Effects of Gamma Radiation on the Degradation and Sterilisation of Poly-(L-Lactide) (PLLA)

Post written by: Diane Samson 

Figure 1. Medical grade PLLA filament from Novus Life Sciences. 

1. Introduction 

Over the past few decades, there has been increasing interest in using Poly(L-lactic acid) (PLA)-based materials, like PLLA (Figure 1), in the biomedical and packaging industries due to their mechanical performance, biocompatibility, and cost-effectiveness (8). One important consideration for the use of PLA-based materials in biomedical applications is the sterilisation process. Irradiation-based sterilisation/disinfection techniques include gamma, electron beam (e-beam), and ultraviolet (UV) irradiation (17). Gamma irradiation is a simple, effective, and highly penetrating sterilisation method.  

Biomaterials can undergo physical and chemical changes under radiation-based sterilisation techniques like gamma radiation (8,17). This may have its disadvantages, along with other traditional sterilisation techniques like autoclaving and ethylene oxide (EtO) (1,17). For example, EtO sterilisation is effective at low temperatures, making it suitable for temperature-sensitive biomaterials (17). However, EtO can leave harmful residues that alter the material's surface and can be toxic to cells (1,17). Extensive aeration is needed to remove EtO residues (17). Heat-based sterilisation, such as steam and dry heat, is effective, simple, and fast (17). However, prolonged high temperatures can damage biodegradable polymers (17). Another option is sterilisation by irradiation, including gamma, electron beam, and UV (17). Gamma irradiation, using 60 Cobalt, is a simple and effective method, but it can alter the biomaterial's properties (17), especially when PLA-based materials exhibit high sensitivity to radiation (8).  

Choosing the right sterilisation approach can maintain biocompatibility and structural integrity (1,8,17). No single "perfect" sterilisation technique exists, so the choice should be based on the specific product, application, and the latest available sterilisation methods (1,8,17).  

2. Properties of Poly(L-lactide) (PLLA)

Figure 2. Middle: Chiral carbon of PLA marked with an asterisk. Left: PLLA, L-lactide. Right: PDLA, D-lactide (Created using chemspider.com). 

PLLA is an FDA-approved polymer known for its low toxicity compared to other synthetic polymers (5). It has demonstrated anti-infective effects and can provide adequate mechanical properties for prolonged regenerative processes during implantation (5). The mechanical behavior of PLLA strongly depends on its molecular weight (MW), crystallinity, and aging characteristics (5). The aging characteristics refer to the changes in its mechanical properties over time, such as embrittlement, increased stiffness, and decreased impact strength. These changes may impact the performance of the final product.  

PLLA is a thermoplastic and transparent polymer (5). The latter depends on the crystal morphologies and crystallinity of PLLA (5). As listed in Table 1, PLLA has a melting temperature between 170 and 190 °C and a glass transition temperature (Tg) of about 60 °C (5,14). Its mechanical strength is around 4.8 Gpa, depending on its MW5. Due to its polyorthoester trait, PLLA is hydrophobic, allowing it to degrade while remaining structurally intact (5) (Table 2). The length of PLLA chains, MW, and structural stability can be significantly reduced under thermal and mechanical stresses (5). The α-crystalline form of PLLA is the most thermodynamically stable, with the C=O dipoles randomly oriented along the main chain, resulting in a non-polar form (5).  

Physical Property	PLLA
Glass Transition Temperature, Tg (°C)	55-60
Melting Temperature, Tm (°C)	170 – 190
∆Hm (J.g−1)	40 – 50
Td (°C)	310
Density (g.cm−3)	1.25 – 1.29
Tensile strength (Mpa)	40 – 70
Elasticity Modulus (Gpa)	7 – 10
Young’s Modulus (Gpa)	2 – 5
Mechanical Strength (Gpa)	4.8
Elongation (%)	2 – 10
Crystallinity (%)	0 – 37
Degradation Period	1.5 – 3 years

Table 1. Physical Properties of PLLA (Data taken from (1,3,6,14, and 22)). 

Pros	Cons
(1) Non-toxic degradation (2) High strength (3) Excellent processability (4) Excellent biocompatibility	(1) Low cell intrusion (2) Long degradation time

Table 2. Summary of the pros and cons of PLLA (Modified from (3)). 

3. Common sterilisation methods 

Sterilisation eliminates microbial life, including viruses, bacteria, and fungi (1). The sterilisation method chosen should maintain the properties of PLLA, as this could lead to adverse responses or functional issues when used in the body (1,17). Table 3 lists existing sterilisation techniques for PLLA and their potential alteration effects. 

