Friend of the earth: How bioplastics can be used in the medical field

Organic and biodegradable, bioplastics can be used in a range of biomedical applications. Pyam Ramnes explains

Familiar to most people, plastics are fossil-based and suffer biodegradability. Bioplastics, however, are organic-based and biodegradable. These kind-to-the-environment materials are from resources like plants and animals which are far more sustainable resources than the fossil-based resources like oil and gas. As a result, the biodegradability of bioplastics combined with the sustainability of their resources, make them a great candidate in the medical field.

Whether for artificial eyes or modern implants, several materials have been used in the medical field. But, a material that benignly disappears in the body would be a panacea. With the emergence of biodegradable and bio-absorbable polymers, temporary prostheses, tissue engineering, and drug delivery vehicles began to rise.

Tissue engineering is a technology that solves donation and transplant rejection problems. It improves tissue function. Biomedically, a malfunctioning or damaged organ may be treated by growing its own cells. This treatment requires a physical support, which is known as scaffold, to guide the formation of new cells. The scaffold role is to facilitate the adhesion of cells and promote the growth of cells. As a result, the scaffold has to degrade as the new cells are grown and formed. Then, the degraded material can be metabolised by the human body.

Auspiciously, poly-lactones such as poly-lactic acid (PLA), poly-glycolic acid (PGA), and poly-caprolactone (PCL) are bio-absorbable, biodegradable and biocompatible. Among the poly-lactones, poly-lactic acid (PLA) is the most desired one for the medical applications due its mechanical properties. PLA is made from lactic acid, which is an organic acid and can be produced from fermentation of sugars which is a sustainable process. This production method is highly desirable by yielding high quality, low cost, and low energy consumption. The fermentation required carbon which can be derived from pure sugar such as glucose, sucrose, or lactose. The carbon source of bacterial fermentation can also be obtained from molasses, whey, sugarcane bagasse, cassava bagasse, starchy materials (potato, tapioca, wheat and barley), or any other material that contains sugar. The process of polymerisation of lactic acid is shown in the below figure: 

PLA can be processed in many ways from injection moulding to extrusion and 3D printing. Additionally, it can be processed by spinning film and casting. PLA melting temperature ranges from 180ᵒC to 220ᵒC which is considered a low melting temperature when it is compared to other plastics. 

Engineering the scaffold lays out the structure of scaffold with the means of architecture, porosity, pore size and distribution, and interconnectivity. This structure needs to be engineered with respect to the cells growth rate, colonisation rate, nutrient delivery, and waste removal. Also, the scaffold can be engineered to emulate specific mechanical and material properties of the tissue of interest. Ideally, the engineering of the scaffold should aim for the facilitation of the attachment, migration, proliferation, differentiation, and 3D spatial organisation of the cell population required for structural and functional replacement of the target organ or tissue.

Currently, the optimal scaffold design is based on the absorbability of the scaffold material as the support is not needed to be removed upon completion of the regeneration. PLA material fulfills this requirement by being capable of being excreted through kidneys or removed in the form of carbon dioxide and water through metabolic processes. Additionally, PLA is highly biocompatible as its degradation product is lactic acid which is not harmful to the metabolic system. Furthermore, PLA performs optimally at a comparably low cost as a minor modification of its structure makes it suitable for additional application. This is a result of chirality of its base molecule – lactic acid – which poses dual asymmetric centres in four different forms.

In some cases, the absorbability of scaffold works against the treatment objectives. In these cases, the scaffold needs to retain the strength while the cells are being slowly regenerated. The examples of this application are ligament and tendon reconstruction or stents for vascular and urological surgery. In these cases PLLA fibres are used. Moreover, PLA composites can be engineered to be capable of simulating cells/tissues for proliferation and osteogenic differentiation in bone tissue engineering. Biologically, ligaments are bone connective tissues with reduced cell density. Among the human body ligaments, the anterior cruciate ligament, ACL, exhibits poor healing potential and limited vascularization. Thereby, ACL repair and regeneration is of an immense concern in the area of tissue engineering. There have been several biocompatible and biodegradable natural polymers used for ACL repair and regeneration, through tissue engineering, such as collagen fiber, silkworm silk-Nano-fibrous matrix, alginate and chitosan polyion complex hybrid nanofibres, collagen platelet-rich plasma, etc. However, ACL regeneration using braided biodegradable scaffold made from synthetic polymers, particularly PGA, PLLA, PLGA, and PCL presented enhanced results. This is due to the thermal and mechanical properties of these materials.

While PLA solves many problems of tissue engineering, there are many is doesn’t. The hydrophilic property of PLA is poor and there are many mechanical properties ideal for scaffold that need to be fulfilled. PLA may disintegrate into small fragments which evoke a foreign body reaction. Additionally, since the surrounding tissue capability in eroding the byproducts is a function of vascularisation and metabolic activity, a low level of such activity would lead to the chemical composition of byproducts and local disturbance such as excess osmotic pressure or pH.

Bioplastics, particularly PLA, are biodegradable materials with organic base which brings about a sustainable resource. Another important aspect of bioplastics is their biocompatibility which makes them usable in biomedical applications. Also, they are bio-absorbable which makes them more ideal in tissue engineering. PLA is made from lactic acid which is produced by bacterial fermentation of sugars. PLA is widely used in tissue engineering, due to its physical and chemical properties, to construct the scaffold. Although PLA presents a decent level of satisfaction in tissue engineering applications, its properties need to be enhanced to meet ideal requirements of tissue engineering.