Comparative Analysis of Irradiation Technologies on
High-Density Polyethylene (HDPE) for
Biopharmaceutical Applications

Blanche Krieguer, Fabien Girard, Samuel Dorey, Nathalie Dupuy, Sylvain

R.A. Marque

► To cite this version:

Blanche Krieguer, Fabien Girard, Samuel Dorey, Nathalie Dupuy, Sylvain R.A. Marque. Comparative Analysis of Irradiation Technologies on High-Density Polyethylene (HDPE) for Biopharmaceutical Applications. European Journal of Pharmaceutical Sciences, 2026, 217, pp.107394. 10.1016/j.ejps.2025.107394. hal-05418551v2

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# Comparative analysis of irradiation technologies on high-density polyethylene (HDPE) for biopharmaceutical applications

Blanche Krieguer<sup>a, *, †</sup>, Fabien Girard<sup>c</sup>, Samuel Dorey<sup>b, *, †</sup>, Nathalie Dupuy<sup>c, *, †</sup>, Sylvain R.A. Marque<sup>a, *, †</sup>

## ARTICLE INFO

### Keywords:

Gamma/X-ray/e-beam technologies

Polyethylene

ESR

DSC

Tensile

HPLC

## ABSTRACT

The impacts of e-beam and X-ray irradiation on medical-grade high-density polyethylene (HDPE) are compared with that of gamma irradiation to evaluate their potential effects on pharmaceutical applications. An extensive suite of analytical techniques was employed to evaluate the chemical and physical transformations of irradiated HDPE. Key findings indicated that irradiation did not significantly alter the mechanical properties, as tensile strength and elongation at break remained stable across all irradiation types and doses. Thermal analysis via Differential Scanning Calorimetry (DSC) revealed a slight decrease in melting temperature at higher doses, with an equivalent melting temperature peak observed for all three irradiation technologies. Electron Spin Resonance (ESR) detected alkyl and allyl radicals, which decreased over time, showing no significant differences attributable to irradiation type or dose. Colorimetric analysis indicated yellowing in the samples, linked to specific additives. The quantification of methionine oxidized byproducts by High-Performance Liquid Chromatography (HPLC) demonstrated that the oxidation potential was equivalent for the three irradiation technologies for HDPE. The study concluded that there was a lack of significant impact of irradiation technologies on several physical, chemical and mechanical properties of HDPE.

## 1. Introduction

This study investigates the effects of gamma, e-beam, and X-ray irradiation on medical-grade polyethylene (PE) to assess its implications for pharmaceutical applications (BPSA Technical Guide, 2021). Polyethylene is widely recognized for its excellent mechanical properties (Chen et al., 2014; Mansour, 2001) and thermal properties (Hu et al., 2016; Tavman et al., 2011), making it a preferred material in the biopharmaceutical industry (Makuuchi and Cheng, 2012). It is commonly used in various components such as pH sensor bodies, connectors and bag ports (BPSA Technical Guide, 2021), highlighting its versatility and importance in medical and biopharmaceutical devices (Gopanna et al., 2019). In parallel to that Ultra-High Molecular Weight Polyethylene (UHMWPE) have been extensively investigated upon irradiation ((Clough, 2001; Kurtz, 1999; Lewis, 2001; Premnath et al., 1996). This material could be used in single use systems potentially interacting with biologics potentially leading to oxidation and to structure modification

(Fricke et al., 2019; Gupta et al., 2022; Jacobitz et al., 2021; Schroeder and Cavacini, 2010; Tavella et al., 2022; Wang et al., 2007).

For radiation processing applications, the primary ionizing radiation sources are gamma rays from cobalt-60, e-beam accelerators, and X-rays (Armenante and Akiti, 2019; Darwis et al., 2015; De Brouwer, 2022; Dupuy et al., 2022). E-beam irradiation occurs when electrons are accelerated by an electromagnetic field within a particle accelerator. X-rays are photons generated using an electron beam accelerator (Loveland et al., 2017). The e-beam is transformed into photons by directing the electrons into a high-density material, such as tungsten or tantalum. While both X-rays and gamma rays are photons, gamma rays originate from radioactive sources and are emitted during radioactive decay (Makuuchi and Cheng, 2012). E-beam irradiation presents a different dose profile and has limited penetration, necessitating adjustments in packaging. The primary distinction among these irradiation technologies lies in the average dose rates delivered, which vary significantly (gamma rays deliver doses in the range of a few kGy/h,

<footnote><sup>a</sup> Aix Marseille Univ, CNRS, ICR, Case 551, 13397 Marseille, France</footnote>

<footnote><sup>b</sup> Sartorius Stedim FMT S.A.S, Z.I. Les paluds, Avenue de Jouques CS91051, 13781, Aubagne Cedex, France</footnote>

<footnote><sup>c</sup> Aix Marseille Univ, Avignon Univ, CNRS, IRD, IMBE, Marseille, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: blanche.krieguer@univ-amu.fr (B. Krieguer), Samuel.dorey@sartorius.com (S. Dorey), nathalie.dupuy@univ-amu.fr (N. Dupuy), sylvain.marque@univ-amu.fr (S.R.A. Marque).</footnote>
X-rays in kGy/min, and e-beam in kGy/s)(Buttafava et al., 2007).

Upon exposure of PE to ionizing radiation, free radicals are generated, leading to modifications within the material. These modifications can include crosslinking, chain scission, and radiation-induced oxidation, which are crucial for understanding the material's behavior post-irradiation (Forster et al., 2019; Goulas et al., 2004; Tian et al., 2023). A previous study using electron spin resonance (ESR) (Krieguer et al., 2024b) demonstrated that the type and quantity of radicals generated in PE are equivalent, regardless of whether the irradiation is carried out with gamma rays or X-rays. For high-density polyethylene (HDPE), radicals such as alkyl, allyl, and polyenyl are observed, with their presence and concentration being equivalent across the aforementioned irradiation technologies.

