Loss and reintroduction of the radical initiator into the FlexyDos3D silicone dosimeter for 3D printing

3D printers allow for the rapid construction of complex 3D objects that would be very time-consuming with traditional casting techniques. Patient-specific objects have been created for years in the fields of dentistry, prosthetics, and surgical guides. However, 3D printed objects using materials that also serve a functional purpose, biologically or chemically, are now finding bio-medical applications. A custom 3D printer has been made that is able to print the FlexyDos3D silicone dosimeter, but the dosimeter’s sensitivity is severely decreased after printing. Testing was performed to determine if chloroform is lost during printing and techniques were tested to reintroduce chloroform into the dosimeter after printing to improve the dose response of the dosimeter. Results showed that the chloroform was completely evaporated from the dosimeter when exposed to conditions similar to that during printing and that chloroform could be reintroduced by soaking or exposure to chloroform vapours which led to an increase in the dose sensitivity. Further testing of the reintroduction of chloroform into the dosimeter is ongoing.


Introduction
Three-dimensional dosimeters have many benefits over one-and two-dimensional dosimeters, but importantly they can record radiation dose contiguously in the three spatial dimensions instead of, at best, a sparsely sampled set of dose values in a 3D space.3D dosimeters can also be made into the shape of an object for treatment validations, such as a human head or breast [1,2].Most 3D dosimeters are cast in moulds to make the shape of the object.However, casting techniques can be very time consuming in initially making the mould, and they are not able to make more complex objects with hollow cavities in a single casting, such as a heart or lungs.As such, making patient-specific dosimeters with casting techniques is not feasible for patient-specific treatment validations.
Additive manufacturing techniques (commonly referred to as 3D printing) allow for the construction of an object by depositing material in successive layers to create a 3D object.As such, more complex shapes can be created that can include cavities.The silicone dosimeter, FlexyDos3D, has been shown to be capable of being 3D printed with the use of a custom-built 3D printer [3].It can print the dosimeter material in its initial uncured state and form a solid shape by extruding the material through a narrow nozzle into a high temperature chamber within the printer that causes the printed material to cure rapidly.
Simple objects, including both solid and hollow cubes and cylinders have been created with the printer but the dosimetric properties of the dosimeter have been found to be far less sensitive than dosimeters created with casting techniques.The difference in the manufacturing techniques involves much higher temperatures during 3D printing of the material to rapidly cure the material, and the much higher surface area to volume ratio involved when extruding the dosimeter through a narrow nozzle.As such, it is believed that the chloroform within the dosimeter, which acts as a radical initiator for the radiation dose response, is likely evaporating out of the dosimeter to a much larger degree than previous tests performed on smaller surface area to volume tests.This would lead to a reduction in the doseresponse [4] Investigations into the evaporation of chloroform from the dosimeter material during printing were performed.However, these tests could not be performed by printing the material as measurements of the mass of the dosimeter material before printing could not be measured.The mass could not be accurately measured because not all of the dosimeter material could be extruded out of the tubing completely.Instead, tests were to be performed on thin castings (roughly 1mm thick) at the temperature used during printing as these could be measured accurately before and after curing at the temperatures used in the 3D printer heated chamber.
Tests were also performed to determine if chloroform could be introduced into the dosimeter after manufacture instead of during and if this would improve the dose response sensitivity towards that of the FlexyDos3D dosimeter with chloroform added during manufacture.

Dosimeter recipe
The standard FlexyDos3D dosimeter uses a silicone elastomer (Sylgard® 184, Merck) as its major constituent with 93% w/w base silicone and 4% w/w curing agent, 3% w/w chloroform, and 0.03% w/w leucomalachite green (LMG).However, to investigate the response of the dosimeter without chloroform and with reintroduction of chloroform into the dosimeter after manufacture, another formulation of the FlexyDos3D dosimeter without chloroform was made.It consisted of the same ratio of silicone base, curing agent, and LMG but no chloroform.The FlexyDos3D formulation without chloroform is referred to as 'un-activated FlexyDos3D' and the FlexyDos3D formulation with chloroform is referred to as 'standard FlexyDos3D'.
For the standard FlexyDos3D, the chloroform is first added to the LMG and mixed until the LMG is dissolved.Then for both samples, the chloroform and LMG or just the chloroform is mixed into the silicone base mixture and then the curing agent is mixed in.Bubbles are introduced into the mixture by the stirring and so a vacuum chamber is used to pull the bubbles from the mixture until there are no remaining bubbles.
The samples are then cured in an oven or the 3D printer oven chamber to rapidly cure the dosimeter.

Chloroform evaporation
The silicone dosimeter cures rapidly from a liquid state to solid when exposed to high temperatures.To be able to print the material in layers without flowing into a puddle, 160°C is used within the printer's oven chamber.However, chloroform has a relatively low boiling temperature (~60°C), and this would likely result in a loss of the chloroform within the dosimeter due to the higher temperatures used.
Two 5g samples of FlexyDos3D dosimeter were made, one of standard FlexyDos3D and the other un-activated FlexyDos3D.Each of the samples were poured into glass petri dishes, which gave a height of roughly 1mm of dosimeter, and then cured in the 3D printer at 160°C for two hours.Two hours was chosen as it currently takes the printer about two hours to print 60mL of dosimeter and so this is how long the first printed layer of dosimeter would likely be exposed to the high temperatures.The samples were weighed before curing and again after curing to determine any mass lost during curing.To determine if the mass loss was the chloroform and not part of the silicone material, a comparison was made between the mass lost in the standard and the un-activated FlexyDos3D castings.