Sterilisation Method	Alteration Effects
Gamma Radiation	Alter MW and chain structure, though these changes are often acceptable (1).
Autoclaving	Significant deformation and structural changes (1).
Ethylene Oxide (EtO)	Minor variation in the characteristics behavior, including increase in crystallinity, a decrease in elongation at break, and in some cases, a variation in wettability (15).  Main issues are toxicity and residues (1).
Electron Beam  (e-beam) Radiation	Less degradation than gamma, but the degree of damage depends on the energy level used (1).
High-Pressure Gas Plasma (HPGP) and Supercritical CO2	More novel techniques.   Show promise as effective yet gentle sterilisation methods (1).

Table 3. A brief overview of the possible effects of each sterilisation method on PLLA.

4. Impact of Gamma Radiation on Properties of PLLA  

4.1 Kinetics of Hydrolytic Degradation 

Gamma irradiation can affect the degradation of PLLA, primarily through hydrolysis (8,16). Significant changes in mechanical properties like Young’s modulus and tensile strength have been reported for PLLA after sterilisation (17). High-energy radiation can break the polymer chains, leading to a decrease in the overall MW of the material (15). As the chains fragment, the material’s degree of crystallinity increases (15).  

The Impact of gamma Irradiation on PLLA can be complex. While It Initially decreases the molecular weight (MW) of PLLA, this can increase the rate of crystallization upon subsequent hydrolytic degradation compared to non-irradiated PLLA (8). This increased crystallinity can then slow down the overall hydrolytic degradation of the PLLA over time (8). In other words, the initial acceleration of degradation due to the radiation-induced MW decrease is eventually counteracted by the formation of more crystalline and hydrolysis-resistant regions (8). The changes in degradation behavior due to irradiation need to be evaluated to ensure the scaffold degrades at the desired rate to support tissue regeneration. Thus, the optimal degradation rate of a 3D support should match the extracellular matrix (ECM) deposition rate of the specific tissue (5). 

4.2 Dose-Dependent Changes in PLLA 

PLLA changes are dose-dependent (5,18), as shown in Table 4. Low gamma radiation doses (up to 30kGy) degrade PLLA, while color and oxygen permeability are unaffected (5). Tm, glass transition temperature (Tg), and number-average molecular weight (Mn) of PLLA decrease sharply, but the rate of change slows down at higher doses (18).  

At higher doses, irradiation can decrease the elongation of PLLA, leading to embrittlement (9). The fraction of PLLA crystals with higher Tm increases with the radiation dose, likely due to the formation of the less ordered α’ crystal form, which then recrystallises during heating (9). Up to a radiation dose of 200 kGy, the PLLA predominantly degrades through random chain scission18. Chain-scission becomes less pronounced above 50-200 kGy, and recombination reactions or partial crosslinking may also occur in the PLLA (9,18).  

Irradiation   Dose (kGy)	Property Changes in PLLA
1 - 30	Sharp decrease in:  Tm, Tg, and Mn of PLA-based materials.  Decrease in:   Elongation.  MW and crystallisation temperature (Tc).  Tensile strength, break elongation, and elastic modulus.  Toughness.  Water vapor and carbon dioxide permeability of PLA-based materials.  Surface tension.  Unaffected:   Oxygen permeability.  Color stability.
> 50	Increase in:   Embrittlement.  Fraction of PLLA crystals with higher Tm.  Decrease in:   MW.  Less pronounced chain scission.  Partial change in surface energy due to chain scission.
≤ 200	Predominantly degrades through random chain-scission.
> 200	Recombination reactions or partial crosslinking may also occur in the PLLA, in addition to chain-scission.

Table 4. Effects of gamma radiation on PLLA material properties at low and higher doses (5,8,9,15,18). 

4.3 Impact of Oxygen on PLLA Degradation 

The decrease in Mn, Tm, and Tg of PLA-based materials irradiated in air is faster than in a vacuum environment (18). This indicates that the presence of oxygen, in combination with the radiation, leads to oxidative chain-scission reactions that accelerate the degradation of the PLLA polymer compared to radiation exposure alone in the absence of oxygen (18).  

4.4 Impact of Gamma Radiation on Shelf Life 

As shelf life is commonly known to be directly affected by the initial material condition before the aging process, Krug et al. demonstrated that the gamma irradiation and EtO sterilisation methods primarily impact the initial condition of PLA-based materials and, subsequently, their shelf life (over a two-year aging period) rather than the actual degradation kinetics over time (16). The degradation of PLA-based materials was reported to occur in a two-stage linear process (see Figure 3), with an accelerated rate of degradation in stage two, likely due to the formation of catalytic end groups (16).  