In this study, irradiation doses of 50 kGy and 100 kGy were employed to compare the effects of the three irradiation technologies. 50 kGy have been selected to represent a worst case to standard routine sterilization range 25–45 kGy. 100 kGy have been selected to amplify materials properties modifications to allow a better detectability. The investigation focuses on the structural modifications in polymers induced by these irradiation technologies, aiming to elucidate their general characteristics for potential use in biopharmaceutical applications or interactions with biopharmaceutical solutions. Long-lived radicals were tracked using ESR, a technique that enables the detection and quantification of radical species present in irradiated materials. Differential Scanning Calorimetry (DSC) was used to assess changes in transition temperatures and heat capacity, serving as indicators of material degradation. The generation of reactive species such as peroxides and peracids due to radical post reactivity is probed by the oxidation of methionine (mimicking the oxidation of proteins) monitored by high performance liquid chromatography (HPLC)(Girard-Perier et al., 2022; Krieguer et al., 2025a, 2025b).

The yellowing was analyzed using colorimetry. Chemical modifications in the polymer structure were identified through Fourier Transform Infrared Spectroscopy (FTIR), which detects changes in chemical bonds and functional groups, providing insights into molecular alterations caused by irradiation. The mechanical properties of the polymers were evaluated using tensile tests, which measure the strength and elasticity of materials under tensile stress. Then all these techniques will show that, in most cases, non-statistical difference are reported.

## 2. Materials and method

### 2.1. Samples

The samples investigated in this article are two high-density polyethylene (HDPE). Their chemical structure is depicted in Table 1 displays their properties: density, melting flow index and tensile strength at yield.

### 2.2. Samples packaging

The samples are placed in cardboard boxes at room temperature at Sartorius Stedim FMT S.A.S, Aubagne. Dose uniformity is monitored

<table><caption>**Table 1**
Properties of Non-irradiated HDPE commercially available (Ineos, 2025, 2015).</caption><thead><tr><th>Polymers</th><th>Identification number</th><th>Density (g/cm³)</th><th>Melt flow index (MFI) (g/10 min) (190 °C/ 2.16Kg)</th><th>Tensile Strength at Yield (MPa)</th></tr></thead><tbody><tr><td rowspan="2">High-density polyethylene (HDPE)</td><td>PE-1¹</td><td>0.960</td><td>7.6</td><td>31</td></tr><tr><td>PE-2¹</td><td>0.953</td><td>26</td><td>26</td></tr></tbody></table>

through dosimetry and is ensured by the organization of plastic components in thin cardboard boxes.

### 2.3. Samples irradiation

#### 2.3.1. Gamma rays

Gamma ray irradiation, sourced from ⁶⁰Co, was conducted at Ionisos in Dagneux, France, with an average dose rate of 2 kGy/h. The irradiation process spanned several days, accumulating the dose over multiple runs within the irradiation bunker in a random schedule.

#### 2.3.2. X-ray

X-ray irradiation was carried out at Aerial-CRT using the feerix® facility, which operates on a Rhodotron (TT300 – IBA). A 7 MV X-ray field was generated by converting a 7 MeV electron beam using a tantalum target. The irradiation was performed horizontally using a pallet conveyor, with multiple runs through the X-ray field to accumulate the dose. This process was completed in single day, achieving an average dose rate of approximately 12 kGy/h.

#### 2.3.3. e-Beam

e-Beam irradiation was carried out at Aerial-CRT using the same feerix® facility as the X-ray treatment. A 10 MeV vertical electron beam was employed, with an average dose rate of 18 MGy·h⁻<fnref n="1" /> (5 kGy/s). The irradiation was conducted in increments of 50 kGy to avoid significant temperature increases in the samples.

### 2.4. Dosimetry

The targeted delivered doses were approximately 50 kGy and 100 kGy, with a dose uniformity of ± 10 %. Table 2 provides a summary of the effective doses. To accurately measure the absorbed dose, alanine pellets combined with ESR spectroscopy (Magnettech MS5000 ESR, Bruker) were used, along with AerEDE® dosimetry software (Aerial, France). Dosimetry readings were performed in a manner traceable to an international standard. Three dosimeters were positioned within the volume of the box containing the samples to ensure dose uniformity within the sample box.

### 2.5. Electron spin resonance (ESR)

ESR measurements were performed using a Magnettech MS5000X ESR spectrometer manufactured by Brucker, Switzerland, and operated with ESR studio software for control. Measurements were performed from 1 to 60 days after sample irradiation, at following days (D): D9, D15, D30, D60. During this time, samples were stored without light and at controlled temperature and humidity (25 °C and 45 % RH). Signal amplitude (highest peak to peak signal) was measured on ESR spectra and calibrated using TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl standard samples at spin concentration between 10¹⁶ and 10¹⁹ spin/g. For more details see SI and (Krieguer et al., 2024a).

### 2.6. Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) was employed to assess the physical and thermal properties of polymers. This technique involves measuring the temperature of the sample with a thermocouple inside the

<table><caption>**Table 2**
Effective doses on samples irradiated by gamma-rays, X-ray and e-beam.</caption><thead><tr><th rowspan="2">Target doses (kGy)</th><th colspan="3">Effective doses (kGy; ± 4 %)</th></tr><tr><th>Gamma</th><th>X-rays</th><th>e-beam</th></tr></thead><tbody><tr><td>50</td><td>51</td><td>50</td><td>55</td></tr><tr><td>100</td><td>90</td><td>99</td><td>109</td></tr></tbody></table>

<footnote n="1">¹ PE-1 and PE-2 are supplied from INEOS. Detailed properties with some of data can be found in the technical sheet (Ineos, 2025, 2015).</footnote>
furnace, while a separate thermocouple measures the temperature of a reference, which is an empty crucible. The variations in heat flow during the melting transition are recorded, producing a thermogram. These tests were conducted using a Sensys Evo131 thermal analyzer from Setaram. Measurements were performed one month after irradiation.

Each sample, weighing about 20 mg, experienced three cycles of heating and cooling during the DSC analysis. For polyethylene samples, the temperature range was set from room temperature to 220 °C, with a heating rate of 10 °C per minute. To ensure accuracy, a stabilization period was included before and after each cycle. Data from the second and third cycles were used for analysis, as the first cycle was intended to remove any previous thermal history of the material. The melting peak temperature and the onset of melting were calculated and integrated using Calisto Processing software.

