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Optimizing polymer scaffold processes at Rutgers University

9 November 2018 No Comment

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About Sanjeeva Murthy and the New Jersey center for Biomaterials at Rutgers University

Fibers braided into guidance conduits for nerve regeneration

Sanjeeva Murthy is an Associate Research Professor at the New Jersey Center for Biomaterials (NJCBM), Rutgers University. He is a Materials Scientist with expertise in polymers, biomaterials and biological structures.  The NJCBM was founded in 1991 by Dr. Joachim Kohn with a mission to improve patient care and public health through the development and commercialization of future generations of biomaterials. Current activities in the laboratory include synthesis and characterization of new polymers for regenerative medicine, and fabricating them into devices, scaffolds for tissue repair and replacement, biomedical implants, surgical meshes, cardiovascular stents, bone regeneration scaffolds and ocular drug delivery systems.

Degradable stents, drug-eluting hernia patches and antibacterial pace-maker pouches are some of the devices that have resulted from this laboratory, and are in clinical use.  Developments in the pipeline includes bone fixation devices and nerve conduits.  Processing polymers into various forms, fibers, pins and films is a common denominator in all these applications.  Optimization of the processing parameters to minimize degradation during processing are essential in these fabrication steps.  Capillary rheometry is an especially important tool for them in the determination of the processing parameters for extrusion and 3D printing of their materials.

Analytical challenges

The most frequent questions the team face in developing methods for fabricating NJCBM’s polymers are all focused on addressing the issues related to the fabrication degradable polymers.  Processing degradable polymers poses special problems.  Effect of temperature, presence of volatiles, duration of processing and shear rate all contribute to the degradation of the polymer.   NJCBM realized the need for 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,’ says Sanjeeva‘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.’

Capillary rheometer  –  key equipment for a processing laboratory

In the laboratory, NJCBM routinely explores new polymers for a large-range of biomedical applications. Before trying to process their polymers, the most important qualification step is to assess whether the polymer is processable, and if so determine the optimum processing parameters, including temperature, pressure (shear rate) and duration, with a minimum amount of material.  The Rosand capillary rheometer has become a go-to tool for both of these steps.  Being a research laboratory, the polymers are often of completely different compositions, and hence 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,’says Sanjeeva. ‘If the polymer is extrudable, then while evaluating the rheology of the polymer, we also typically make approx.  100mm 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 evaluation process, they often compare the processability and performance by extruding commonly used polymers such as polylactides.

The utility of the Rosand capillary rheometer in the evaluation of degradable polymers cannot be overstressed according to Sanjeeva, as these polymers have a tight window for processing.  Too low a temperature makes it non extrudable while too high a temperature causes degradation. Too short a time in the barrel may not be sufficient to equilibrate the polymer but too large a time might crosslink 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,’ states 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 ageing experiments.’

One other innovative application they use rheology for is to referee the molecular weight determinations of a series of polymers of the same composition by two different Gel Permeation Chromatography (GPC) methods (they have a Malvern Panalytical Viscotek instrument).  While GPC measures 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.

Some examples of recent successes using the Rosand rheometer are:

  • The successful extrusion of two tyrosine derived polymers, one for nerve conduit applications, and the other meniscus application
  • Extrusion of 4 – 5 mm rods using poly(L-lactic acid) that were machined into bone fixation screws.
  • Evaluating 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.

Sanjeeva will be presenting his work in a live Malvern Panalytical webinar on 13th November, which you can register for here.

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