Mechanically gradient polymers

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Polymer gradient materials (PGM) are a class of polymers with gradually changing mechanical properties along a defined direction creating an anisotropic material. These materials can be defined based upon the direction and the steepness of the gradient used and can display gradient or graded transitions. [1] A wide range of methods can be used to create these gradients including gradients in reinforcing fillers, cross-link density or porosity to name a few. 3D printing has been used to combine several of these methods in one manufacturing technique. [2]

Contents

These materials can be inspired by nature where mechanical gradients are used commonly to improve interfaces between two dissimilar surfaces. When two materials that have different moduli are connected together in a bilayer, this can create a weak junction, whereas a mechanical gradient can reduce the stress and strain of the connection. In contrast, a butt joint that gives a junction between materials with little to no gradient has been shown to be weaker than the homogenous components. [3]

Mechanically gradient polymers are not manufactured as extensively as can be found in nature, and there can also be unintended gradients during the manufacturing process. Materials are not always completely uniform despite the intentions of the manufacturer, and these unintended gradients may weaken the material rather than improve. Therefore, mechanical gradients must be properly applied to the particular application to prevent introduction of instabilities. [4]

Methods

Gradient in reinforcing filler

Reinforcing fillers such as carbon nanotubes that have high mechanical moduli have been used commonly to create polymer composites with high strength and toughness. [5] Since the modulus and filler amount are linked, by varying the amount of filler across the polymer the modulus will similarly change. [6] Additionally, since long nanofillers create anisotropic moduli, if the direction of the nanofiller could be modified along the length of the polymer, the modulus gradient could also be tuned in this manner.

A common approach to increasing the mechanical strength of polymers includes changing the crosslinking density of the polymer. Crosslinks connect the polymer chains creating a web that resists deformation. Therefore, increasing the crosslinking density in a section of a polymer will increase the modulus in this location. This can be used to create a mechanical gradient if the crosslinking density changes across the polymer. A common approach to achieving this is using a photopolymerization process which with changes in UV exposure you can change the degree of crosslinking or polymerization in the area exposed. [7] In a similar manner, the amount of initiator or crosslinker could be varied across the sample creating a similar effect.

Gradient in porosity

Porosity can be used to decrease the modulus of a polymer as seen in polymer foams, and as seen in bone, a change in porosity and therefore density can also be used to create a mechanical gradient along its cross-section. [8]

Applications

Nature

There are many examples in nature where soft tissue and hard surfaces are connected by a mechanical gradient to improve the fracture and impact resistance. Examples include mussels that connect to hard rocks by the mussel byssus which connects back to the soft muscle of the foot. [9] A more extreme example is the squid beak which has an extremely hard tip required to kill and dismember its prey which connects back to the soft flesh of the body of the squid. Without the mechanically gradient in the beak, the squid would be unable to withstand the high impacts despite its hardness since it would break off from the body at the junction between the materials. [10]

Biomedical Implants

As mentioned above, many systems in nature incorporate mechanical gradients, and similarly for biomedical implants these gradients can be useful. Many implants are stiff and can cause damage to the surrounding tissues due to this difference in stiffness. This is a problem for instance in microelectrodes implanted into the brain which is extremely soft. The damage caused can create a buildup of fibrous tissue which can then interfere with the signal between the electrode and the brain. [11] Similarly, in knee and hip implants, there is a need for high integration between the strong bone and the cartilage and tissue. Otherwise problems such as stress shielding can occur where the bone degenerates due to the implant having too strong of a modulus. [12]

Related Research Articles

Gel Highly viscous liquid exhibiting a kind of semi-solid behavior

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Thermosetting polymer Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Hydrogel

A hydrogel is a crosslinked hydrophilic polymer that does not dissolve in water. They are highly absorbent yet maintain well defined structures. These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature.

Trabecula

A trabecula is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has a mechanical function, and is usually composed of dense collagenous tissue. They can be composed of other materials such as muscle and bone. In the heart, muscles form trabeculae carneae and septomarginal trabecula. Cancellous bone is formed from groupings of trabeculated bone tissue.

Durotaxis is a form of cell migration in which cells are guided by rigidity gradients, which arise from differential structural properties of the extracellular matrix (ECM). Most normal cells migrate up rigidity gradients.