## 2.7. Tensile test

Elongation, ultimate tensile strength (UTS), toughness, and the yields were measured on the test sample. The mechanical properties of irradiated and non-irradiated samples were measured using a universal testing machine (Zwick Z005). Measurements were performed one month after irradiation. The crosshead speed used was set at 50 mm/min. The initial gauge length was 100 mm. Five to eight specimens were tested for each sample. Tensile strength tests were conducted at a temperature of 23 ± 3 °C and a pressure of 1 atm. UTS is defined as the maximum stress the material can endure, calculated by dividing the maximum load by the original cross-sectional area of the specimen (Dorey et al., 2019; Girard-Perier et al., 2023). For more details see SI.

## 2.8. Colorimetry

Color measurements of the polymeric materials were outsourced to AERIAL. Color measurements were performed using the CIELAB color space also referred to as L*a*b* with the CR 400 Konica Minolta chromameter. Verification measurements were done with white plaque CR A43 Konica Minolta. L*, a*, b* values were recorded for each sample. L* indicates the brightness level between light and dark, a* represents the balance between green and red coloration, and b* represents the balance between blue and yellow coloration. According to the L*, a*, b* color diagram presented in the literature (X-rite and Pantone, 2016), when a color is expressed in CIELAB, a shift in the +α* direction indicates a red shift, while a shift in the +b* direction indicates a shift towards yellow. The central L* axis shows L* = 0 (black or total absorption) at the bottom. At the center of this plane is neutral gray.

The measurement is valid only if the sample surface is plane, so that, measurements (from 2 to 5) were performed at different locations on the sample depending on the sample shape. The l*, a*, b* were measured prior to irradiation and after one month after gamma, e-beam and X-ray irradiation at 50 kGy.

## 2.9. Infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) was employed to analyze alterations in the chemical properties of the polymer surface. Samples were placed on the Bruker "Golden Gate" ATR accessory with a diamond crystal, capturing ATR-FTIR spectra from 4000 to 650 cm⁻¹ using a Thermo Nicolet IS10 spectrometer. Measurements were performed from ten days after irradiation. To address polymer inhomogeneity, three spectra per sample were recorded, with background scans ensuring cleanliness. For the FTIR fingerprint spectral region [1800–700 cm⁻¹], the spectra were baseline corrected and normalized. The method, involving minimal penetration depth, was described in the literature and in SI (Krieguer et al., 2025c, 2024c). For more details see SI.

## 2.10. High performance liquid chromatography (HPLC)

After cutting each sample into small pieces (2 mm), they were placed in vials filled with a 50 µM methionine solution in buffer (10 mM NaH₂PO₄, 10 mM Na₂B₄O₇•10H₂O, 5 mM NaN₃, pH=8.2). The concentrations of methionine and methionine sulfoxide were determined using an Agilent 1260 HPLC system equipped with a quaternary pump, an autosampler, and a fluorescence detector. The gradient program, the separation details and the HPLC system conditions were described in SI and in a previous article (Krieguer et al., 2025c, 2025a).

## 2.11. Principal component analysis (PCA)

Principal component analysis (PCA) is a commonly used method in chemometrics, and has been previously described in previous study (Krieguer et al., 2025c). Each principal component (PC) captures the highest remaining variance in the dataset. By projecting scores onto the PC-defined space, one can visualize the similarities and differences among samples. The loadings indicate which variables contribute most to each PC. PCA models were created using CAMO's UNSCRAMBLER X.1 software (Computer Aided Modelling, Trondheim, Norway).

## 2.12. Equivalency analysis

### 2.12.1. Regression models

Regression models were performed using Minitab software to assess linear multiparametric models that identify and quantify the factors influencing the properties of polymers. Data is collected for several independent variables that may affect polymer properties. Coefficients are estimated through linear regression, quantifying the impact of each independent variable on polymer properties. This model allows for the evaluation of the significant impact of each independent variable, with a 95 % confidence interval (α=0.05). The Pareto chart displays the absolute values of standardized effects from the largest to the smallest effect. The chart also plots a reference dashed line indicating whether the effects are statistically significant. The reference line for statistical significance depends on the significance level (denoted by α), the number of samples, and the degree of freedom. The model can then be used to optimize processing conditions or polymer composition (Malashin et al., 2023; Minitab Statistical software, 2022).

### 2.12.2. TOST equivalence test

To assess the equivalency between gamma, X-rays, and e-beam irradiated samples, equivalence tests (TOST, two one-sided t-test (Girard-Perier et al., 2023)) were performed with Minitab® software. For the tensile tests, the equivalency is claimed when the ratio [Radials]X-ray or e-beam/[Radicals]gamma or [Radicals]e-beam/[Radicals]X-ray is above the lower limit and below the upper limit. For the elongation, the criterion is 25 % and for the yield the criterion is 15 %. The TOST (Two One-Sided T-Test) is particularly valuable in the context of evaluating irradiation technologies as it provides us with the ability to demonstrate the equivalence of the effects of different irradiation technologies on material properties. This approach is essential to ensure that the observed variabilities do not compromise the functionality of the materials. Unlike statistical tests, such as t-test, that focus solely on detecting significant differences, the TOST allows validation that the differences between irradiation technologies remain within acceptable limits defined by industrial equivalence criteria on the intended application range. These criteria are established based on the practical relevance of the results, rather than relying exclusively on statistical thresholds (Ni et al., 2023).
# 3. Results and discussion

## 3.1. Electron spin resonance

ESR analysis was performed after gamma, X-ray and e-beam radia-
tion at 50 and 100 kGy. For HDPE, radicals such as alkyl, allyl, and
polyenyl (Fig. 1) have been observed in a preliminary study, after irra-
diation by X-ray and gamma at 50 kGy (Krieguer et al., 2024b). Various
types of PE were also analyzed including HDPE, linear low-density
polyethylene (LLDPE), and low-density polyethylene (LDPE) after
gamma and X-ray irradiation. The ESR signals for LLDPE and LDPE
samples could not be included in the quantitative analysis due to their
short lifetimes, with concentrations rapidly diminishing below the limit
of detection (LOD). The absence or weak signals observed in LLDPE and
LDPE samples may be attributed to their low degree of crystallinity.
Radicals in these materials can easily self-terminate due to higher
mobility, which is a result of the greater amorphous phases present in
LLDPE and LDPE compared to HDPE (Yamamoto et al., 2005).

