Time-of-Flight Secondary Ion Mass Spectrometry Studies of in Vitro Hydrolytic Degradation of Biodegradable Polymers

Analytical tools to assess polymer biodegradation: A critical review and recommendations

Many petroleum-derived plastic materials are highly recalcitrant and persistent in the environment, posing significant threats to human and ecological receptors due to their accumulation in ecosystems. In recent years, research efforts have focused on advancing biological methods for polymer degradation. Enzymatic depolymerization has emerged as particularly relevant for biobased plastic recycling, potentially scalable for industrial use. Biodegradation involves adsorption to the plastic solid surface, followed by an interfacial reaction, resulting in cleavage of bonds of polymer chains exposed on the surface. Here, widely varying substrate-specific kinetics are observed, with the polymer's properties possessing a significant impact on the rate of this interfacial catalysis. Thus, there is a critical need for sensitive and accurate characterization of the material surface during and after interfacial depolymerization to fully understand the reaction mechanisms. Here, we provide a critical review of a range of techniques used in the analysis of material surfaces to characterize the chemical, topological, and morphological features relevant to the study of enzymatic biocatalysis, including microscopy techniques, spectroscopic techniques (e.g., X-ray diffraction analysis, Fourier transform infrared attenuated total reflectance spectroscopy, and mass spectrometry detection of analytes associated with degradation). Techniques for evaluation of surface energy and topology in their relevancy for sensitive detection of biological surface modifications are also discussed. In addition, this paper provides an overview of the strengths of these techniques and compares their performance in both sensitivity and throughput, including emerging techniques, which can be useful, particularly for the rapid analysis of the surface properties of polymeric materials in high-throughput screening of candidate biocatalysts. This research serves as a starting point in selecting and applying appropriate methodologies that provide direct evidence to the ongoing biotic degradation of polymeric materials.

Reactive Desorption Electrospray Ionization Mass Spectrometry To Determine Intrinsic Degradability of Poly(lactic-co-glycolic acid) Chains

Because of its speed, sensitivity, and ability to scrutinize individual species, mass spectrometry (MS) has become an essential tool in analytical strategies aimed at studying the degradation behavior of polyesters. MS analyses can be performed prior to the degradation event for structural characterization of initial substrates or after it has occurred to measure the decreasing size of products as a function of time. Here, we show that MS can also be usefully employed during the degradation process by online monitoring the chain solvolysis induced by reactive desorption electrospray ionization (DESI). Cleavage of ester bonds in random copolymers of lactic acid (LA) and glycolic acid (GA) was achieved by electrospraying methanol-containing NaOH onto the substrates. Experimental conditions were optimized to generate methanolysis products of high abundance so that mass spectra can be conveniently processed using Kendrick-based approaches. The same reactive-DESI performance was demonstrated for two sample preparations, solvent casting for soluble samples or pressed pellets for highly crystalline substrates, permitting to compare polymers with LA/GA ratios ranging from 100/0 to 5/95. Analysis of sample fractions collected by size exclusion chromatography showed that methanolysis occurs independently of the original chain size, so data recorded for poly(LA-co-GA) (PLAGA) copolymers with the average molecular weight ranging from 10 to 180 kDa could be safely compared. The average mass of methanolysis products was observed to decrease linearly (R² = 0.9900) as the GA content increases in PLAGA substrates, consistent with the susceptibility of ester bonds toward solvolysis being higher in GA than in LA. Because DESI only explores the surface of solids, these data do not reflect bulk degradability of the copolymers but, instead, their relative degradability at the molecular level. Based on a "reactive-DESI degradability scale" such as that established here for PLAGA, the proposed method offers interesting perspectives to qualify intrinsic degradability of different polyesters and evaluate their erosion susceptibility or to determine the degradability of those polymers known to degrade via erosion only.