A direct correlation was also observed between the absorption of moisture and the progression of degradation (see Figure 4) (16). The moisture content measured in the PLLA granules increased over the course of the aging/degradation period, regardless of the specific PLA type (16). The data on moisture content changes could be described by either a two-stage linear process or an exponential function (16).  

Figure 3. The degradation of the PLA materials in a two-stage process, between one and two years of accelerated aging. The study obtained data by accelerated aging (ACC) (Graph taken from (16)).  

Figure 4. Over time increase in moisture content of the PLA granules through a two-stage linear process during the aging/degradation period across all PLA types (Graph taken from (16)). 

As the initial material condition impacts shelf life more than the method of sterilisation, and the hydrolytic degradation of PLLA is accelerated by the presence of moisture, it is important for manufacturers to carefully control the initial material properties, processing conditions, and packaging/storage environment to minimize exposure to moisture and maximize the long-term shelf life of the devices. The sterilization method, while important, has less impact than the overall material history and storage conditions. 

5. Examples of PLLA-based 3D Printed Medical Devices 

The unique thermal, mechanical, and degradation characteristics of PLLA make it versatile and widely used in tissue engineering and biomedical applications that require tailored properties (5) (Figure 5). The main applications of PLLA discussed here are tissue engineering, drug delivery, and implants.  

Figure 5. Common clinical applications of 3D-printed PLLA (Created using biorender.com). 

5.1 Orthopaedic Implants and Devices  

Figure 6. Examples of 3D-printed implant solutions provided by Novus Life Sciences. 

PLLA can be used for the fixation of fractured bones in the form of screws, pins, wires, and plates (Figure 4). Bioabsorbable implants have been shown to provide fracture fixation results that are comparable to using metallic implants for hand injuries in terms of healing and patient satisfaction (12). For example, Voutilainen et al. conducted a study where they performed 24 wrist joint fusion procedures using self-reinforced poly-L-lactic acid (SR-PLLA) pins in patients with rheumatoid arthritis. The results demonstrated that 21 out of the 24 treated wrists were able to maintain bony union successfully, and 22 out of the 24 patients reported satisfactory pain relief following the surgical intervention (21), supporting the use of SR-PLLA pinning as a favorable treatment option. PLLA facilitates proper bone healing by enabling bony union while eliminating the complications that necessitate hardware removal (12). By minimising stress shielding, PLLA can actively promote the healing of the fracture site and encourage the growth of the surrounding cortical bone. The design flexibility of 3D printing allows for PLLA-based patient-specific implants, such as craniofacial or orthopedic devices (see Figure 5). 

Figure 7. Medical images of a 16-year-old male patient with a transverse fracture of the 5th metacarpal in his left hand. (A) Preoperative X-ray displaying the fracture site. (B) Postoperative X-ray demonstrating fixation of the fracture using a bioabsorbable plate made of u-HA/PLLA. (C) X-ray at the final 5-year, 8-month follow-up visit shows the long-term outcome after the surgical treatment. (D) 3D CT reconstruction of the 5th digit during the final follow-up assessment. (Figure taken from (12)). 

5.2 Tissue Engineering Scaffolds 

PLLA may not be an optimal choice for suture materials due to its relatively slow degradation rate. However, due to their tunable mechanical properties and extended retention of mechanical strength, PLLA fibres are often the preferred material in tissue engineering applications (1,5,7). Tissue engineering applications of PLLA filaments that have been extensively researched include bone and dermis regeneration, such as ligament and tendon reconstruction procedures or stents for vascular and urological surgeries. As a form of tissue engineering through injectable microspheres, PLLA can also be used in facial reconstruction surgeries, embolic material for arteriovenous fistula and malformations, massive hemorrhage, and tumours (1,7,11). 

Scaffolds fabricated from this bioresorbable material typically exhibit higher tensile strength (60–70 MPa) and modulus (2–4 GPa) but lower elongation at break (2–6%) compared to other synthetic polymers like PCL and PDLA (5,11). This makes PLLA-based scaffolds suitable for supporting the healing of high-strength tissues, such as bone, ligaments, and dermis (5). 

5.3 Drug Delivery Systems  

PLLA microspheres have been used as drug-delivery vehicles. A variety of drugs have been successfully incorporated into and released from PLA-based materials (20). These include Ibuprofen, Amphotericin B, Paclitaxel, 5-fluorouracil, Doxorubicin, Ketoprofen, Rifampicin, Salinomycin, Ciprofloxacin, and Ornidazole (20). The hydrophobic nature of PLLA allows it to be modified to accommodate both hydrophobic and hydrophilic drugs (20). PLLA-based drug delivery fabricated with 3D printing allows controlled release and customised dosages (4,10,13), with examples seen in Figures 6 and 7.  