Fig. 2 displays the ESR signals of PE-1 and PE-2 post-irradiation. Measurements after gamma irradiation were performed several days later due to the time required for sample transportation. No ESR signals were detected in non-irradiated samples. The observed multiplet with a g-factor of 2.003 corresponds to alkyl radicals, which are attributed to a mixture of overlapping signals of alkyl, allyl, and polyenyl radicals (Fig. 1)(Jahan and McKinny, 1999; Kornacka et al., 2013; Krieguer et al., 2024b; Mehmood et al., 2013; Silva et al., 2003; Stadler, 2011a). Over time, the multiplet signal evolved into a single line, attributed to polyenyl radicals, as evidenced by the signal at 60 days and no change in the g-value (Jahan et al., 2001; Jahan and McKinny, 1999; Seguchi and Tamura, 1973; Silva et al., 2003; Stadler, 2011b).

Fig. 3 shows the decrease in radicals generated after irradiation of PE-1 and PE-2. The structures of alkyl, allyl and polyenyl radicals were demonstrated in our previous studies (Krieguer et al., 2024b).

Regression models (Pareto charts) were developed for each sample to evaluate the impact of dose, ageing, and irradiation technology on radical production (Fig. 4). Post-irradiation time (i.e., kinetics) has a significant effect, with radical levels decreasing as post-irradiation time increases for both PE (Fig. 4). When comparing X-ray and e-beam with gamma irradiation, the Pareto charts for each sample revealed no statistical differences among the irradiation technologies, and no statistical difference in the dose's effect on radical concentration.

## 3.2. Differential Scanning Calorimetry (DSC)

The DSC results (Fig. 5) of polyethylene’s thermal properties indicate that the melting temperature is around 135 °C ± 5 °C agreeing with the literature (Hu et al., 2016). Regression models were developed for each sample to evaluate the impact of dose and irradiation technology on the melting peak and onset temperature (Fig. 6 and in Section 3.2. in SI). When comparing X-ray and e-beam with gamma irradiation, the Pareto charts for each sample revealed no statistical differences among the irradiation technologies, but there was a statistical difference in the dose’s effect on the melting peak temperature and melting onset temperature. Both samples exhibited erratic behavior in their thermal properties after irradiation. This phenomenon could indicate a competition between chain scission and cross-linking. Some authors reported that cross-linking could lead surprisingly to a decrease of the melting temperature (Charlesby and Ross, 1953; Lewis, 2001). The competition

between chain scission and cross linking is also affected by the materials
recipe and especially by the stabilization package.

## 3.3. Tensile test

Tensile tests were performed on both HDPE. Values of melt flow index (MFI) are directly related to the reverse of the melt viscosity and to molecular weight. Hence, grade with low MFI generally display greater stiffness and reduced elongation at break, whereas those with high MFI demonstrate increased deformability due to their lower molecular weight and decreased entanglement density as highlighted by PE-1 and PE-2 (Table 1)(Ineos, 2025, 2015).

The stress-strain curves of PE obtained after each irradiation technology (i.e. gamma, X-ray, and e-beam) are displayed in Figure S2 in SI. Fig. 7 displays elongation at yield and Tensile strength at Yield for both HDPEs. Regression models were developed for both polyethylene samples and pareto chart are displayed in Section 3.3 in SI. For elongation at break and Ultimate tensile strength, the Pareto charts revealed no statistical differences among the irradiation technologies and the dose.

Hypothesis tests (TOST) were performed to compare elongation at yield and tensile strength at yield (p-values in Table S1). For all conditions, the p-values from TOST are less than 0.05, indicating equivalences for tensile strength at yield and elongation at yield of both PE are equivalent. The results presented in SI show that the three technologies and the two doses are equivalent for both PE and for elongation and tensile strength at yield. This means that even with observed variations, they are deemed insignificant for the practical application of the material, thus ensuring that the product's performances are not affected.

## 3.4. Colorimetry

This study focused on measuring the L*, a*, and b* values prior and post-irradiation at 50 kGy. A PCA was performed using the results from the color analysis (Fig. 8) and displaying three distinct groups: non-irradiated samples, irradiated PE-1, and irradiated PE-2.

The group corresponding to non-irradiated PE is negatively projected on the first component (PC-1, 68 % of the total variance) and the second component (PC-2, 32 % of the total variance). The group corresponding to PE-1 irradiated is positively projected on PC-2 and is projected negatively on PC-1 (Fig. 8a). The examination of the corresponding loading (Fig. 8b) shows that these samples are characterized by a high value of L* and a low value of b*. This points out an increase of opacity for PE-1 regardless of the irradiation technology and also points out a slight increase in yellowness for gamma irradiated samples. The group corresponding to PE-2 irradiated is positively projected on PC-1 (Fig. 8a). The examination of the corresponding loading (Fig. 8b) shows that these samples are characterized by a high value of b*, increasing significantly when PE-2 is irradiated, resulting in a more pronounced yellowing compared to non-irradiated samples. The yellowing effect and the brightness are more pronounced for the gamma and X-ray irradiated samples than for the e-beam irradiated samples.

This yellowing is attributed to the formation of chromophore centers linked to free radicals and oxidative transformations of phenolic antioxidants (Gaston et al., 2018). PE-1 samples exhibit less yellowing than PE-2 after irradiation, which can be explained by the presence of specific additives that stabilize the material and reduce the formation of color centers (Kömmling et al., 2017). The additives in PE-1 likely enhance the stability of antioxidants, preventing their decomposition into colored

<figure><img src="image_3.png" /><figcaption>Fig. 1. Structure of (a) alkyl radical, (b) allyl radical, (c) polyenyl in HDPE.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. ESR signal for PE-1 (a) nine days after irradiation and (b) 60 days after irradiation and ESR signal for PE-2 (c) one day after irradiation and (d) 60 days after irradiation for gamma (red), e-beam (green) and X-rays (blue) at 50 kGy (dashed line) and 100 kGy (solid line).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Monitoring of the concentration of radicals for PE-1 (left) and PE-2 (right) by ESR as concentration (spin per g) vs. post irradiation ageing (days). Squares correspond to samples irradiated at 50 kGy and empty triangles correspond to samples irradiated at 100 kGy, red for gamma irradiation, blue for X-rays and green for e-beam. Error bars are included in the dimension of symbols.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Pareto analysis of significant effects on radical concentration: ageing after irradiation (blue), dose (red) and irradiation technologies (green). (a) PE-1. (b) PE-2. The red lines correspond to the significance threshold given by the software MODDE for 95 % confidence with p-value < 0.05. The equations and coefficients of the regression models obtained for each sample are provided in the SI.</figcaption></figure>

compounds, whereas PE-2, with different additive compositions, shows a greater increase in b* values, indicating more significant yellowing.