Capillary zone electrophoresis as a tool to monitor the last stages of the degradation of water-sensitive polymers

In order to monitor the formation of the water-soluble by-products from chain-scission of degradable polymers used in the biomedical field, four capillary electrophoresis methods are discussed with the aim of giving the limits and performance for each. Three of them (electroosmotic flow reversal by dynamic adsorption of a polycation, multilayer polyelectrolyte coatings and physical binding of polyethylene oxide) are based on the use of dynamic coatings onto the inner surface of a fused silica capillary, a simple means to adapt performance to specific separations via modification and control of the electroosmotic flow of fused capillary. Using oligomers of lactic acid considered as standards the methods are compared. Other examples of ester-containing macromolecules (poly(hydroxybutyrate)), as well as degradable polyanions are described, namely N-acetylneuraminate polymer and poly(beta-malic acid).

Time-of-flight secondary-ion mass spectrometry for the surface characterization of solid-state pharmaceuticals

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) is a highly surface sensitive analytical method for surface chemical identification and surface chemical distribution analysis (mapping). Here we have explored the application of ToF-SIMS for the characterization of solid-state pharmaceuticals and highlight specific case studies concerning the distribution and stability of pharmaceutical actives within solid matrices (pellets and polymeric carriers) and the face-specific properties of pharmaceutical crystals.

Quantitative ToF-SIMS Studies of Protein Drug Release from Biodegradable Polymer Drug Delivery Membranes

Biodegradable polymers are of interest in developing strategies to control protein drug delivery. The protein that was used in this study is Keratinocyte Growth Factor (KGF) which is a protein involved in the re-epithelialization process. The protein is stabilized in the biodegradable polymer matrix during formulation and over the course of polymer degradation with the use of an ionic surfactant Aerosol-OT (AOT) which will encapsulate the protein in an aqueous environment. The release kinetics of the protein from the surface of these materials requires precise timing which is a crucial factor in the efficacy of this drug delivery system.Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was used in the same capacity to identify the molecular ion peak of the surfactant and polymer and use this to determine surface concentration. In the polymer matrix, the surfactant molecular ion peak was observed in the positive and negative mode at m/z 467 and 421, respectively. These peaks were determined to be [AOT + Na+] and [AOT-Na+]-. These methods are used to identify the surfactant and protein from the polymer matrix and are used to measure the rate of surface accumulation. The second step was to compare this accumulation rate with the release rate of the protein into an aqueous solution during the degradation of the biodegradable film. This rate is compared to that from fluorescence spectroscopy measurements using the protein autofluorescence from that released into aqueous solution.

Static secondary ion mass spectrometry for biological and biomedical research

Static secondary ion mass spectrometry (SSIMS) is a capable of providing detailed atomic and molecular characterization of the surface chemistry of biological and biomedical materials. The technique is particularly suited to the detection and imaging of small molecules such as membrane lipids, metabolites, and drugs. A limit of detection in the ppm range and spatial resolution <1 microm can be obtained. Recent progress in instrumental developments, notably cluster ion beams, and the application of multivariate data analysis protocols, promise further advances. This chapter presents a brief overview of the theory and instrumentation of static secondary ion mass spectrometry followed by examples of a range of biological and biomedical applications. Because of the ultrahigh vacuum requirements and extreme surface sensitivity of the technique, appropriate sample preparation and handling is essential. These protocols, and the analysis methodology required to ensure high-quality, reliable data are described.

Characterization of linear and cyclic polylactic acids and their solvolysis products by electrospray ionization mass spectrometry

Linear and cyclic polylactic acids (PLAs) were characterized using electrospray ionization mass spectrometry (ESI-MS) as part of our ongoing investigation of the hydrolysis mechanism of biodegradable polymers. The condensation oligomers of linear polylactic acid (LPLA) were synthesized by thermal dehydration of L-lactic acid. The trimer and tetramer base polymers of cyclic polylactic acid (CPLA) were obtained by cyclization reactions of lactic acid trimers and tetramers, respectively. In the ESI-MS/MS measurement, LPLA yielded three types of product ion series, while CPLA yielded only one type, from which the repeated units of CPLA were removed. The MS/MS spectrum of the NH4+ adduct ion for both cyclic and linear PLA showed loss of one ammonia molecule. The postsource decay (PSD) spectrum of CPLA by matrix-assisted laser desorption ionization (MALDI) mass spectrometry was similar to the ESI-MS/MS spectrum, while that of LPLA was different. In addition, the degradation of cyclic and linear PLAs by solvolysis was investigated. Solvolysis with anhydrous MeOH was quite feasible, but did not readily occur in the presence of even a small amount of water in the MeOH solvent.

