Using Multi-Detection GPC/SEC to Determine Impact of Sterilization on Medical-Grade Polymers

Jun 06, 2014
Volume 10, Issue 10

Sterilization is a core part of the medical-grade polymer formulation process. However, high-energy sterilization techniques can have a destructive influence on core polymer features inducing chain scission, bond cleavage, and changes in molecular weight (M w ). For high-end polymers with engineered functionality, such as novel medical materials for timed drug release, the influence of sterilization must be carefully monitored because changes to polymer structure can substantially alter performance. Traditional single detection gel permeation/size-exclusion chromatography (GPC/SEC) is limited in its application for novel material analysis because there are not the appropriate reference standards needed and it does not give the advanced structural analysis required. Multi-detection GPC/SEC overcomes these issues by combining light scattering, refractive index (RI), and viscometry detection to determine absolute Mw and detailed structural information. This article gives an introduction to multi-detection GPC/SEC and presents new data showing how the technique is being used to determine the impact of sterilization on medical grade poly(L-lactide).

Polymer-based controlled drug release systems are used to deliver discrete therapeutic drug dosages to patients over days, weeks, and even months following a single injection. These systems avoid the need for repeated administration, and so decrease patient discomfort and save on healthcare resources. Certain polymer materials can be manipulated to dissolve at selective rates in vivo to ensure that a precise quantity of drug is released at the required time. Such behaviour is tailored to a given specification by precisely engineering performance-defining polymer characteristics — including molecular weight (Mw), molecular weight distribution, and structural features such as branching — during the formulation process.

It is vital to monitor molecular weight parameters throughout the preparatory stages leading up to administration, particularly following sterilization. Energy applied by processes such as gamma irradiation and electron collision sterilization often exceed the level needed to induce polymer bond breakage and chain scission, which means that sterilization can directly affect molecular weight characteristics. Appropriate polymer characterization techniques provide the detailed insight required to quantify these changes.

Multi-Detection GPC/SEC

GPC/SEC enables the determination of molecular weight for polymers, proteins, and macromolecules. The first stage of the analysis involves the separation of a dissolved sample on the basis of size using a column containing microporous packing material. The second step involves detection of eluting compounds, and this determines the amount of information generated.

Figure 1: (a) Smaller, isotropic scattering molecules scatter light evenly in all directions while (b) anisotropic scatterers scatter light at different angles at different intensities.
Traditional GPC/SEC techniques incorporate a single refractive index (RI) detector that determines the concentration of samples in each size fraction and produces a relative molecular weight distribution via a calibration curve. It relies on external reference standards for calibration because an RI detector, in isolation, is unable to measure absolute molecular weight. In addition, single detection GPC/SEC offers no information about the material structural characteristics. For custom-engineered polymers without easily assessable reference standards, and where structure strongly influences functionality, single detection GPC/SEC cannot provide the data needed to support development.

Modern GPC/SEC instruments often incorporate additional detectors alongside RI to increase the information flow from each experiment. For example, triple detection GPC/SEC also includes a light scattering detector and a viscometer. A light-scattering detector uses the direct relationship between the angle and intensity at which molecules scatter light, and the molecular mass to calculate absolute molecular weight data without any requirement for column calibration. A viscometer determines intrinsic viscosity (IV) from measurements of the pressure drop across a capillary bridge as a function of sample solution flow rate. IV is an important molecular characteristic directly related to the density of the macromolecules and is essential in determining accurate structural information.

These additional detectors give valuable information when used individually but also work in tandem to maximize analytical insight. For example, absolute molecular weight data derived from light scattering can be used alongside IV measurements to investigate the structural characteristics of a polymer by using a Mark-Houwink (MH) plot. An MH plot is a log–log graph of intrinsic viscosity against molecular weight distribution and its y intercept, gradient, and overall shape yield information. For example, a material with a high degree of branching is likely to have a shallower slope than one with a more linear structure. The ability of multi-detection GPC/SEC to deliver this information makes it a valuable technique for medical polymer analysis and development.

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