Development and Analysis of Pluronic Gels For Scar Prevention

Kayla Hom, Dominic Patrizio, Dale Sugimoto Chemical and Molecular Engineering Program, Stony Brook University

AbstractAfter a lumbar laminectomy or microdiscectomy, cell proliferation from damaged muscle tissue in the area

surrounding the spinal column may result in epidural fibrosis and scar tissue adhesions, leading to intense pain and adversely affecting quality of postoperative life. Epidural fibrosis may, however, be prevented by the insertion of a material at the surgical site that physically blocks cell proliferation. Previous research has shown promising results with gelatin gels and semipermeable membranes.

We suggest that Pluronic F127 gels could have greater relevance due to their injectability and biodegradability. It is also a non-cell-adhesive, non-cytotoxic material that is liquid below physiological temperatures and exhibits a cubic micellar gel structure above approximately 21˚C. Thus, Pluronic F127 gels could be used for post-surgery application as a barrier for scar formation prevention. For this purpose, gel properties and behavior were analyzed in conditions mimicking in vivo interactions.

Rheological measurements of the elastic moduli of F127 gels prepared in phosphate buffer saline (PBS) at 37˚C showed that a concentration greater than approximately 17-wt% is critical for gelation. Furthermore, gels containing additional entrained protein showed that the concentration of entrained protein is a critical factor for determining gel integrity and elasticity.

Gel dissolution tests demonstrated that regardless of volume of the fluid added and time elapsed, lower concentrations of F127 gels (20-25%) dissolved faster when in contact with PBS than 30% gels, which swelled up to twice their original volume before deteriorating. The dissolution efficiency depends mostly on the volume to liquid contact surface area of the gel. Rheological measurement conducted after swelling showed lower elastic moduli than control measurements on untreated control gel.

Using data extrapolated from CT scans and a 3D spine model, we plan to create a system in which to conduct a tangential flow test using a gel volume, fluid flow, and surface area comparable to conditions present in the area of interest.

Keywords: pluronic, controlled release, post-discectomy syndrome, microdiscectomy, epidural scarring, spinal cord, responsive polymers

IntroductionIn a laminectomy, or decompression surgery, the back part of the vertebra that covers the spinal

canal, known as a lamina, is removed. This enlarges the spinal canal in order to relieve pressure on the spinal cord. This pressure is often the result of bony growths, which may occur in the case of arthritis.

However, a complication may occur in which epidural scar tissue grows into the gap caused by removal of the lamina, potentially adhering to spinal nerve roots. This causes severe pain, resulting in what is known as Failed Back Surgery Syndrome, or FBSS.1

As the purpose of the surgery is to remove pain and relieve pressure, this is a major problem. It is necessary, then, to find a way to mitigate or eliminate the growth of scar tissue adhesions within the spinal column.

The simplest and most obvious solution to this problem is to fill the gap caused by a laminectomy with a material that physically blocks cell proliferation. Research into this option has been carried out using gelatin gels and semipermeable membranes.2

We decided to look at Pluronic F127 as a relevant alternative. Pluronic F127 has several physiologically and structurally relevant properties, as it is non-cell-adhesive, non-cytotoxic, and gels at body temperature above concentrations of approximately 18 wt% in solution. Pluronic F127 is a member of a family of polymer materials known as poloxamers, with BASF-produced poloxamers known as a Pluronics. These materials consist of A-B-A triblock copolymers of ethylene oxide and propylene oxide, and are used as antifoaming agents, wetting agents, dispersants, thickeners, and emulsifiers.3 Pluronics form micelles in solution above a Critical Micellar Concentration (CMC). As A-B-A triblock copolymers

in the form of PEO-PPO-PEO, pluronics isolate their hydrophobic PPO blocks from a polar solvent (e.g. water, cell media, PBS, etc.) while exposing hydrophilic PEO blocks.

This means that Pluronic F127 will form a micellar gel in the body after injection, creating a barrier to which cells cannot attach within an occupied space. Furthermore, because Pluronic F127 gels as a result of weak interactions between extra-micellar PEO chains, the barrier it forms will be semi-permanent, blocking cell proliferation for a sufficient amount of time, but dissolving more quickly than other options. This could provide a significant advantage over traditional crosslinked hydrogels or semipermeable membranes, which may remain in the body indefinitely.

In short, we see Pluronic F127 gels as a non-cell-adhesive, non-cytotoxic material that could be loaded into the vertebral column post-surgery, forming a semi-permanent barrier in place of the lamina to prevent epidural scarring and adhesions. If developed, such a gel would prevent epidural scarring by blocking cell proliferation within the area without killing damaged tissues present at the surgical site.

