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
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.
(PHOTO CREDIT: MIGUEL MALO/GETTY IMAGES)
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.
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.
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.
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.
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.