Addressing Polystyrene Manufacturing Issues at the Micro/Nano Scale


Challenge: A manufacturer was experiencing production issues associated with polystyrene vessels in a life science application. Our challenge was to investigate the problem, which was impacting multiple product lines, to determine the root cause of failure and provide a means of resolution.

Material: Polystyrene is critical to biomedical research and science. Its optical clarity and ability to be molded and sterilized make it ideal for this purpose. Polystyrene, however, has a hydrophobic or water-repellant surface so the surface must be treated to allow adhesion, spreading and growth of cell cultures.

Process: In a typical production process, the polystyrene is injection molded, then sterilized either by irradiation or an ethylene oxide treatment, and then the surface is modified usually with oxygen-rich plasmas. This last surface treatment step is critical to cellular adhesion, growth, and the overall success of the polystyrene product.

What Do We Know about the Surface of Polystyrene and Its Wettability?

In “Why the dish makes a difference: Quantitative comparison of polystyrene culture surfaces,” Zeiger, A. S. et al Acta Biomater. 2013, 9(7), 7354-7361, surface analysis studies were conducted on cell culture vessels to calculate root mean square (RMS) surface roughness on vessels from different commercial sources and manufacturers. The authors discovered fiber-like and pit-like features on the material surface that they labeled as “signatures of the manufacturing process.” They also noted that there were little differences in surface features related to the manufacturing process among polystyrene vessels from the same commercial source/manufacturer. They determined that several polystyrene characteristics impacted cell response including surface chemistry, topography (a result of the manufacturing process) and surface roughness. Changes in wettability (contact angle) were found to be a result of surface chemistry and the manufacturing processes.

RJ Lee Group’s surface science laboratory scientists had been conducting root cause failure analysis studies previously on the surface analysis of polystyrene materials for use as cell culture vessels. Our scientists had used atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) to investigate the surface of the polystyrene. Survey scans and high resolution scans were conducted using the XPS to determine surface chemistry. Using the AFM and following the parameters set by Zeiger, RMS roughness values were extracted and the polystyrene surfaces were determined to be heterogeneous in terms of both chemistry (XPS) and topology (AFM). Observations were consistent with the cited literature and several qualitative types of features were detected using AFM. This work gave both RJ Lee Group and the client new insight into variables that impact the polystyrene surface. This insight enabled us to re-evaluate the entire process and advise the client of specific process and specification changes that could be used to resolve the issue.

XPS map of oxygen distribution on the surface of treated polystyrene

XPS map of oxygen distribution on the surface of treated polystyrene.

What Did We Learn?

We learned that:

  • Differences exist between vessels from different manufacturers but they may also exist for the same manufacturing process and even within the same vessel.
  • Surface chemistry and topology at the nanoscale are both critical features for consistent, reproducible surface properties.
  • Protein and/or cell interaction intimately depend on the surface chemistry, mechanics and surface topography of the tissue culture polystyrene (TCPS) dishes/trays, within which the surface topography plays an important role as well.
  • Machine marks, cleaning and polishing inherently have an impact on the surface finish and properties of the various molds used. Those marks mirror onto the polystyrene surface in TCPS production.

 

AFM map of surface morphology of injection molded polystyrene showing fiber-like surface features and pits.

AFM map of surface morphology of injection molded polystyrene showing fiber-like surface features and pits.

How Do We Proceed Now that We Understand the Problem?

In this particular study, the containers were part of a failure analysis or manufacturing anomaly analysis. The material surface was not hydrophilic enough and there were also issues with the surface morphology at the nano level. Once we began the investigation, we understood more about the materials and requirements of the component from the manufacturer, open literature and product information. We found that the surface oxygen content, typically determined from XPS spot scans, is critical for success but not a sole sufficient testing method. Cell interactions intimately depend on the surface chemistry and topography of the cell culture containers and so using XPS, surface mapping and AFM are also requirements. We tied those requirements for performance to new component testing approaches. Using the data from our analyses, we provided the client with a range of steps in the product life cycle for them to investigate to guide them to a solution. They were able to resolve the issues in each of their processes which they discovered were occurring at the trial stages we jointly designed, and where each particular component was then tested.

Conclusion

We need to better understand the product life cycle in terms of requirements defined by stakeholders, regulations, specifications, modeling and testing. We can then align those requirements with testing. Multiple steps exist across the production process. Each step requires specific characterization techniques and quality control that tie into the requirements and success of the end application.

  • Identify characterization techniques that can be used across the entire production process and highlight priority tests at each stage.
  • Address the quality of raw materials, comprehensive characterization of resins, additives, fillers, processing aides, mold release agents using chromatography, spectroscopy, thermal and surface analysis, physical and mechanical testing, and biological testing with cell culture simulation studies.
  • Understand the product’s life cycle to ensure effective troubleshooting and guidance for failure prevention.

Our studies have stimulated considerable discussion and planning related to resolving production problems. Over the past six months our clients continue to improve their production approaches by revamping their processes.