Figure 6. An orthodontic retainer designed to deliver the drug clonidine. The figure shows the computer-aided design (A), the printed device (B), and the retainer worn as an example (C) (Figures taken from (13)). 

Figure 8. Vaginal rings designed to release the drug progesterone. The computer-aided designs and printed versions are shown for O-shaped (A and D), Y-shaped (B and E), and M-shaped ring (C and F) geometries (Figures taken from (10)). 

6. Conclusion 

Gamma radiation can have a significant impact on the properties and degradation of PLLA. While gamma radiation is a common sterilisation method, the high-energy ionising radiation can induce chain scission, cross-linking, and other structural changes in PLLA that alter its mechanical, thermal, and biodegradation characteristics.  

Gamma radiation dose and specific PLLA formulation are important factors to consider when determining the degree of property changes. The effects of radiation on PLLA properties are dose-dependent. At low doses, there is a sharp decrease in Tm, Tg, and Mn, followed by a slower rate of change. Up to 200 kGy, the degradation is mainly due to random chain-scission, while above 200 kGy, recombination or partial crosslinking may also occur. The presence of oxygen during irradiation accelerates the degradation through oxidative chain-scission18. Higher radiation doses tend to accelerate the degradation of PLLA, which could be advantageous for certain short-term applications but may compromise the long-term stability and performance of PLLA-based implants and devices.  

Given the widespread use of PLLA in 3D-printed medical devices, careful evaluation of the effects of gamma sterilisation on the material properties is needed, and alternative sterilisation techniques that may better preserve the intended functionality of the final product should be considered. 

7. References 

1. Abd Alsaheb, Ramzi A & Aladdin, Azzam & Othman, Nor & Malek, Roslinda Abd & Leng, Ong & Aziz, R. & El Enshasy, Hesham. (2015). Recent applications of polylactic acid in pharmaceutical and medical industries. Journal of Chemical and Pharmaceutical Research. 2015. 51-63. 

   
   

2. Alonso-Fernández, I., Haugen, H. J., López-Peña, M., González-Cantalapiedra, A., & Muñoz, F. (2023). Use of 3D-printed polylactic acid/bioceramic composite scaffolds for bone tissue engineering in preclinical in vivo studies: A systematic review. Acta Biomaterialia, 168, 1–21. https://doi.org/10.1016/j.actbio.2023.07.013  

   
   

3. Arif, Z. U., Khalid, M. Y., Noroozi, R., Sadeghianmaryan, A., Jalalvand, M., & Hossain, M. (2022). Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications. International Journal of Biological Macromolecules, 218, 930–968. https://doi.org/10.1016/j.ijbiomac.2022.07.140  

   
   

4. Barcena, A. J., Ravi, P., Kundu, S., & Tappa, K. (2024). Emerging biomedical and clinical applications of 3D-printed poly(lactic acid)-based devices and Delivery Systems. Bioengineering, 11(7), 705. https://doi.org/10.3390/bioengineering11070705  

   
   

5. Benyathiar, P., Selke, S. E., Harte, B. R., & Mishra, D. K. (2020). The effect of irradiation sterilization on poly(lactic) acid films. Journal of Polymers and the Environment, 29(2), 460–471. https://doi.org/10.1007/s10924-020-01892-8  

   
   

6. Capuana, E., Lopresti, F., Ceraulo, M., & La Carrubba, V. (2022). Poly-L-lactic acid (plla)-based biomaterials for Regenerative Medicine: A review on processing and applications. Polymers, 14(6), 1153. https://doi.org/10.3390/polym14061153  

   
   

7. Dhandayuthapani, B., Yoshida, Y., Maekawa, T., & Kumar, D. S. (2011). Polymeric scaffolds in tissue engineering application: A Review. International Journal of Polymer Science, 2011, 1–19. https://doi.org/10.1155/2011/290602  

   
   

8. Fedorenko, A. A., Grinyuk, E. V., Salnikova, I. A., & Kostjuk, S. V. (2022). Effect of gamma-irradiation on hydrolysis of commercial poly(L-Lactide) at elevated temperature. Polymer Degradation and Stability, 206, 110202. https://doi.org/10.1016/j.polymdegradstab.2022.110202  

   
   

