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consideration point for systemic toxicity and biopolymers is regarding the dose. The standard biocompatibility test is performed on the basis of surface area or mass to volume; these ratios are spelled out in Table 1.4.
Table 1.4 Standard device extraction ratios used for biocompatibility (per ISO 10993‐12).
| Thickness (mm) | Extraction ratio |
|---|---|
| <0.5 | 6 cm2/ml |
| 0.5–1.0 | 3 cm2/ml |
| >1.0 | 3 cm2/ml |
| >1.0 (elastomeric devices) | 1.25 cm2/ml |
| Irregular solid devices | 0.2 g/ml |
| Irregular porous devices | 0.1 g/ml |
As Table 1.4 points out, the more surface area or mass a device has, the more extraction volume is added to the device during sample preparation. This approach works well for solid, stable materials such as metals and hard plastics but can be challenging with materials such as biopolymers, especially if they are produced with a porous microarchitecture or are biodegradable.
Another giant gap in the approach that uses surface area or mass for calculating the extraction volume is that it does not take into consideration the actual dose that a single patient will be exposed to. Typically, each biological test requires a certain minimal volume of fluid to run, and because of this limitation the sample amount needed for the testing is directly portioned to the logistics demanded by the test itself and not on the actual clinical use of the device. For example, let us say during a surgical procedure, a patient will only receive one PLA screw that is 0.5 g in weight. For the biocompatibility assessment of the screw, a standard subacute study was run. For testing, up to 112 screws were included in order to conform to the required sample volumes that were repeatedly dosed to the test animals, resulting in an exposure that is in actuality multiple times the clinical mass to body weight dose. This leads to a vast overestimation of the exposure risks of the biopolymer.
A better way to design the different systemic toxicity studies of biopolymers is based on dose per body weight of the patient. The standard weights per patient population are described in Table 1.5. In this case, one would determine the appropriate worst‐case target population for the medical device or material and determine a dose per kg of body weight based on that criterion. Subsequently, the testing would be done with a sample size that would expose the specific animal to a safety‐factor‐corrected dose that represents the appropriate clinical dose.
Table 1.5 Standard body weight parameters.
| Population | Standard body weight used (kg) |
|---|---|
| Adult man | 70 |
| Adult woman | 58 |
| Children | 10 |
| Neonates (<1 yr) | 3.5 |
An example of a test design according to the clinical dose approach would be as follows: a surgical procedure where up to two screw PLA screws (each weighing 0.5 g) will be implanted into a patient, the worst‐case exposure per patient will be 1 g of PLA, and the specific clinical prescribed doses are outlined in Table 1.6.
Table 1.6 Example of specific population doses for 1 g PLA screw.
| Population | Gram of screw per kg body weight | With 10X safety factor |
|---|---|---|
| Adult man | 0.01 | 0.14 |
| Adult woman | 0.02 | 0.17 |
| Children | 0.10 | 1.00 |
| Neonates (<1 yr) | 0.29 | 2.86 |
In a rat subchronic study, if the worst‐case target population is adult women and the test rat weighs 500 g, the dose would be calculated as follows:
Desired ratio with safety factor = 0.17 g of screw per body weight
This approach would ensure an accurate exposure dose to the animal and would present a more clinically relevant evaluation for the risks of systemic toxicity for the device.
1.4.3 Implantation
The most difficult and complex test design for many biopolymers revolves around implantation risks. It is important not to walk into an implant study with haste and without careful planning. Indeed, in this case, failing to plan could lead to a failing test. It is important that the study is planned in sufficient detail such that all relevant information can be extracted from the study, as the implant test is usually the longest test in the biocompatibility suite, and therefore, it is imperative to have the design right up front.
The main issue with testing a biopolymer in an implant test is the absorption profile. Physical characteristics (such as form, absorption rate, metabolism characteristics, density, and surface hardness) can all influence the tissue response to the test material. Also, the choice of control articles should be matched as closely as reasonably possible to the test sample physical characteristics. This is recommended in order to allow comparison of the specific tissue reaction(s) with that of a similar material whose clinical acceptability and biocompatibility characteristics have been established to determine acceptance criteria for the test.
Another key consideration for the implant test for a biopolymer is with the implantation time points. ISO 10993‐6 states: “For absorbable materials, the test period shall be related to the estimated degradation time of the test product at a clinically relevant implantation site. When determining the time points for sample evaluation, an estimation of the degradation time shall be made.” Usually, in practice we try to estimate the absorption profile based on the specific metabolism rate and method of the material and the implant system. After this, we set three time periods: one where we first see degradation (usually between two and four weeks), second when half the sample is degraded, and third when we see a “steady state” in the sample material. A steady state is defined as a point in time where the body is no longer interacting with the material and no additional changes are happening. For example, in vivo implantation tests with a PLLA density scaffold demonstrated fast degradation in the first three weeks, after which the degradation rate progressively decreased [20]. This milestone is reached when the body has either encapsulated or otherwise dealt with the foreign material or when full degradation of the material has occurred.
As mentioned above, an appropriate control