As such, we have developed standard operating procedures (SOPs) for Pluronic F127 gel formulation and have conducted tests on Pluronic F127 gels created using those SOPs. These tests include measurement of the elastic modulus, G’, of gels in PBS, measurement of G’ with Bovine Serum Albumin (BSA) added to the gel, gel dissolution, dissolution of a dye contained within the gel, and dissolution rate of gels in a tangential flow system created based on conditions and spacing evident in a 3D printed model based on the CT of a spine.

LiteratureIn order to determine a material for post-surgery scar tissue prevention and drug delivery, we

need a non-cell-adhesive, non-cytotoxic material. Given the properties indicated in the literature, we believe that Pluronic F127 is a viable option for post-surgery scar tissue prevention in the spinal column.

Pluronic F127, also known as Poloxamer 407, is an FDA-approved material currently used in a variety of personal care products and pharmaceuticals, including cosmetics, contact lens cleaning solutions, and mouthwash.4 Research involving Pluronic F127 has explored several novel applications, many of which involve injection and drug delivery.

In Evaluation of the poly(lactic-co-glycolic acid)/pluronic F127 for injection laryngoplasty in rabbits, Lee et.al. tested the biocompatibility of PLGA/Pluronic F127 solutions in the vocal fold through injection in rabbits. Results showed that not only did every rabbit survive the injection, but also none showed an inflammatory response to the PLGA/Pluronic F127 solution.5 For our purposes, this serves to highlight the safety and viability of Pluronic F127 gels as an injectable material.

In The effect of physiologically relevant additives on the rheological properties of concentrated Pluronic copolymer gels, Jiang et. al. tested the effect of various additives on the elastic (G’) and viscous (G’’) moduli of Pluronic F127 gels. Their work shows that the presence of salts may affect the elastic and viscous moduli of the gels.6 This is a potential problem, as it suggests that introduction of the gels into the body could effectively decrease their integrity by exposing them to certain salts. However, since we are conducting our tests in media, we expect that differences in gel elasticity and integrity between our gels and the final gels will be minimal.

Figure 1. Two examples of salts, NaCl and CaCl2, and how their concentration affects the elastic modulus of pluronic.[4]

In Gels of Pluronic F127 and nonionic surfactants from rheological characterization to controlled drug permeation, Antunes et.al. added vesicles to the pluronic as a transdermal drug delivery vehicle and tested the rheological effects by using a rheometer to measure the elastic and viscous moduli. 7

They observed that, at lower temperatures, the mixture behaved as a Newtonian fluid, characteristic of a micellar gel, and transitioned to a cubic phase at higher temperatures, where viscosity and shear rate appeared to have an inverse relationship. They also stated that thermal gelation was weak but observable at 15%, becoming more clearly visible at 18%. Antunes et. al, also concluded that the presence of amphiphiles in the polymer network enhances network strength, possibly due to the polymer–vesicle hydrophobic association. Hydrophobic association between hydrophobic segments of the polymer and vesicles caused the rheological response of the mixed polymer–surfactant system to be more solid-like than polymer alone systems. They concluded that this effect could be related to an increase of density of active links present in the polymer network.

Figure 2. Comparison of the different moduli of each sample made. [5]

As Antunes et. al. suggested that polymer-vesicle systems increased the storage modulus and the density of active links in the system, vesicles, or other additives with similar interactions, are a potential means to increase the integrity of our gels, and may slow degradation in the body.

Since the gel will be placed within the vertebral column, sterility of the gel is vital. In Evaluation of the Effect of a Gamma Irradiated DBM-Pluronic F127 Composite on Bone Regeneration in Wistar Rat, Kayal et. al. sterilized Pluronic mixtures through gamma irradiation. However, since gamma irradiation caused the modulus of the gel to change slightly and Cobalt-60 is unavailable to us, this method of sterilization has limited relevance.8

In Controlled Release of High Molecular Weight Hyaluronic Acid from Molecularly Imprinted Hydrogel Contact Lenses Ali and Bern performed tests to aid in development of hydrogel-based contact lenses made from high weight hyaluronic acid.9 standing liquid swelling tests were among what they performed. They chose this kind of test due to the environment contact lenses experience; continuous contact with liquid for several hours at a time.

Our gel is also going to be surrounded by bodily fluids for its entire lifetime, so performing tests similar to that which Ali and Bern carried out can help determine how well our gel will function in the vertebral column. Our desired environment also has potential for flow, so not only standing liquid tests, but flow tests could be performed as well.