### 3.5. Infrared spectroscopy

The infrared spectra of the PE non-irradiated and irradiated by gamma, e-beam, and X-ray at 50 and 100 kGy are displayed in Fig. 9. The IR assignments of the PE spectra are in agreement with the literature (Al-Ghamdi et al., 2022; Hu et al., 2016) and do not show striking difference between irradiation technologies. PCA was performed on the

spectra representing the effects of all irradiation doses and technologies for both PE samples (Figure S3 in SI). For both polyethylene samples, no significant effect of irradiation dose or technology was observed.

### 3.6. Oxidation assay by high performance liquid chromatography

The presence of reactive oxygen species is evaluated indirectly by detecting oxidized methionine in solution through HPLC (Girard-Perier et al., 2022). Interestingly, the oxidation levels in the non-irradiated HPDE samples were almost as high as those in the irradiated ones,
<figure><img src="image_7.png" /><figcaption>Fig. 5. Box plots of (a) melting peak temperature and (b) melting onset temperature for HDPE samples non irradiated (NS in black) and one month after irradiation by gamma (red), X-ray (blue) and e-beam (green) at 50 and 100 kGy. Outliers are represented by the black diamonds.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Pareto analysis of significant effects on (a) melting peak temperature and (b) melting onset temperature on PE-2 (data from the Fig. 5): dose (red) and irradiation technologies (green). The red lines correspond to the significance threshold given by the software MODDE for 95 % confidence with p-value < 0.05. The equations and coefficients of the regression models obtained for each sample are provided in the SI.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Box plots for (a) elongation at yield and (b) tensile strength at Yield plotted for HDPE samples one month after irradiation by gamma (red), X-ray (blue) and e-beam (green) at 50 and 100 kGy.</figcaption></figure>

showing no significant difference between them (Fig. 10). We might foresee a difference with the PE-1 at 63 days, however not confirmed with a pareto test (see SI). This oxidation may be attributed to ageing-induced oxidation of methionine solution.

For both PE samples, neither the irradiation technology nor the dose significantly affected the methionine sulfoxide concentration. However, the solution contact time significantly increased the concentration (Fig. 11). A Pareto chart analysis was performed for both PE samples using data measured exclusively after 63 days of contact with the

solution (refer to SI). The results indicate that both the dose and the irradiation technologies do not statistically impact the concentration of methionine oxide in the solution, while methionine oxidation can occur over time as an ageing effect.

### 3.7. Study overview

Table 3 summarizes the results obtained from all analytical techniques applied to both HDPE samples.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** PCA of L*, a* and b* values obtained by color analysis on PE non irradiated and one month after irradiation by gamma, e-beam and X-ray at 50 kGy. (a) Score plot of PCA with irradiation technology (black square: Non irradiated (NS), red circle: gamma, blue triangle: X-ray, green diamond: e-beam). (b) Loading plot of PC-1.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** FTIR-ATR spectra of PE-1 non-irradiated (Non irradiated in black line), ten days after irradiation by gamma (red line), by X-ray (blue line) and by e-beam (green line) at 50 kGy (dashed lines) and 100 kGy (solid lines).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Methionine sulfoxide concentration (µM) in stored solution after 1, 10, 21, 42 and 63 days of contact analyzed by HPLC for all PE samples. Black circles correspond to non-irradiated samples. Squares correspond to samples irradiated at 50 kGy and triangles correspond to samples irradiated at 100 kGy, red for gamma irradiation, blue for X-rays and green for e-beam. Error bars are included in the dimension of symbols.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Pareto analysis of significant effects on methionine sulfoxide concentration: Methionine contact day (days) (A in purple), Dose (kGy) (B in red) and Irradiation technologies (C in green). (a) PE-1. (b) PE-2. The red lines correspond to the significance threshold given by the software MODDE for 95 % confidence with p-value < 0.05. The equations and coefficients of the regression models obtained for each sample are provided in the SI.</figcaption></figure>

<table><caption>**Table 3**
Summary of multi-scale analysis results on both PE.</caption><thead><tr><th rowspan="2"></th><th colspan="2">PE-1</th><th colspan="2">PE-2</th></tr><tr><th>Irradiation technologies</th><th>Dose</th><th>Irradiation technologies</th><th>Dose</th></tr></thead><tbody><tr><td>ESR</td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td></tr><tr><td>DSC</td><td>NSD<sup>1</sup></td><td>SS<sup>2</sup></td><td>NSD<sup>1</sup></td><td>SS<sup>2</sup></td></tr><tr><td>Tensile tests</td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td></tr><tr><td>FTIR</td><td>ND<sup>3</sup></td><td></td><td>ND<sup>3</sup></td><td></td></tr><tr><td>HPLC</td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td><td>NSD<sup>1</sup></td></tr></tbody></table>

Hence, the irradiation technologies do not significantly affect the properties of HDPE up to a dose of 100 kGy, as highlighted by the six analytical techniques utilized in this research.

## 4. Conclusion

The comprehensive multi-scale and multi-technique analytical approach employed in this study provides an in-depth understanding of the irradiation effects on high-density polyethylene (HDPE). By investigating alterations at the molecular, macromolecular, and material levels, this methodology elucidates the chemical and physical transformations on HPDEs following irradiation, facilitating the evaluation of the impact and equivalence of the three irradiation technologies.

