extruders, and using up hundreds of grams of polymers.
Our material is $20 per g since they are custom synthesized. With 5-10 g of polymers we were able to determine the processing conditions before using 200 g on a large extruder. Melt indexer uses the same amount of materials but does not provide as much information as the rheometer. The understanding of the shear rate, viscosity, temperature and the pressure required for processing are valuable in our decision to proceed to large scale operation.
Sanjeeva Murthy, Associate Research Professor, The New Jersey Center for Biomaterials (NJCBM), Rutgers University
Capillary Rheometer - Key Equipment for a Processing Laboratory
In the lab, NJCBM is routinely involved in the exploration of new polymers for a wide variety of biomedical applications. Before trying to process the new polymers, a key qualification step is to know the processability of the polymer, and if so identify the optimum processing parameters, including duration, pressure (shear rate) and temperature, with a minimum amount of material. The Rosand capillary rheometer is a go-to tool for both of these steps. Being a research lab, the polymers are often of entirely different compositions and therefore, their thermal characteristics are unknown.
After initial evaluations using DSC and TGA, the first thing we do as soon as we get approx. 10 g of the polymer, is to try and extrude the polymer. If the polymer is extrudable, then while evaluating the rheology of the polymer, we also typically make approx. 100 µm diameter fibers to evaluate the mechanical and degradation behavior of the polymer. This way, the capillary rheometer enables us to screen a large number of polymers and choose the most appropriate polymer for a desired application.
In the assessment process, the performance and processability are often compared by extruding commonly used polymers such as polylactides.
According to Sanjeeva, the utility of the Rosand capillary rheometer in the assessment of degradable polymers cannot be overstressed as they have a tight window for processing. Degradation of the polymer occurs if the temperature is too high and the polymer becomes non extrudable if the temperature is too low. Too large a time in the barrel might cross-link the polymer but too short a time may not be sufficient to equilibrate the polymer.
We use the rheometer to evaluate the additives that might stabilize the polymer during degradation. We also use the rheometer to evaluate the stability of the drugs that are incorporated into the polymer,” informs Sanjeeva. “We take advantage of the temperature stability of the rheometer to assess the thermal stability of the polymer by keeping the polymer for various times and temperatures, and then measuring their rheology. Such studies are useful in scaling up the extrusion process to large batches of polymers, and also serve as accelerated aging experiments.
One other innovative application used in the laboratory for rheology is to referee the molecular weight determinations of a series of polymers possessing the same composition by two different GPC methods (the lab has a Malvern Panalytical Viscotek instrument in addition to other GPC equipment). While GPC determines the swollen volume and not the length of the polymer chains, melt viscosity is directly related to the polymer chain length with no interference from solvent and column characteristics present in GPC measurements.
The following are some of the recent successes achieved using the Rosand rheometer:
- Assessing the suitability of polymers for 3D printing applications and extruding the required diameter (1.75 mm) filaments for use with Fusion Deposition Modeling 3D printer
- Extrusion of 4 - 5 mm rods using poly(L-lactic acid) that were machined into bone fixation screws
- The successful extrusion of two tyrosine derived polymers, one for meniscus application and the other for nerve conduit applications
Choosing a Malvern Panalytical Instrument
NJCBM was in need of an instrument that could be employed as a rheometer, as well as an extruder, and was easy to handle by students with little supervision. The center considered both a capillary rheometer and a rotational rheometer but selected the Rosand capillary rheometer. Compared to rotational rheometer, a lot more material is required for a capillary rheometer, but it serves a dual purpose, as a rheometer as well as a small-scale extruder. A small-bore diameter (9.5 mm) rheometer was selected to further minimize the amount of polymer required to operate the instrument and it has been modified to facilitate filament extrusion.
Even undergraduates are able to produce high quality data with minimal training. In fact, this was used by high school students during the summer of 2017 as a part of the New Jersey Governor's School of Engineering & Technology in a project - “Fabrication and Characterization of Polymeric Sutures and Brain Stents”.
Malvern Panalytical Panlytical's reputation for technical support and service was another reason behind the selection of the Rosand rheometer and Sanjeeva expressed his gratitude about the support provided by Malvern Panalytical to date, commenting that, “Malvern Panalytical customer service is outstanding. Their technology consultant is always on call with any help that is required with the instrument. In the future, we plan to use the rheometer for measuring the viscosity of protein solutions and gels.”
Download the Full Article
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.