In Prevention of Epidural Scarring After Microdiscectomy: A Randomized Clinical Trial Comparing Gel and Expanded Polytetrafluoroethylene Membrane Gerd et. al. ran clinical trials on two

different methods of post-discectomy scar prevention. The two materials were ADCON-L Gel (ALG)Preclude Spinal Membrane (PSM).2 They had a trial size of 31 subjects, where after discectomy, one of the two implants were placed into the patient. 18 received PSM and the remaining 13 received ALG. All patients had follow up MRIs done 3-6 months after surgery. Through surveys and the results from MRIs Gerd et. al. found that both of the methods for prevention provided the same amount of scar prevention, and most patients experienced less pain, than if no implant was used. This shows that any material that can block cell proliferation will succeed in lowering pain and scar formation. So if pluronic can handle the conditions and environment experienced in the vertebral column, then it should be successful in blocking cell proliferation, due to its non-cell-adhesive properties.

HypothesisIn order to determine a material for post-surgery scar tissue prevention and drug delivery, we

need a non-cell-adhesive, non-cytotoxic material.Given the properties indicated in the literature, we believe that Pluronic F127 is a viable option

for post-surgery scar tissue prevention in the spinal column.In order to test this, tested the degradability and integrity of the gel under realistic conditions

based on stress, flow, and the exposed surface area of the gel. We plan to use a system designed to simulate the conditions in the vertebral column based on a 3D printed model.

Materials and MethodsThe FDA considers safety precautions and procedures of paramount importance when dealing

with the human body. In our case, the FDA’s CFR title 21 is relevant, as our Pluronic F127 gel was considered as an implantable medical device. Therefore, good laboratory practice (GLP) was maintained throughout the development of the gel samples, so that any experimental results are repeatable, and a good manufacturing practice (GMP) process can be created based on our procedures. This means that our SOPs needed to be in accordance with GMP/GLP requirements, with a focus on sterilization, cleanliness, and safety. As such, all tests and formulations were performed in an organized fashion, and in compliance with regulations.

OverviewInitially, the solvent chosen for the pluronic was cell media, but due to its protein content, an

alternative avenue was required, as any protein present in pluronic changes the physical properties, and would introduce more variables. Thus phosphate buffered saline (PBS) was used to dissolve the pluronic. It is non-toxic to cells, and it lacks any of the proteins cell media has that could potentially change the gel’s properties. Red food dye was added to the pluronic (diluted to 1:8000, dye:PBS), as the pluronic used has too low of an optical density to be detected by the spectrophotometer.

In accordance to SOPs developed, several batches of pluronic gel were produced with varying weight percents and solvents. Initially the focus was on 20-, and 25-wt% pluronic gels, but eventually included 30% as well as we found it to handle swelling/dissolution better. The effect of bovine serum albumin as an additive was also tested, added at 5%, eventually discovering that solubility of BSA in gel appeared to be lower than 5%.

FormulationGels were prepared by adding Pluronic to solvent and stirring overnight in a 4 cold room, and℃

stored in syringes. As Pluronic F127 solutions gel above 37 , gels were stored in liquid form in syringes℃ and converted to gel by increasing the temperature (usually via incubation) after transferring gels to the requisite container for the test to be performed. Solutions were sterilized by microfiltration prior to gelation.

SpectroscopySpectroscopic tests used a Beckman DU 530 UV-Vis Spectrometer to measure the solubility and

release of BSA in Pluronic gels. We conducted a Bradford Assay at 595 nm wavelength using BioRad Protein Assay to measure BSA content in gels over time. We also determined total BSA content in gels by lowering temperature, causing gels become a sol, and measuring the amount present, in both 20- and 25- wt% gels.

RheologyRheological tests used a Malvern Instruments Gemini HR Nano Rotational Rheometer equipped

with peltier plates to measure the elastic (G’) and shear (G’’) moduli of 2 mL gel samples by varying shear at constant amplitude. All samples were tested at a constant frequency of 1 Hz, as this would be similar to conditions in a physiological environment. The gels were tested at 20-, 25-, and 30- Pluronic wt%, in both PBS and media, and the effect of BSA was tested as an additive in 20-, and 25- Pluronic wt% gels in media.