ESR analyses, corroborating previous findings by Krieger et al. (2024c), demonstrated that the formation of alkyl, allyl, and polyenyl radicals in irradiated samples is largely unaffected by the irradiation type or dose, with these radicals diminishing over time. DSC revealed that the thermal properties of the samples remained consistent across all irradiation technologies, with only a minor reduction in melting temperature at higher doses, thereby preserving overall thermal stability. Mechanical testing verified that tensile strength and elongation at break were statistically equivalent across irradiation types, ensuring the mechanical performance of HDPE. By emphasizing equivalence rather than difference, the TOST helps to analyze experimental variability that might otherwise be interpreted as significant in difference tests. This approach is crucial to support the transition to alternative sterilization technologies, such as electron beam or X-ray irradiation, without compromising the quality and functionality of materials used in health care devices. Colorimetric analysis identified changes in brightness for one HDPE sample and alterations in color balance, with HDPE-2 exhibiting a yellowing effect post-irradiation attributed to specific additives' package. The study also assessed oxidation potential, with methionine sulfoxide concentrations indicating effect equivalence across the three irradiation technologies for both samples.

## Declaration of generative AI in scientific writing

During the preparation of this work the author(s) used generative AI in order to improve the readability and language of the manuscript. After using this tool/service, the author(s) reviewed and edited the content as needed and took(s) full responsibility for the content of the published article.

## CRediT authorship contribution statement

**Blanche Krieger:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Resources, Formal analysis, Data curation, Conceptualization. **Fabien Girard:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Samuel Dorey:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Nathalie Dupuy:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Sylvain R.A. Marque:** Writing – review & editing, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

## Acknowledgements

N.D., F.G. and S.R.A.M. are thankful to Aix-Marseille University (AMU), Institut de Recherche pour le Développement (IRD) and Centre National pour la Recherche Scientifique (CNRS) A.M.U., I.R.D. and C.N. R.S. for support.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ejps.2025.107394.

## Data availability

Data will be made available on request.

## References

Al-Ghamdi, H., Farah, K., Almuqrin, A., Hosni, F., 2022. FTIR study of gamma and electron irradiated high-density polyethylene for high dose measurements. Nucl. Eng. Technol. 54, 255-261. https://doi.org/10.1016/j.net.2021.07.023.

Armenante, P.M., Akiti, O., 2019. Sterilization processes in the pharmaceutical industry. In: am Ende, D.J., am Ende, M.T. (Eds.), Chemical Engineering in the Pharmaceutical Industry. John Wiley & Sons, Inc., Hoboken, NJ, USA, pp. 311-379. https://doi.org/10.1002/9781119600800.ch64.

<footnote n="1">¹ NSD: Non-statistically significant difference on the dose range investigated (up to 100 kGy).</footnote>

<footnote n="2">² SS: Statistically significant on the property evaluated.</footnote>

<footnote n="3">³ ND: No difference observed.</footnote>
BPSA Technical, Guide, 2021. X-rays Sterilization of single-Use BioProcess Equipment, Part 1: Industry Need. RequirementsRisk Evaluation ed.

Buttafava, A., Tavares, A., Arimondi, M., Zaapo, N., Nesti, S., Dondi, D., Mariani, M.,
Faucitano, A., 2007. Dose rate effects on the radiation induced oxidation of
polyethylene. Nuclear Instruments Methods Phys. Res. Section B: Beam Interactions
Materials Atoms 265, 221–226. https://doi.org/10.1016/j.nimb.2007.08.091.

Charlesby, A., Ross, M., 1953. The effect of cross-linking on the density and melting of Polythene. Proc. R. Soc. Lond. A 217, 122–135. https://doi.org/10.1098/rspa.1953.0049.

Chen, P.-Y., Chen, C.C., Harmon, J.P., Lee, S., 2014. The effect of gamma radiation on hardness evolution in high density polyethylene at elevated temperatures. Mater. Chem. Phys. 146, 369–373. https://doi.org/10.1016/j.matchemphys.2014.03.039.

Clough, R.L., 2001. High-energy radiation and polymers: a review of commercial processes and emerging applications. Nucl. Instrum. Methods Phys. Res., Sect. B 185, 8–33. https://doi.org/10.1016/S0168-583X(01)00966-1.

Darwis, D., Erizal, Abbas, B., Nurlidar, F., Putra, D.P., 2015. Radiation Processing of Polymers for Medical and Pharmaceutical Applications. Macromol. Symp. 353, 15–23. https://doi.org/10.1002/masy.201550302.

De Brouwer, H., 2022. Comparison of the effects of x-ray and gamma irradiation on engineering thermoplastics. Radiat. Phys. Chem. 193, 109999. https://doi.org/10.1016/j.radphyschem.2022.109999.

Dorey, S., Gaston, F., Girard-Perier, N., Dupuy, N., Marque, S.R.A., Delaunay, L., 2019. Generation of O₂ permeation barrier during the gamma-irradiation of polyethylene/ethylene-vinyl alcohol/polyethylene multilayer film. Ind. Eng. Chem. Res. 58, 14115–14123. https://doi.org/10.1012/acs.iecr.9b02145.

Dupuy, N., Marque, S.R.A., Fifield, L.S., Pharr, M., Staack, D., Pillai, S.D., Nichols, L., Murphy, M.K., Dorey, S., 2022. Preparing Gamma sterilization with x-ray and e-beam technologies. Bioprocess Technical 20, 24–28.

Forster, A.L., Tsinas, Z., Al-Sheikhly, M., 2019. Effect of irradiation and detection of long-lived polynenyl radicals in highly crystalline ultra-high molar mass polyethylene (UHMMPE) fibers. Polymers (Basel) 11. https://doi.org/10.3390/polym11050924.

Fricke, T.C., Echtermeyer, F., Zielke, J., De La Roche, J., Filipovic, M.R., Claverol, S., Herzog, C., Tominaga, M., Pumroy, R.A., Moiseenkova-Bell, V.Y., Zygmont, P.M., Leffler, A., Eberhardt, M.J., 2019. Oxidation of methionine residues activates the high-threshold heat-sensitive ion channel TRPV2. Proc. Natl. Acad. Sci. U.S.A. 116, 24359–24365. https://doi.org/10.1073/pnas.1904332116.

