Materials Report: Polylactic Acid

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By Emily Schmitt

As businesses seek to reduce their environmental impact, they are analyzing the materials and methods they use to consider less harmful alternatives. The most researched and widely used biodegradable material in O&P is polylactic acid (PLA).3 PLA is a thermoplastic polyester material that is much less toxic in production and degradation than materials such as polypropylene or acrylonitrile butadiene styrene (ABS). These common materials are petroleum-based plastics, and though they are FDA-approved as safe for use in many different applications, their production is dependent on a limited resource, and they are non-degradable.

PLA, however, is produced from renewable resources such as sugarcane or cornstarch and is fully biodegradable and compostable. Sugar from these plants is fermented to get lactic acid, which is then polymerized into PLA, or polylactide. The formation of PLA does not involve toxic chemicals, and it will break down over time into fully organic components.3,5 Whether bioplastics such as PLA truly are less harmful to the environment than traditional plastics used in O&P is a complicated question, due to the environmental impact of the farming resources required to produce large amounts of PLA. Considering first the needs of O&P users and keeping the quality of their care as the priority, it is most important to consider whether this material can create devices equal in caliber to the current standards of care.

PLA has become such a prominent biodegradable plastic because of its many desirable material properties. It has a high tensile strength, high elastic modulus, good flexural strength, and great optical properties. These qualities make PLA a great option for disposable products; with only 10 percent elongation at break, though, PLA is too brittle for many applications involving deformation at high stress.5

The lack of ductility makes PLA an unsuitable choice for many orthotic applications, but the weaknesses of the material can be improved by making composites. Reinforcing fibers, and micro- and nano-fillers and additives can be used with a PLA polyester matrix to create a composite material with much more desirable characteristics while retaining the environmental benefits of PLA.6 Some PLA composites being used in O&P applications include PLA-CaCO3, thermoplastic polyurethane (TPU)/PLA composite, and PLA+.7,8 According to Varga et al., when used to create upper-limb orthoses, PLA-CaCO3 is "potentially viable in [the] clinical environment, but further laboratory and clinical investigations are necessary."

Within the context of O&P, polylactic acid or composites of it are most often applied to the production of rigid or semi-rigid components of custom-fabricated upper- and lower-limb orthoses. PLA comes in sheets and can be thermoformed as polypropylene often is, but academic literature supports that PLA orthoses are mainly being manufactured through 3D printing.

In a review analyzing the effectiveness of additive manufacturing versus conventional manufacturing of AFOs, Shahar et al. conclude that additive manufacturing produces AFOs with comparable strength as conventionally manufactured devices. Their study involved producing AFOs out of several materials using both thermoforming and 3D printing. PLA was the one material to only be 3D printed and not thermoformed, but it was noted that PLA has the highest tensile strength and lowest cost of the thermoplastics used, as well as minimal warping, making it the best material for additive manufacturing of AFOs.4

Because AFOs support the ankle joint, they are subjected to a high amount of force and cyclical loading, making it important that the material they are constructed of will not fail under load or deformation. The successful outcome of a 3D-printed PLA AFO in this study is promising for the use of this material for patients. A similar study done by Tao et al. determined that PLA can be successfully used to 3D print custom-finger orthoses.7

Research on the feasibility of PLA 3D-printed lower-limb prosthetic sockets was published in the 2019 article "Exploring 3D-printing in Prosthetics." Over six weeks of follow-up on subjects using 3D-printed sockets concluded that the PLA sockets performed as well as the standard-of-care devices. While this result is promising for the use of PLA, research assistant Nicolette Chamberlain-Simon states that PLA is "not well suited for most O&P applications" and suggests that 3D-printeddevices offer great potential to the field with the use of materials with better properties. Though the PLA sockets had great outcomes over six weeks, the strength-to-weight and strength-to-thickness ratios are not as high as carbon devices, and therefore further validation should be done to evaluate the long-term efficacy of using PLA.2

Despite the potential promise, outside of academic research there are few examples of PLA being used as a replacement for petroleum-based materials in clinical practice. There are some encouraging examples, such as the Victoria Hand Project, a nonprofit organization that produces low-cost, 3D-printed upper-limb O&P devices that are used in less resourced countries. Two of their prosthetic designs, the Victoria Hand and the LimbForge arm are both printed using PLA filament, chosen because of the material's high strength, biocompatibility, and ease of printing. While there may be no PLA composite worthy of replacing current gold-standard materials, in areas where people may not have easy access to O&P care, having an inexpensive 3D-printed option may be of great benefit.

While this use of PLA is encouraging, the Victoria Hand Project does not seem to have any published research documenting the long-term outcomes of their devices. PLA may be making strides in improving patients' lives in situations where speed, efficiency, and low cost of manufacturing are high priorities, but it seems that it will take more development and research of this material to make the same impact in US-based clinical settings.

O&P is constantly evolving and adapting with new technologies and adapting practices to higher environmental standards will hopefully occur in time as well. These shifts may open the door for materials like PLA to become part of common practice.

Emily Schmitt is a first-year MSPO student at the University of Hartford.

References

1. Brasov UT, Repanovici A, Brasov UT, Nadinne R, Brasov UT. Biomaterials : Polylactic Acid and 3D Printing Processes for Orthosis and Prosthesis. 2017;(March). doi:10.37358/MP.17.1.4794

2. Exploring 3D printing in Prosthetics | Lower Extremity Review Magazine. Accessed April 24, 2021. https://lermagazine.com/special-section/conference-coverage/exploring-3d-printing-in-prosthetics

3. Farah S, Anderson DG, Langer R. Physical and Mechanical Properties of PLA, and Their Functions in Widespread Applications-A Comprehensive Review. Advanced Drug Delivery Reviews. 2016;107:367-392. doi:10.1016/J.ADDR.2016.06.012

4. Farah S, Sultan MTH, A.U. S, Safri SNA. View of A Comparative Analysis between Conventional Manufacturing and Additive Manufacturing of Ankle-Foot Orthosis. Applied Science and Engineering Progress. https://ph02.tci-thaijo.org/index.php/ijast/article/view/240576/163766. Published 2020. Accessed April 25, 2021.

5. Murariu M, Dubois P. PLA composites: From production to properties. Adv Drug Deliv Rev. 2016;107:17-46. doi:10.1016/j.addr.2016.04.003

6. Sangeetha VH, Deka H, Varghese TO, Nayak SK. State of the art and future prospectives of poly(lactic acid) based blends and composites. Polymer Composites. 2018;39(1):81-101. doi:10.1002/pc.23906

7. Tao Y, Li P, Shao J, Shi SQ. Application of A Thermoplastic Polyurethane/Polylactic Acid Composite Filament for 3d-Printed Personalized Orthosis Y. TAO et al.: Application Of A Thermoplastic Polyurethane/Polylactic Acid Composite Filament For 3d-Printed Personalized Orthosis. Published online 2019. doi:10.17222/mit.2018.180

8. Varga P, Lorinczy D, Toth L, Pentek A, Nyitrai M, Maroti P. Novel pla-caco3 composites in additive manufacturing of upper limb casts and orthotics-a feasibility study. Materials Research Express. 2019;6(4):045317. doi:10.1088/2053-1591/aafdbc

9. 3D Printed Hand | Victoria Hand Project. Accessed April 29, 2021. https://www.victoriahandproject.com/