Chloroform penetration
Previous research into the FlexyDos3D dosimeter had shown that chloroform was able to evaporate out of the dosimeter over time, even at room temperature and that the chloroform concentration affected the dose response of the dosimeter.The opposite case was considered where a FlexyDos3D dosimeter without any chloroform in it could be exposed to chloroform, either by soaking in chloroform or by sealing the dosimeter in a chamber with chloroform vapour, and if the FlexyDos3D dosimeter would then absorb this chloroform and cause it to exhibit a dose-response.
One sample of standard FlexyDos3D and 3 samples of un-activated FlexyDos3D were made each consisting of 20g of their respective dosimeter in a plastic cylindrical vial measuring roughly 25 mm on its inner diameter.Each sample was cured in a 60°C oven for two hours.The lower temperature was used for curing the samples as the plastic containers would melt at higher temperatures and the container had to be broken away from the dosimeter after casting for the later testing.After curing, two of the unactivated samples, were removed from the cylinders.Each sample was weighed again before each sample was placed in a separate sealed container.One sample was suspended in the container above chloroform amounting to 3% w/w of the samples mass (0.6g).The other sample was immersed completely in chloroform for two minutes, before being removed from the chloroform, patted dry, and then weighed to determine the amount of chloroform absorbed in the dosimeter.Both treated dosimeters were then sealed into separate air-tight containers.The un-activated FlexyDos3D sample and the standard FlexyDos3D sample were used as controls and were not exposed to chloroform after manufacture but were placed in sealed containers.All the samples were also stored in an opaque container to eliminate exposure to light sources before irradiation.
The samples were irradiated with 10Gy each with a 1cm x 1cm square beam.After irradiation, all the samples were again stored within the opaque container.
Readout of the samples was performed with a modified dual-wavelength cone-beam optical scanner based off a Modus Vista™ scanner (Modus Medical Devices Inc, London, Canada).The samples were rotated within the optical scanner through a full rotation under two different wavelengths of light and 256 images were taken in each revolution with a resolution of 0.12 mm / pixel at the centre of the scanner's rotation.
A filtered back-projection reconstruction was performed in the Matlab software (The Math Works, Inc).

Results and Discussion
The masses of the evaporation samples cured at 160°C for 2 hours are shown in Table 1 for both before and after irradiation.The un-activated FlexyDos3D did not have any chloroform and showed a minimal change in mass as compared to the standard FlexyDos3D sample which did have 0.175g of chloroform within the sample before curing.The mass of the standard FlexyDos3D sample changed by roughly the mass of chloroform added to it after curing at high temperatures within the printer oven.
For the irradiation of the un-activated FlexyDos3D samples, a significant dose response was seen after the delivery of 10Gy in both the soaked and vapour exposed samples as compared to the sample with no chloroform treatment.The standard FlexyDos3D sample showed the greatest optical density change due to radiation.The reconstructed optical CT images of the optical density difference can be seen in Figure 1 and the optical density at the maximum optical density change for each sample is shown in Table 2.The greater response seen in the FlexyDos3D sample as compared to the treated samples is likely due to a few factors.The LMG in the un-activated samples was unable to dissolve fully within the silicone mixture and could be observed visually as opaque clumps within the silicon matrix.As there was less LMG dispersed throughout the cured sample, it would seem reasonable that the dose response of the colour change would also be lessened.

Conclusion
The FlexyDos3D dosimeter loses its chloroform component when heated to high temperatures and having a high surface area to volume ratio.The chloroform is also likely lost during the 3D printing of the dosimeter where the extrusion of the material through a 0.6mm nozzle into a high temperature chamber meet the conditions of high temperature and high surface area to volume ratio.However, whilst the testing had shown that chloroform was able to evaporate out of the dosimeter, further tests showed that chloroform could also absorbed by the dosimeter when the dosimeter was briefly immersed in chloroform or if in a sealed container with chloroform vapours.This method could be used for dosimeters produced by the 3D printer that would lose their chloroform during printing to restore their dosimetric properties.The LMG will likely need to be initially dissolved into some chloroform as the LMG did not dissolve within the dosimeter even after soaking or exposure to chloroform vapours.The inhomogeneity of LMG led to imaging artifacts in the current study and may also lead to dose response errors if it is not homogeneously dispersed within the dosimeter.
Further testing will investigate the homogeneity of the chloroform penetration in larger phantoms and quantitative measurements of the dose response for the chloroform penetrated samples.The time required to expose the samples to chloroform will also be investigated to ensure homogeneity of chloroform distributed in the dosimeter.

Figure 1 .
Figure 1.Optical CT reconstructions of the irradiated samples.Each sample was irradiated with a 1cm x 1cm square beam to 10Gy (Dmax).(a) A FlexyDos3D sample with no chloroform added, (b) chloroform was added by exposing the dosimeter to chloroform vapours for 12 hours, (c) chloroform was added by soaking the dosimeter in chloroform for 2 minutes, (d) a standard FlexyDos3D sample with the chloroform added during manufacture.The streaking artifacts seen in (a -c) are due to the LMG clumps that were present throughout the samples because the LMG was unable to dissolve completely within the silicone mixture during manufacture without chloroform.

Table 1 .
Change in mass of cured samples

Table 2 .
Optical density at Dmax in the treated samples