Cross-link Bond that links one polymer chain to another

In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural polymers.

Implant (medicine) Device surgically placed within the body for medical purposes

An implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue. The surface of implants that contact the body might be made of a biomedical material such as titanium, silicone, or apatite depending on what is the most functional. In some cases implants contain electronics, e.g. artificial pacemaker and cochlear implants. Some implants are bioactive, such as subcutaneous drug delivery devices in the form of implantable pills or drug-eluting stents.

Natural fiber Fibers obtained from natural sources such as plants, animals or minerals without any synthesizing

Natural fibers or natural fibres are fibers that are produced by geological processes, or from the bodies of plants or animals. They can be used as a component of composite materials, where the orientation of fibers impacts the properties. Natural fibers can also be matted into sheets to make paper or felt.

Nanocomposite

Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nano-scale repeat distances between the different phases that make up the material.

Biomaterial Any substance that has been engineered to interact with biological systems for a medical purpose

A biomaterial is a substance that has been engineered to interact with biological systems for a medical purpose, either a therapeutic or a diagnostic one. As a science, biomaterials is about fifty years old. The study of biomaterials is called biomaterials science or biomaterials engineering. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into the development of new products. Biomaterials science encompasses elements of medicine, biology, chemistry, tissue engineering and materials science.

Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state to their original (permanent) shape when induced by an external stimulus (trigger), such as temperature change.

Filler (materials)

Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler which are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Nano-scaffolding is a medical process used to regrow tissue and bone, including limbs and organs. The nano-scaffold is a three-dimensional structure composed of polymer fibers very small that are scaled from a Nanometer scale. Developed by the American military, the medical technology uses a microscopic apparatus made of fine polymer fibers called a scaffold. Damaged cells grip to the scaffold and begin to rebuild missing bone and tissue through tiny holes in the scaffold. As tissue grows, the scaffold is absorbed into the body and disappears completely.

Artificial bone

Artificial bone refers to bone-like material created in a laboratory that can be used in bone grafts, to replace human bone that was lost due to severe fractures, disease, etc.

Materials that are used for biomedical or clinical applications are known as biomaterials. The following article deals with fifth generation biomaterials that are used for bone structure replacement. For any material to be classified for biomedical application three requirements must be met. The first requirement is that the material must be biocompatible; it means that the organism should not treat it as a foreign object. Secondly, the material should be biodegradable ; the material should harmlessly degrade or dissolve in the body of the organism to allow it to resume natural functioning. Thirdly, the material should be mechanically sound; for the replacement of load bearing structures, the material should possess equivalent or greater mechanical stability to ensure high reliability of the graft.

Surface chemistry of neural implants

As with any material implanted in the body, it is important to minimize or eliminate foreign body response and maximize effectual integration. Neural implants have the potential to increase the quality of life for patients with such disabilities as Alzheimer's, Parkinson's, epilepsy, depression, and migraines. With the complexity of interfaces between a neural implant and brain tissue, adverse reactions such as fibrous tissue encapsulation that hinder the functionality, occur. Surface modifications to these implants can help improve the tissue-implant interface, increasing the lifetime and effectiveness of the implant.

Self-healing hydrogels

Self-healing hydrogels are a specialized type of polymer hydrogel. A hydrogel is a macromolecular polymer gel constructed of a network of crosslinked polymer chains. Hydrogels are synthesized from hydrophilic monomers by either chain or step growth, along with a functional crosslinker to promote network formation. A net-like structure along with void imperfections enhance the hydrogel's ability to absorb large amounts of water via hydrogen bonding. As a result, hydrogels, self-healing alike, develop characteristic firm yet elastic mechanical properties. Self-healing refers to the spontaneous formation of new bonds when old bonds are broken within a material. The structure of the hydrogel along with electrostatic attraction forces drive new bond formation through reconstructive covalent dangling side chain or non-covalent hydrogen bonding. These flesh-like properties have motivated the research and development of self-healing hydrogels in fields such as reconstructive tissue engineering as scaffolding, as well as use in passive and preventive applications.

