Biomaterials in tissue engineering

Transcription

1 Biomaterials in tissue engineering S. Swaminathan Director Centre for Nanotechnology & Advanced Biomaterials School of Chemical & Biotechnology SASTRA University Thanjavur Tamil Nadu Page 1 of 6

2 Table of Contents 1. BIOMATERIALS IN TISSUE ENGINEERING Contact angle measurement using goniometer Fourier Transform Infrared Spectroscopy (FTIR)... 5 Page 2 of 6

3 1. Biomaterials in tissue engineering A Biomaterial is defined a material used in medical device intended to interact with the biological systems without provoking any harmful effects. A good example is calcium sulphate (Plaster of Paris), which has been used as a bone graft substitute. Similarly hip-joint prostheses have been fabricated from stainless steel, or titanium, special alloys, ceramics and ultrahigh molecular weight polyethylene. Expanded polytetrafluoroethylene (eptfe) has been used as non-stick coating in the cooking vessel and has also been exploited as small artery grafts. Most of the skate shoes use polyurethane to provide maximum grip on the skateboard. The same material can be used to fabricate the hemodialysis access graft. Another good example is the use of biomaterials in the fabrication of intraocular lens. About 50% of elder population suffers from the cataracts where the natural lens loses its transparency and become cloudy. Hence biomaterials such as poly (methyl methacrylate), silicone elastomer, soft acrylic polymers has been used to fabricate the intraocular lens, substitute for the natural lens. Although most of the identified biomaterials have improved the quality of patient s life, each manmade construct has its own limitations due to the variations in genes, body chemistries, sex, life style and degree of physical exercise from individual to individual. Hence choice of the biomaterial for the intended application plays a vital role for the success rate. For example use of calcium sulphate as a bone substitute cannot support the patient weight completely. Moreover, this will degrade faster than the healing rate of bone. Stress shielding, implant loosening and corrosion are the major drawbacks of using metal alloys in hip prostheses. Use of eptfe graft in small arteries can provoke foreign body response and exhibit complaint mismatch. Polyethylene terephthalate (PET) is highly thrombogenic promoting thrombosis and catastrophic complications in prosthetic vascular graft implantation. Additionally use of polyurethane as a graft can leach some harmful products. Hence a biomaterial should not be toxic to the system and provide mechanical support to the cells. Page 3 of 6

4 Mostly polymers have gained more attention for the use of biomaterials in biomedical applications such as orthopaedics, dental applications, soft tissue and cardiovascular regeneration. Polymer is a macromolecule composed of repeating units of same or different monomers. The polymers are mainly classified as natural (those which are derived from the living organism) and synthetic (those which are synthesised chemically). Molecular weight is the term used to describe the average length of the polymer chains, which are expressed as Daltons or atomic mass units. Number average molecular weight (Mn) and weight average molecular weight (Mw) are the major types of molecular weights. Number average is defined as the distribution of molecular weight over the number of molecules whereas weight average is the distribution of molecular weight over the weight of each chain. The ratio of Mw to Mn is known as polydispersity index (PDI), measures the breadth of the distribution. Usually PDI is around for most of the commercial polymers and polymers, whose PDI is is said to be monodisperse. The biomaterial properties such as bulk, surface, mechanical and biological have found to influence dynamic interactions at the tissue implant interface. In the bulk, solid materials differ from other state of the materials such as liquid and gas due to the strength of interatomic forces, which brings the atoms together. There are three primary or strong inter atomic bonds (ionic, covalent, and metallic) and two secondary or weak inter atomic bonds (van der Waals and hydrogen bonding). Surface property differs from that of the bulk since the atoms at the surface have high reactivity as well as special organization. Such surface can be modified for the intended function that can drive the biological functions such as protein adsorption, cell adhesion, cell growth, and blood compatibility in response to biomaterial. Hence it is very essential to characterise the surface using different methods of characterization. Page 4 of 6

5 1.1 Contact angle measurement using goniometer Surface energy and wettable property of the biomaterial is evaluated by contact angle measurements. The angle of liquid droplet between the liquid and the surface is used to determine the surface energy. Assume that the drop of water is placed on both polished and unpolished surface of the automobile body. The water beads up on the polished surface whereas the droplet flows over the unpolished surface. This is because highly polished surface possess hydrocarbons at its surface whereas unpolished surface rich in oxidized layer. When the liquid spreads over the surface without any droplet formation, then the surface is said to be wetted or hydrophilic. Hence adhesive force (between water and surface) is greater than the cohesive force (between the molecules in the liquid). For hydrophobic material, the cohesive force is greater as compared to adhesive force, hence the water beads up on the surface. The contact angle less than 30 is designated as hydrophilic since adhesive force is nearly equal to cohesive force. If the contact angle is less than 10, then the water spreads over the surface, named as superhydrophilic. The contact angle increases with the hydrophobicity and angle greater than 90 are called as hydrophobic. Lotus effect or superhydrophobicity will be seen only when the contact angle exceeds 150. The success of scaffold mainly depends upon how cells can interact with the scaffold since cells mainly prefer to sit on the hydrophilic surface. That s why we need to know the contact angle for a specific material. The major advantages of this method are that it is inexpensive, quick and very easy process. However, it also has some limitations such as operator dependent, surface heterogeneity and roughness can alter the results. Even material property has an influence for example some material may absorb water and swell in an aqueous environment or some may dissolve. 1.2 Fourier Transform Infrared Spectroscopy (FTIR) FTIR is a characterization technique used to determine the molecular fingerprint of the biomaterial. When an IR radiation is passed through the sample, the radiation should be either absorbed or transmitted by the atoms in Page 5 of 6

6 the molecule. Each atom has a specific characteristic vibration frequency and no two unique materials produce a fingerprint of IR spectrum. FTIR has benefits such as 1-5 µm penetration depth, greater signal to noise ratio, identify the existence of the unknown molecules, and provide the unique molecular fingerprint of the material. The major disadvantage is that the FTIR instrument measures only interferograms but not spectra. Page 6 of 6