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The cardiomyocytes, which are the primary cell types present within the heart, are relatively sensitive to electrical signal stimulation. This inherent sensitivity has inspired researchers to develop electroactive polymers to repair damaged cardiomyocytes following cardiovascular events such as stroke and myocardial infarctions.
How Cardiac Tissue Death Occurs
In 2019, cardiovascular diseases (CVDs) remained the leading killer of people around the world and accounted for the deaths of almost 18 million individuals.
Regardless of the type of CVD, it is estimated that up to 85% of all related deaths are the result of stroke or myocardial infarction (MI). Within a few hours of a MI, otherwise referred to as a heart attack, approximately one billion cardiomyocytes, which are the muscular cells that make up the heart, will die within the left ventricular wall. Unfortunately, the adult heart has limited regenerative capabilities, causing any lost myocardial tissue to be replaced by fibrotic scar tissue. This type of fibrotic tissue prevents the heart from contracting at a normal pace, increasing the likelihood of MI survivors to ultimately succumb to heart failure.
What are Cardiac Patches?
To overcome and even possibly reverse the heart damage that cardiovascular events such as MIs and strokes cause, several researchers have begun engineering cardiac patches comprised of a wide range of materials.
The healing mechanisms behind these materials vary, ranging from biomaterials that deliver cells and/or growth factors directly to damaged tissue, to electromechanical materials aimed at restoring the biomechanical needs of the heart muscle.
More recently, researchers within the tissue engineering community have found that electroactive biomaterials such as piezoelectrics, photovoltaic materials, electrets, and conductive polymers are capable of promoting the propagation of electrical signals between cardiac cells at a greater extent than when insulated biomaterials are used.
Essential Characteristics of a Biomedical Polymer Patch
Conductive polymers are a unique class of electroactive biomaterials that are associated with numerous advantages for cardiac patch applications. For example, these materials have a naturally conductive nature, are easy to process, and can be selectively engineered to either provide transient or static electrical charges.
For biomedical applications, a considerable amount of research has been done on polypyrrole (PPy). This polymer is biocompatible, chemically stable and associated with high conductivity capabilities of up to 4 x 103 S/m when present in environmental conditions.
In addition to having inherent conductive properties, a biomedical polymer patch used for cardiac regeneration should also be prepared to meet the complex biomechanical demands of the myocardium.
While some polymer materials, such as those derived from polyester thermoplastics, have been investigated for tissue engineering applications, their lack of elasticity limits their usefulness for maintaining the contraction and normal function of the heart.
Some ways in which researchers have attempted to improve these essential mechanical properties is by manipulating the structural geometries of polymeric materials through the use of nanofiber interlocking, hollow-sphere structures, surface cracks and pyramidal structures.
Click here to read more about the key developments in polymer science.
Melt Electrospinning Writing for Polymeric Cardiac Patch Development
One of the most widely used additive manufacturing technique is electrospinning, which has been used for both solutions and melt phases of polymers.
Melt electrospinning writing (MEW) is a unique electrospinning process that is often utilized for polymers of high viscosity and low conductivity characteristics.
MEW is particularly useful for the three-dimensional (3D) printing of porous polymeric materials that require distinct geometrical features.
As a result of the inherent advantages associated with MEW, a recent study interested in developing a mechanically functional polymeric patch used MEW to create an auxetic pattern.
In their work, MEW facilitated the creation of a missing-rib auxetic design of poly(ε-caprolactone) (PCL) material. The missing-rib design involves the selective removal of ribs within the network of the material while simultaneously ensuring that the internal angles are unchanged. No cardiac cells were incorporated into this patch, and any positive results were solely achieved by the geometrical structure of the PCL material.
To test whether the auxetic patches exhibited similar mechanical properties to that of the human myocardium, uniaxial tensile loading conditions were applied to the patches to assess stress-strain behavior. During these studies, researchers found that the PCL patches had a J-shaped stress-strain response that closely resembled the straightening and progressive tensioning experienced by cardiac collagen fibrils. In addition to having similar biomechanical behaviors, the PCL patches were also found to exhibit conductivity levels that averaged 2 to 2.5 S/m.
Overall, the MEW technique was found to enhance the biomechanical and conductive properties of the PCL material to closely resemble that which is present in the heart. While future work must still be done to evaluate the in vivo therapeutic applicability of these patches for the treatment of CVDs, the findings of this study are expected to change how biomaterials, particularly those comprised of polymers, are engineered.
How have polymers been used in other applications?
References and Further Reading
World Health Organization [Online] Available at: https://www.who.int/health-topics/cardiovascular-diseases/#tab=tab_1 (Accessed on 13 March 2020).
Olvera, D., Molina, M. S., Hendy, G., & Monaghan, M. G. (2020). Electroconductive Melt Electrowritten Patches Matching the Mechanical Anisotropy of Human Myocardium. Advanced Functional Materials. DOI: 10.1002/adfm.201909880.
Ning, C., Zhou, Z., Tan, G., Zhu, Y., & Mao, C. (2018). Electroactive polymers for tissue regeneration: Developments and perspectives. Progress in Polymer Science 81; 144-162. DOI: 10.1016/j.progpolymsci.2018.01.001.
Dayan, C. B., Afgah, F., Okan, B. S., Yildiz, M., et al. (2018). Modeling 3D melt electrospinning writing by response surface methodology. Materials & Design 148; 87-95. DOI: 10.1016/j.matdes.2018.03.053.