Dissolution/SwellingTo test how the gel’s physical properties are affected by the amount of liquid it absorbs, we

performed simple standing tests. Several 20-, 25-, and 30- wt% pluronic samples were made, where each sample was 1.5 ml of the gel plated on a cell culture plate, and recorded the weight of the gel after incubating at body temperature for 5 minutes. To ensure that the gel would maintain liquid form before plating, syringes and pipette tips were kept cold until used. Then either 0.5 mL, 1.0 mL, 1.5 mL PBS were added to the samples, directly on top of the gel, then incubated the samples for either 30, 60 or 90 minutes. Some samples were also left overnight. The PBS was kept in the incubator until used, to prevent liquifying the gel due to its thermal properties.

Once all the samples were completed, any excess liquid left on top of the gel was removed and rheological tests were performed to characterize how the gel swells and its dissolution as a function of liquid-to-gel ratio over different durations of time. Spectroscopic tests were performed on the liquid removed to determine the optical density of the dye released by the gel. This was done as a means to see how much of the gel was lost to the PBS

After completion of the PBS tests, more samples of 30-wt% pluronic were made, and 0.5 mL, 1.0 mL or 1.5 mL cell media were added. The cell media was used to see the effect that liquid closer in composition to the spinal area would have on the gel. 30% was chosen exclusively for this test as it had the most promising data from the previous standing tests.

Results and DiscussionFood Dye Concentrations

Figure 3. Concentration of food dye retained in the 20-wt% and 30-wt% F127 samples after 90 minutes

Figure 3 shows that the 20-wt% F127 gels with the least amount of PBS added retained the greatest amount of food dye for the longest duration, which was consistent across all the samples. In general, 20-wt% F127 gels retained a higher concentration of food dye than the 30-wt% F127 gels.

Dissolution and swellingAfter adding PBS to the F127 samples and heating them, they were organized by the time

elapsed, and then by the volume of PBS added. Measurements and calculations were made based on the initial weight of the gel, the weight of the gel and the different volumes of PBS, and the final weight after heating the mixture at 37 ºC. These were calculated as percentages of the initial pluronic gel remaining, or of the initial pluronic gel that swelled due to the addition of PBS.

Figure 4. Percentage of the 20-wt%, 25-wt%, and 30wt-% Pluronic F127 gels that dissolved or swelled, following the addition of

PBS and heating at 37 ºC for 60 and 90 minutes.

Table 1. A summary of the percentage of the 20-wt%, 25-wt%, and 30-wt% Pluronic F127 gels that dissolved or swelled, following the addition of PBS and heating at 37 ºC for 60 minutes and 90 minutes.

% Pluronic + mL PBS 60 minutes 90 minutes

20-wt% + 0.5 mL PBS 48.66% ± 18.05% 61.29% ± 15.15%

20-wt% + 1.0 mL PBS 12.20% ± 0.82% 39.06% ± 7.97%

20-wt% + 1.5 mL PBS 20.57% ± 13.23% 70.08% ± 44.88%

25-wt% + 0.5 mL PBS 118.16% ± 8.16% 108.55% ± 17.04%

25-wt% + 1.0 mL PBS 95.35% ± 10.07% 105.15% ± 10.06%

25-wt% + 1.5 mL PBS 76.40% ± 13.02% 67.11% ± 21.39%

30-wt% + 0.5 mL PBS 102.55% ± 23.84% 105.89% ± 4.78%

30-wt% + 1.0 mL PBS 122.53% ± 4.35% 114.01% ± 17.27%

30-wt% + 1.5 mL PBS 115.79% ± 1.60% 120.64% ± 7.85%

Figure 4 and Table 1 indicate that the 20-wt% F127 gels will always dissolve when PBS is added, regardless of the time elapsed and volume. The 25-wt% F127 gels swell, but once 1.5 mL of PBS are added it begins to degrade after the first hour. The 30-wt% F127 gels swell, regardless of the volume of PBS added and the time elapsed.

Figure 5. The percentage of the 30-wt% Pluronic F127 gels that dissolved or swelled overnight, following the addition of PBS or cell media.

The addition of cell media was done to stimulate an environment more realistic to that found near the spine, in terms of protein interactions with the gel. The gels placed in this environment swelled faster,

but degraded quicker, as well.Swelling is an interesting characteristic that was not an intended goal, so further investigation of

this property was required. As a result, the strength of the gels needed to be compared. Rheology had been performed on 20-wt% and 25-wt% F127 samples as preliminary data. The 30-wt% F127 gels were tested for their rheological properties.

Rheology

Figure 6. The rheogram for a 30-wt% sample with 0.5 mL PBS, showing both the viscous and elastic moduli.