Artificial cartilage is a synthetic material made of hydrogels or polymers that aims to mimic the functional properties of natural cartilage in the human body. Tissue engineering principles are used in order to create a non-degradable and biocompatible material that can replace cartilage. While creating a useful synthetic cartilage material, certain challenges need to be overcome. First, cartilage is an avascular structure in the body and therefore does not repair itself. This creates issues in regeneration of the tissue. Synthetic cartilage also needs to be stably attached to its underlying surface, bone. Lastly, in the case of creating synthetic cartilage to be used in joint spaces, high mechanical strength under compression needs to be an intrinsic property of the material.

Titanium foams exhibit high specific strength, high energy absorption, excellent corrosion resistance and biocompatibility. These materials are ideally suited for applications within the aerospace industry. An inherent resistance to corrosion allows the foam to be a desirable candidate for various filtering applications. Further, titanium's physiological inertness makes its porous form a promising candidate for biomedical implantation devices. The largest advantage in fabricating titanium foams is that the mechanical and functional properties can be adjusted through manufacturing manipulations that vary porosity and cell morphology. The high appeal of titanium foams is directly correlated to a multi-industry demand for advancement in this technology.

Thermally induced shape-memory effect (polymers)

The thermally induced unidirectional shape-shape-memory effect is an effect classified within the new so-called smart materials. Polymers with thermally induced shape-memory effect are new materials, whose applications are recently being studied in different fields of science, communications and entertainment.

References

  1. ↑ Claussen, K. U.; Giesa, R.; Schmidt, H. W., Longitudinal polymer gradient materials based on crosslinked polymers. Polymer 2014, 55 (1), 29-38.
  2. ↑ Dizon, J. R. C.; Espera, A. H.; Chen, Q.; Advincula, R. C., Mechanical characterization of 3D-printed polymers. Additive Manufacturing 2018, 20, 44-67.
  3. ↑ Claussen, K. U.; Giesa, R.; Schmidt, H. W., Longitudinal polymer gradient materials based on crosslinked polymers. Polymer 2014, 55 (1), 29-38.
  4. ↑ Callister, W.; Rethwisch D. Fundamentals of Materials Science and Engineering: An Integrated Approach, 2012, 4, 466.
  5. ↑ Andrews, R.; Weisenberger, M. C., Carbon nanotube polymer composites. Curr Opin Solid St M 2004, 8 (1), 31-37.
  6. ↑ Fox, J. D.; Capadona, J. R.; Marasco, P. D.; Rowan, S. J., Bioinspired Water-Enhanced Mechanical Gradient Nanocomposite Films That Mimic the Architecture and Properties of the Squid Beak. Journal of the American Chemical Society 2013, 135 (13), 5167-5174.
  7. ↑ Sunyer, R.; Jin, A. J.; Nossal, R.; Sackett, D. L., Fabrication of Hydrogels with Steep Stiffness Gradients for Studying Cell Mechanical Response. PLOS One 2012, 7 (10).
  8. ↑ Sherwood, J.K.; Griffith, L.G.; Brown, S. U.S. Patent No. 6454811, 2002.
  9. ↑ Harrington, M. J.; Waite, J. H., How Nature Modulates a Fiber's Mechanical Properties: Mechanically Distinct Fibers Drawn from Natural Mesogenic Block Copolymer Variants. Adv Mater 2009, 21 (4), 440-+.
  10. ↑ Tan, Y. P.; Hoon, S.; Guerette, P. A.; Wei, W.; Ghadban, A.; Hao, C.; Miserez, A.; Waite, J. H., Infiltration of chitin by protein coacervates defines the squid beak mechanical gradient. Nat Chem Biol 2015, 11 (7), 488-+.
  11. ↑ Tate, M. L. K.; Detamore, M.; Capadona, J. R.; Woolley, A.; Knothe, U., Engineering and commercialization of human-device interfaces, from bone to brain. Biomaterials 2016, 95, 35-46.
  12. ↑ Huiskes, R.; Weinans, H.; Vanrietbergen, B., The Relationship between Stress Shielding and Bone-Resorption around Total Hip Stems and the Effects of Flexible Materials. Clin Orthop Relat R 1992, (274), 124-134.
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