Characterization of a homologous series of D,L-lactic acid oligomers; a mechanistic study on the degradation kinetics in vitro

A series of low molecular weight polymers of D,L-lactic acid has been synthesized. The oligomers were characterized with respect to molecular weight, glass transition temperature and solubility. The number average molecular weight of the oligomers ranged from 290 to 1320Da. Oligomers with an M(n)<800Da were soluble in buffer at pH 7.4 but insoluble in water and acidic medium. Kinetic studies were performed at pH 1.5, 4.5 and 7.4 using an accelerated in vitro monomer release test. The hydrolytic rate was dependent on molecular weight of oligomer, temperature and pH of the media, with the lowest rate found around pH 4.5. The activation energy was dependent on molecular weight and ranged from 47 to 67kJmol(-1). Random ester cleavage was identified as mechanism of hydrolysis in basic media whereas in acidic media chain-end cleavage ("unzipping") was the mode of action.

Surface analysis methods for characterizing polymeric biomaterials

Surface properties have an enormous effect on the success or failure of a biomaterial device, thus signifying the considerable importance of and the need for adequate characterization of the biomaterial surface. Microscopy techniques used in the analysis of biomaterial surfaces include scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and confocal microscopy. Spectroscopic techniques include X-ray photoelectron spectroscopy, Fourier Transform infrared attenuated total reflection and secondary ion mass spectrometry. The measurement of contact angles, although one of the earlier techniques developed remains a very useful tool in the evaluation of surface hydrophobicity/hydrophilicity. This paper provides a brief, easy to understand synopsis of these and other techniques including emerging techniques, which are proving useful in the analysis of the surface properties of polymeric biomaterials. Cautionary statements have been made, numerous authors referenced and examples used to show the specific type of information that can be acquired from the different techniques used in the characterization of polymeric biomaterials surfaces.

Quantitative TOF-SIMS analysis of oligomeric degradation products at the surface of biodegradable poly(alpha-hydroxy acid)s

This paper reports the development of a new method for quantification of the hydrolytic surface degradation kinetics of biodegradable poly(alpha-hydroxy acid)s using time-of-flight secondary ion mass spectrometry (TOF-SIMS). We report results from static SIMS spectra of a series of poly(alpha-hydroxy acid)s including poly(glycolic acid), poly(L-lactic acid), and random poly(D,L-lactic acid-co-glycolic acid) hydrolyzed in various buffer systems. The distribution of the most intense peak intensities of ions generated in high mass range of the spectrum reflects the intact degradation products (oligomeric hydrolysis products) of each biodegradable polymer. First, a detailed analysis of the oligomeric ions is given based on rearrangement of the intact hydrolysis products. The pattern of ions can distinguish both degradation-generated intact oligomers and their fragment ion peaks with a variety of combinations of each repeat unit. Then, the integration and summation of the area of all ion peaks with the same number of repeat units is proposed as a measurement that provides a more accurate MW average than the typically used method which counts only the most intense peak. The multiple ion summation method described in this paper would be practical in the improvement of quantitative TOF-SIMS studies as a better data reduction method, especially in the surface degradation kinetics of biodegradable polymers.

Mass spectra of copolymers

Recent and older literature (covering the last 12-13 years) in the field of mass spectra of random and block copolymers is reviewed. A detailed description is given of the information on copolymer properties that can be recovered from the analysis of the low-mass region of the spectrum (the region below 500 Da) and the high-mass region. The features of mass spectra of copolymers obtained by different synthetic routes are discussed, such as free radical, condensation, ring-chain equilibration, microbial synthesis, ring-opening, simple anionic, cationic, Ziegler-Natta, and/or metallocene catalysis, along with some random and block copolymers that occur in Nature. The emphasis is on copolymer composition and average molar mass determination, and on the benefits of coupling mass spectrometry (MS) with separation techniques such as size-exclusion chromatography (SEC) and high performance liquid chromatography (HPLC).