Tensile Strength of Resorbable Biomaterials

Sutures have been used for thousands of years for medical intervention, with the earliest materials being linen or cat gut.

The sutures used today are now classified by two different categories, first by composition, either natural or synthetic materials, and by absorption, either non-resorbable or resorbable. Resorbable materials degrade in the body primarily through programmed degradation caused by the interaction of water with specific chemical groups in the polymer chain. Thus, these materials are used to hold a wound together for a timeframe long enough for healing without the need for removal.

In this video, we will discuss the mechanisms behind resorbable material degradation and demonstrate how to evaluate the change in strength of the materials over time as they are exposed to different environments.

Resorbable materials primarily degrade in the body by oxidative, hydrolytic, and enzymatic degradation. Materials may undergo oxidation in vivo as the body reacts to the foreign object and releases oxidative species to attack it. The oxidative effect on polymers can cause chain scission and contribute to degradation. In hydrolytic degradation, water attacks susceptible bonds in the polymer to generate oligomers and finally monomers.

Polyesters like polydioxanone are commonly utilized as resorbable materials because the ester group is easily degraded via hydrolysis. After the material is implanted, it begins to absorb water. Hydrolytic scission then begins wherever the material is in contact with water. Hydrophilic materials absorb more water, and therefore degrade more rapidly throughout. However, hydrophobic materials absorb water more slowly and tend to degrade from the outside in.

Enzymes in the body catalyze various reactions and thus, catalyze the hydrolytic degradation of materials as well. The hydrolysis reaction is catalyzed by enzymes called hydrolases which can increase the rate of hydrolytic degradation by as much as 10 times. As the material degrades, the mechanical properties of the material change as well.

Let's take a look at how to analyze the change in strength of resorbable materials over time due to hydrolytic degradation in acidic, neutral, and alkaline environments.

For this experiment, obtain two types of resorbable sutures. Here, we use polyglyconate and polydioxanone.

Prepare six screw cap sample tubes each labeled with the date, sample type, and solution that the sample will be placed in. There should be one acidic, one alkaline, and one neutral solution for each sample type. Here, we show one of each sample. However, you should prepare three samples of each suture type for every time point.

Next, open the suture packaging and remove the suture. Cut the needle off the suture and dispose of it in the sharps container. Cut each suture into three pieces approximately 10 to 12 inches long. Make note of the physical characteristics of the suture. Use a caliper to measure the diameter of each suture and note the initial dimension.

Finally, weigh each suture, record the weight, and place one suture in each sample tube. Fill the neutral sample tubes with enough deionized water so that the suture is fully submerged, and cap the tube. Then, fill the acidic tubes with dilute hydrochloric acid and fill the alkaline sample tubes with dilute sodium hydroxide solution. Finally, place all six sample tubes in a rack in an incubator at 37 degrees Celsius.

Now let's take a look at how to determine the strength of the sutures using tensile testing. The tensile test loads a sample by stretching it until failure, enabling the determination of material strength.

First, test fresh sutures that have not been incubating in test solutions. Place the suture in the fixture of the instrument and secure it in place. The control sample should be the same length as the instrument which is approximately 10 to 12 inches. Next, zero the instrument and record the displacement speed setting. Ensure that peak hold is displayed on the control panel. Then initiate tension on the suture. The force and displacement will start to change on the instrument. Load the suture until failure. Then, turn off the instrument and record the peak force from the display panel.

Now let's measure the tensile strength of the samples that had been exposed to solutions at varying pH.

After the specified amount of time, remove the samples from the oven. Measure the pH of the solution in each tube using pH paper. After the pH of all solutions have been measured, remove the suture to be tested and rinse it with deionized water. Make note of the physical characteristics of the material.

Pat the sample dry with a paper towel, then weigh it and record the new mass. Next, place the specimen in the grips of the tensile tester and lock it into place. Zero the instrument and make sure the displacement speed is the same as used for the control sample. Also check that peak hold is displayed. Now, load the specimen until failure. Record the peak force from the display. Repeat the tensile test for each sample over the course of the time study.

Now let's see how to analyze the data to determine the strength of the samples.

First, calculate the average tensile stress of each sample by dividing the peak force by the cross-sectional area of the suture. Then, calculate the percent tensile strength retained by the suture after incubation using the formula shown. A plot of tensile strength over time for each sample shows that the strength of both types of sutures decreased over time in acidic, neutral, and alkaline solutions.

The polydioxanone structures degraded more in the acidic solution, with only 41% of the original tensile strength retained after five weeks, while 49 and 78% of strength was retained for the neutral and alkaline solutions, respectively. The polyglyconate sutures degraded similarly in all three solutions retaining around 42% of strength in acidic, neutral, and alkaline solutions after five weeks. The results are expected as the materials both possess ester bonds that are susceptible to hydrolytic scission, which is enhanced at high and low pH.

Now let's take a look at where resorbable materials are used in the biomedical engineering field.

Resorbable materials like the sutures tested in this video are most commonly used in surgical procedures to enable the healing of surgical sites while eliminating the need for suture removal. However, resorbable materials also play a role in tissue engineering as the scaffold for engineered tissue. Resorbable tissue scaffolds provide the initial three-dimensional structure for tissue, but degrade slowly as the cells grow and create their own structural material. Eventually, the initial scaffold is no longer needed and the engineered tissue more closely resembles native tissue.

Bone grafting involves replacing missing or damaged bone in order to help large fractures heal. In this study, researchers created a defect in the skull by drilling a five-millimeter hole. The bone fragment was detached and the bone graft attached to the bone using fibrin glue. Although donor bone is often used, resorbable materials present an alternative enabling the graft to degrade away as native bone grows.

You've just watched JoVE's introduction to resorbable materials. You should now understand how these materials degrade in vivo and in vitro, how to test for strength changes due to degradation, and some applications of these materials in the biomedical engineering field. Thanks for watching!