Gaston, F., Dupuy, N., Marque, S.R.A., Gigmes, D., Dorey, S., 2018. Monitoring of the discoloration on γ-irradiated PE and EVA films to evaluate antioxidant stability. J. Appl. Polym. Sci. 135, 46111. https://doi.org/10.1002/app.46114.

Girard-Perier, N., Claeys-Bruno, M., Marque, S.R.A., Dupuy, N., Gaston, F., Dorey, S., 2022. Monitoring of peroxide in gamma irradiated PE/EVOH/PE multilayer film using methionine probe. Food Bioprod. Process. 132, 226–232. https://doi.org/10.1016/j.fbp.2022.02.001.

Girard-Perier, N., Marque, S.R.A., Dupuy, N., Girardier, B., Dorey, S., 2023. Gamma, e-beam and x-ray irradiaions on pe/evoh/pe multilayer film: an industrial point of view regarding the impact on mechanical properties. Polymers (Basel) 15, 2799. https://doi.org/10.3390/polym15132799.

Gopanna, A., Rajan, K.P., Thomas, S.P., Chavali, M., 2019. Polyethylene and polypropylene matrix composites for biomedical applications. Materials For Biomedical Engineering. Elsevier, pp. 175–216. https://doi.org/10.1016/B978-0-12-816874-5.00006-2.

Goulas, A.E., Riganakos, K.A., Kontominas, M.G., 2004. Effect of ionizing radiation on physicochemical and mechanical properties of commercial monolayer and multilayer semirigid plastics packaging materials. Radiat. Phys. Chem. 69, 411–417. https://doi.org/10.1016/j.radphyschem.2003.08.013.

Gupta, S., Jiskoot, W., Schöneich, C., Rathore, A.S., 2022. Oxidation and Deamidation of monoclonal antibody products: potential impact on stability, biological activity, and efficacy. J. Pharm. Sci. 111, 903–918. https://doi.org/10.1016/j.xphs.2021.11.024.

Hu, P., Zhao, P.-P., Zhang, G.-W., Wang, X.-H., 2016. Thermal properties of 60Co irradiated crosslinked high density polyethylene. Sol. Energy Mater. Sol. Cells 149, 55–59. https://doi.org/10.1016/j.solmat.2015.12.042.

Ineos, 2025. Technical information of ELTEX® HD6070EA-B [WWW Document]. URL https://www.ineos.com/show-document/?grade=HD6070EA-B&bu=INEOS20 O202620P20Europe&documentType=Technical%20Data%20Sheet&docLanguag e=EN&version=71a625f93a1c1dbf77a34c98c9dbf844.

Ineos, 2015. Technical Information of ELTEX® MED HD5226EA-M [WWW Document]. URL https://www.ineos.com/show-document/?grade=HD5226EA-M&bu =INEOS20O202620P20Europe&documentType=Technical%20Data%20Sheet&d oLanguage=EN&version=aea02ac4a1e19fc00bcecb57aa31b1a7c.

Jacobitz, A.W., Liu, Q., Suravajila, S., Agrawal, N.J., 2021. Tryptophan oxidation of a monoclonal antibody under diverse oxidative stress conditions: distinct oxidative pathways favor specific tryptophan residues. J. Pharm. Sci. 110, 719–726. https:// doi.org/10.1016/j.xphs.2020.10.039.

Jahan, M.S., King, M.C., Haggard, W.O., Sevo, K.L., Parr, J.E., 2001. A study of long-lived free radicals in gamma-irradiated medical grade polyethylene. Radiat. Phys. Chem. 62, 141–144. https://doi.org/10.1016/S0969-806X(01)00431-5.

Jahan, M.S., McKinny, K.S., 1999. Radiation sterilization and subsequent oxidation of medical grade polyethylene: an ESR study. Nucl. Instrum. Methods Phys. Res., Sect. B 151, 207–212. https://doi.org/10.1016/S0168-583X(99)00083-X.

Kömmling, A., Chatzigiannakis, E., Beckmann, J., Wächtendorf, V., Von Der Ehe, K., Braun, U., Jaunich, M., Schade, U., Wolff, D., 2017. Discoloration Effects of High-Dose γ-Irradiation and Long-Term Thermal Aging of (U)HMW-PE. Int. J. Polym. Sci. 2017, 1–10. https://doi.org/10.1155/2017/1362491.

Kornacka, E.M., Przybytniak, G., Święszkowski, W., 2013. The influence of crystallinity on radiation stability of UHMWPE. Radiat. Phys. Chem. 84, 151–156. https://doi.org/10.1016/j.radphyschem.2012.06.027.

Krieger, B., Dorey, S., Dupuy, N., Girard, F., Kuntz, F., Ludwig, N., Marque, S.R.A., 2025a. Investigating dose rate effects and reactive species formation in irradiated multilayer films – part 1 EVA/EVOH/EVA. Phys. Chem. Chem. Phys. 27, 18935–18941. https://doi.org/10.1039/D4CP04152F.

Krieger, B., Dorey, S., Ludwig, N., Kuntz, F., Marque, S.R.A., Dupuy, N., Girard, F., 2025b. Investigating dose rate effects and reactive species formation in irradiated multilayer films – part 2 PE/EVOH/PE. Phys. Chem. Chem. Phys. 27, 18942–18948. https://doi.org/10.1039/D4CP04153D.

Krieger, B., Ludwig, N., Dorey, S., Dupuy, N., Girard, F., Girard-Perier, N., Kuntz, F., Marque, S.R.A., 2024a. Thermal properties and radical monitoring after gamma, X-ray, and electron beam irradiation in polyamides. Phys. Chem. Chem. Phys. 10, 1039. https://doi.org/10.1039/D4CP02358G. D4CP02358G.

Krieger, B., Marque, S.R.A., Dorey, S., Dupuy, N., Girard, F., Girard-Perier, N., Kuntz, F., Ludwig, N., 2024b. Radical detection and electron-spin resonance (ESR) monitoring in polymer materials irradiated with gamma and X-rays: polyethylene and polypropylene. J. Appl. Polym. Sci., e55098 https://doi.org/10.1002/app.55098.