Figure 6 shows a rheological plot. The elastic modulus, G’, shows the gel’s ability to withstand deformation as a function of shear stress. Once the viscous modulus, G’’, is greater than the elastic modulus, it indicates that this is the shear rate at which the gel transitions from its micellar gel state to a liquid state. All samples were tested at a constant frequency of 1 Hz. Rheograms for several samples were analyzed, with the results summarized in Figures 7-10.

Figure 7. Plateau elastic moduli of previous samples of different F127 concentrated gels, with varying concentrations to determine formation of micelles.

Figure 8.. Plateau elastic moduli of samples of 20%- and 25%-F127 concentrated gels, with varying additives and concentrations

Figures 7 and 8 were preliminary data obtained. Figure 7 confirms that the critical micelle concentration occurs at F127 concentrations greater than 17-wt%. It also shows that adding the food dye did not have a significant effect on the elastic modulus of the 30-wt% F127 gels.

Figure 8 demonstrates that gels prepared in PBS were much stronger in terms of the elastic modulus, than those prepared with entrained BSA. Thus, the addition of protein weakens the gel’s integrity. As a result, in further experimental procedures, it was decided to make the 30-wt% F127 samples in PBS and not in BSA so as to prevent a quick dissolution of the gel.

Figure 9. The intersection at which the elastic and viscous moduli crossover for samples of different F127 wt% concentrated gels, with varying additives.

*

Figure 10. Plateau elastic moduli of samples of 30-wt% F127 with varying volumes of PBS added, and plateau elastic moduli of samples of 30-wt% F127 with varying volumes of cell media added. Samples were all incubated for 22 hours.

Figures 9 and 10 indicate that the gels that swell have a lower elastic modulus than those that dissolve quicker. Figure 9 compares the gels in terms of the point at which they transition between their elastic and viscous states in terms of the shear stress. Figure 10 shows the samples’ abilities to withstand deformation after different additives were used after being incubated 22 hours, compared to control gels in which no additives were added.

The 20-wt% F127 gels demonstrate the highest elastic moduli, while the 30-wt% F127 gels

displayed the lowest. The 30-wt% F127 samples with PBS added tended to have a greater ability to withstand shearing stress, as well demonstrated a higher elastic modulus than those with cell media. It was therefore concluded that a gel that swells would not be able to withstand as much deformation as a gel that deteriorates faster.

ConclusionsBy testing F127 gels over a range of concentrations, it was confirmed that in order to obtain a

micellar network, a concentration of 18-wt% or greater was needed for gelation at body temperature. With the introduction of protein to the gels, there was an effect on the integrity and elasticity. Regardless of the volume of PBS added, lower concentrations of F127 gels (20-25%) dissolved faster than 30% gels, which always swelled in volume before deteriorating. Otherwise, adding PBS to F127 gels will always result in a degree dissolution, independent of volume and of time elapsed. It was surmised that the efficiency of dissolution depended on the volume to liquid contact surface area of the gel.

In order to stimulate a protein environment similar to that found in the area of interest, samples were placed in cell media. These gels swelled quicker than those placed in PBS, but also degraded faster.

Rheology performed on these gels indicated that the gels that swelled demonstrated lower elastic moduli than measurements on untreated control gels. The addition of food dye as a visual tracker did not have a large effect on the elastic modulus of the 30-wt% gels, but adding PBS only resulted in much higher elastic moduli when compared to adding BSA. With respect to adding PBS, adding cell media lowered the shear stress at which 30-wt% gels would transition between their gel and liquid states. The control samples without the addition of PBS had a higher shear stress limitation, as well as elastic modulus. It was therefore concluded that a gel that swells prior to deterioration would not be able to withstand as much deformation as a gel that deteriorates faster.

Using data extrapolated from CT scans and a 3D spine model, we plan to design a system in which to conduct a tangential flow test while comparing gel volume, fluid flow, and surface area similar to conditions present in the surgical site.

AcknowledgementsThis work was made possible through support from Stony Brook University’s Department of

Materials Science and Engineering, the program of Chemical and Molecular Engineering, and the Department of Pharmacological Sciences. We would like to thank the graduate students and mentors for our research group, especially Adriana Pinkas-Sarafova (Chemical and Molecular Engineering Program) and Clement Marmorat (Department of Materials Science and Engineering), for their guidance and the opportunity to work in their labs, and Yuval Shmueli (Department of Materials Science and Engineering) for providing us with the 3D scans and models with which we were able to mimic the conditions found in the spine. Additional thanks to Michael Caponegro (Department of Pharmacological Sciences) and Stella Tszyrka (Department of Pharmacological Sciences). We appreciate the rest of the Chemical and Molecular Engineering program for their support over the last four years.

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