9. Fedorenko, A., Grinyuk, E., Salnikova, I., Emeliyanova, O., Dudchik, N., Sychik, S., Tychinskaya, L., Skakovsky, E., Liubimau, A., & Kostjuk, S. (2024). Combined Effect of Antimicrobial Agent and Γ-Irradiation on Properties of Poly(L-Lactide) Active Films. https://doi.org/10.2139/ssrn.4755508  

   
   

10. Fu, J., Yu, X., & Jin, Y. (2018). 3D printing of vaginal rings with personalized shapes for controlled release of progesterone. International Journal of Pharmaceutics, 539(1–2), 75–82. https://doi.org/10.1016/j.ijpharm.2018.01.036 

    
    

11. Haider, A., Haider, S., Rao Kummara, M., Kamal, T., Alghyamah, A.-A. A., Jan Iftikhar, F., Bano, B., Khan, N., Amjid Afridi, M., Soo Han, S., Alrahlah, A., & Khan, R. (2020). Advances in the scaffolds fabrication techniques using biocompatible polymers and their biomedical application: A technical and statistical review. Journal of Saudi Chemical Society, 24(2), 186–215. https://doi.org/10.1016/j.jscs.2020.01.002  

    
    

12. Jee, E., Robichaux-Edwards, L., Montgomery, C., Bilderback, K., Perry, K., & Massey, P. A. (2024). Polylactic acid bioabsorbable implants of the hand: A Review. Journal of Hand and Microsurgery, 16(3), 100053. https://doi.org/10.1016/j.jham.2024.100053 

    
    

13. Jiang, H., Fu, J., Li, M., Wang, S., Zhuang, B., Sun, H., Ge, C., Feng, B., & Jin, Y. (2019). 3D-printed wearable personalized orthodontic retainers for sustained release of clonidine hydrochloride. AAPS PharmSciTech, 20(7). https://doi.org/10.1208/s12249-019-1460-6  

    
    

14. Khouri, N. G., Bahú, J. O., Blanco-Llamero, C., Severino, P., Concha, V. O. C., & Souto, E. B. (2024). Polylactic acid (PLA): Properties, synthesis, and biomedical applications – a review of the literature. Journal of Molecular Structure, 1309, 138243. https://doi.org/10.1016/j.molstruc.2024.138243  

    
    

15. Krug, N., Zarges, J.-C., & Heim, H.-P. (2023). Influence of ethylene oxide and gamma irradiation sterilization processes on the properties of poly-L-lactic-acid (PLLA) materials. Polymers, 15(16), 3461. https://doi.org/10.3390/polym15163461  

    
    

16. Krug, N., Zarges, J.-C., & Heim, H.-P. (2024). Influence of ethylene oxide and gamma irradiation sterilization processes on the degradation behaviour of poly(lactic acid) (PLA) in the course of artificially accelerated aging. Polymer Testing, 132, 108362. https://doi.org/10.1016/j.polymertesting.2024.108362  

    
    

17. Łopianiak, I., & Butruk-Raszeja, B. A. (2020). Evaluation of sterilization/disinfection methods of fibrous polyurethane scaffolds designed for tissue engineering applications. International Journal of Molecular Sciences, 21(21), 8092. https://doi.org/10.3390/ijms21218092  

    
    

18. Nugroho, P., Mitomo, H., Yoshii, F., & Kume, T. (2001). Degradation of poly(l-lactic acid) by γ-irradiation. Polymer Degradation and Stability, 72(2), 337–343. https://doi.org/10.1016/s0141-3910(01)00030-1  

    
    

19. Pérez Davila, S., González Rodríguez, L., Chiussi, S., Serra, J., & González, P. (2021). How to sterilize polylactic acid based medical devices? Polymers, 13(13), 2115. https://doi.org/10.3390/polym13132115  

    
    

20. Sahini, M. G. (2023). Polylactic acid (pla)-based materials: A review on the synthesis and Drug Delivery Applications. Emergent Materials, 6(5), 1461–1479. https://doi.org/10.1007/s42247-023-00551-7  

    
    

21. Voutilainen, N., Juutilainen, T., Patiala, H., & Rokkanen, P. (2002). Arthrodesis of the wrist with bioabsorbable fixation in patients with rheumatoid arthritis. Journal of Hand Surgery, 27(6), 563–567. https://doi.org/10.1054/jhsb.2002.0806  

    
    

22. Zan, J., Qian, G., Deng, F., Zhang, J., Zeng, Z., Peng, S., & Shuai, C. (2022). Dilemma and breakthrough of biodegradable poly-L-lactic acid in bone tissue repair. Journal of Materials Research and Technology, 17, 2369–2387. https://doi.org/10.1016/j.jmrt.2022.01.164