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 The polymer, Poly ethylene glycol (PEG) is a great candidate for soft tissue replacement.  PEG is formed by linking repeating units of ethylene glycol to form polymers that are linear or branched molecules with different masses. PEG is a synthetic non degradable polymer that is widely used as an artificial scaffold in tissue engineering research.  Therefore, PEG is an excellent candidate because when used as a hydrogel, it is a great scaffolding material for repairing and regenerating tissues.  This is because of the 3d environment it creates that is very similar to that of soft tissues.  Thus, some of PEG’s advantageous properties include good biocompatibility, non-immunogenicity, and resistance to protein absorption which are key in tissue replacement.  However, one big con of using PEG is its bio-inert nature (it does not initiate a response when introduced to a biological tissue). Thus, when using PEG as a hydrogel it can not individually provide an ideal environment that supports cell adhesion and tissue reformation. As a result, researchers have developed bioactive modified PEG gels to mimic ECM or extracellular matrix.  The ECM components play a critical role in mediating cell functions and have important biological functions.  Density is an important physical characteristic property of a substance.  All objects have density, but there are many factors that contribute to how high or low the density is.  The density of an object is defined as the relationship between the mass of the substance and how much space it occupies, this is also known as volume.  Density is determined by the atoms that make up an object.  The atoms arrangement, size and mass all contribute to the density of an object.  Ultimate Tensile Strength is also known as tensile strength or the ultimate, and it is the maximum amount of stress an object can undergo before it pulled or stretched.  To determine the Ultimate Tensile Strength of an object you would divide the cross section of a material tested by the stress placed on the material.  The Ultimate Tensile Strength is the final amount of stress experience before the object is fractured.  The Modulus of Elasticity also known as the Young’s Modulus is the elastic limit of any solid material.  It can also be referred to as the stiffness of a material.  Although it is a fundamental property that cannot be changed it is dependent on temperature and pressure.  To determine the modulus of elasticity you would divide the stress by the strain.  The elastic modulus of a material is known as the slope of its stress-strain curve.  The stiffer the material the higher the modulus of elasticity is.  The melting point of an object is a very important physical characteristic.  It can be defined as the temperature at which a solid substance melts to become a liquid.  There is no universal temperature that determines that an object will melt.  Every substance has a different melting point due to many factors such as the size and shape of molecules, ionic bonds and intermolecular forces.   While comparing the two polymers, PEG and PGA I noticed they had very different values.  PGA had very high numbers none of which were negative. Meanwhile, PEG had low values including negatives.  PGA had a density of 1.5 g/cc, meanwhile PEG had a density of 1.083 g/cc.  These two values were not too different, however PGA was slightly more dense.  The ultimate tensile strength for PGA was 890 MPa, meanwhile PEG had an ultimate tensile strength of 18 MPa.  These two values are very far apart, PGA is almost 50 times stronger than PEG.  The modulus of elasticity of PGA is 8.4 GPa, meanwhile PEG has a modulus of elasticity of 0.0047.  Again these two values were very different, this showed PGA is a stiffer material because it has a higher value.  Lastly, the melting point of PGA is 224-230 °C meanwhile PEG had a melting of -50?.  This shows that PGA can withstand higher temperatures.  All of this data shows that PGA is more dense, stronger, stiffer, and can withstand higher temperatures.  Although, it has all of those advantageous characteristics in the situation of a soft tissue replacement they are not necessary.

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