Krieger, B., Marque, S.R.A., Dorey, S., Girard, F., Ni, Y., Li, D., Murphy, M.K., Fifield, L. S., Dupuy, N., 2024c. Comparison of effects from gamma, e-beam and x-ray radiation on multilayer polymer films used in biopharmaceutical devices by chemometric treatment of spectroscopic data. Radiat. Phys. 218, 111607. https://doi.org/10.1016/j.radphyschem.2024.111607.

Krieger, B., Marque, S.R.A., Girard, F., Dupuy, N., Ludwig, N., Kuntz, F., Dorey, S., 2025c. Investigation of X-Rays, e-Beam, and Gamma Irradiation on Chemical, Thermal, and Mechanical Properties of Polypropylene-Derivatives. Macro Chem. Phys., e00130 https://doi.org/10.1002/mcap.202500130.

Kurtz, S., 1999. Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty. Biomaterials 20, 1659–1688. https://doi.org/10.1016/S0142-9612(99)00053-8.

Lewis, G., 2001. Properties of crosslinked ultra-high-molecular-weight polyethylene. Biomaterials 22, 371–401. https://doi.org/10.1016/S0142-9612(00)00195-2.

Loveland, W.D., Morrissey, D.J., Seaborg, G.T., 2017. Modern Nuclear Chemistry, 1st ed. Wiley. https://doi.org/10.1002/9781119348450.

Makuuchi, K., Cheng, S., 2012. Radiation Processing of Polymer Materials and Its Industrial Applications. 1st ed. Wiley. https://doi.org/10.1002/9781118162798.

Malashin, I.P., Tynchenko, V.S., Nelyub, V.A., Borodulin, A.S., Gantimurov, A.P., 2023. Estimation and Prediction of the Polymers' Physical Characteristics Using the Machine Learning Models. Polymers (Basel) 16, 115. https://doi.org/10.3390/polym16010115.

Mansour, S.A., 2001. Effects of γ-irradiation on the mechanical and relaxation behaviour of high and low density polyethylene. Egypt. J. Sol. 24, 89–100. https://doi.org/10.21608/ejs.2001.150496.

Mehmood, M.S., Shah, J.M., Mishra, S.R., Walters, B.M., 2013. The effect of high dose on residual radicals in open air irradiated α-T UHMWPE resin powder. Radiat. Phys. Chem. 84, 100–104. https://doi.org/10.1016/j.radphyschem.2012.06.037.

Minitab Statistical software, 2022. Minitab Help. Retrieved from. https://support.minitab.com/en-us/minitab/21/help-and-how-to/.

Ni, Y., Bisel, T.T., Hasan, M.K., Li, D., Fuchs, W.K., Cooley, S.K., Nichols, L., Pharr, M., Dupuy, N., Marque, S.R.A., Murphy, M.K., Pillai, S.D., Dorey, S., Fifield, L.S., 2023. Compatibility of ethylene vinyl acetate (EVA)/ethylene vinyl alcohol (EVOH)/EVA films with gamma, electron-beam, and X-ray irradiation. Npj Mater. Degrad. 7. https://doi.org/10.1038/s41529-023-00413-x.

Premnath, V., Harris, W.H., Jasty, M., Merrill, E.W., 1996. Gamma sterilization of UHMWPE articular implants: an analysis of the oxidation problem. Biomaterials 17, 1741–1753. https://doi.org/10.1016/0142-9612(95)00349-5.

Schroeder, H.W., Cavacini, L., 2010. Structure and function of immunoglobulins. J. Allergy Clinic. Immunol. 125, S41–S52. https://doi.org/10.1016/j.jaci.2009.09.046.

Seguchi, T., Tamura, N., 1973. Mechanism of decay of alkyl radicals in irradiated polyethylene on exposure to air as studied by electron spin resonance. J. Phys. Chem. 77, 40–44. https://doi.org/10.1021/j100620a008.

Silva, P., Albano, C., Lovera, D., Perera, R., 2003. Characterization by EPR of radicals in HDPE, PA6 and HDPE/PA6 blend irradiated with gamma rays. Revista Mexicana De Fisica 49, 192–194.

Stadler, F.J., 2011a. Dynamic-mechanical behavior of polyethylenes and ethene/ α-olefin-copolymers: part II. α- and β-relaxation. Korean J. Chem. Eng. 28, 954–963. https://doi.org/10.1007/s11814-010-0411-4.

Stadler, F.J., 2011b. Dynamic-mechanical behavior of polyethylenes and ethene/- α-olefin-copolymers: part III. γ-relaxation. Korean J. Chem. Eng. 28, 2057. https:// doi.org/10.1007/s11814-011-0080-y.

Tavella, D., Ouellette, D.R., Garofalo, R., Zhu, K., Xu, J., Oloo, E.O., Negron, C., Ihnat, P. M., 2022. A novel method for in silico assessment of Methionine oxidation risk in monoclonal antibodies: improvement over the 2-shell model. PLoS One 17, e0279689. https://doi.org/10.1371/journal.pone.0279689.

Tavman, I., Aydogdu, Y., Kök, M., Turgut, A., Ezan, A., 2011. Measurement of heat capacity and thermal conductivity of HDPE/expanded graphite nanocomposites by differential scanning calorimetry. ISJ 50, 56–60.

Tian, R., Li, K., Lin, Y., Lu, C., Duan, X., 2023. Characterization techniques of polymer aging: from beginning to end. Chem. Rev. 123, 3007–3088. https://doi.org/10.1021/acs.chemrev.2c00750.

Wang, W., Singh, S., Zeng, D.L., King, K., Nema, S., 2007. Antibody structure, instability, and formulation. J. Pharm. Sci. 96, 1–26. https://doi.org/10.1002/jps.20727.

X-rite, Pantone, 2016. A guide to understanding color.

Yamamoto, K., Kato, K., Sugino, Y., Hara, S., Miwa, Y., Sakaguchi, M., Shimada, S., 2005. ESR Study on segmental motion of polyethylene in amorphous region, dependent on crystallinity, molecular weight, and labeled site. Macromolecules 38, 4737–4743. https://doi.org/10.1021/ma050062f.