3.1. Design of suprachoroidal spacer implant
The suprachoroidal spacer implant should be: (1) water permeable, facilitating free water flow within the SCS; (2) sufficiently rigid to easily advance within the SCS; (3) not so rigid as to inadvertently penetrate the choroid and retina; (4) capable of remaining within the SCS long term; and (5) most importantly, non-toxic and biocompatible.
To meet these criteria, we designed the implant as a monolithic crosslinked hydrogel based on PEG derivatives (Fig. 3). PEG is recognized as biologically safe by the United States Food and Drug Administration (FDA) and is widely used in various fields including, chemical, cosmetics, food, and pharmaceutical industries18,19. In particular, crosslinked PEG-based hydrogels have shown therapeutic potentials due to the biocompatibility, permeability, and negligible cytotoxicity20,21. The fabrication of ideal PEG-based hydrogels as a suprachoroidal spacer implant was optimized by testing different hydrogels prepared with different parameters including arrangement of methacrylate functional groups (PEG dimethacrylate and 4-arm PEG methacrylate), MW of PEG (5, 10, and 20 kDa), and weight percentage (w/v) of PEG (10, 20, 30, and 40%) in pre-gel solutions.
3.2. Mechanical testing of PEG hydrogels
We tested cylindrical PEG hydrogel blocks, fabricated with varying parameters, for stiffness at 5% strain in compression mode. Hydrogels made from 4-arm PEG methacrylate (10 kDa MW, 30 weight %) showed higher stiffness compared to those made from PEG dimethacrylate (Fig. 4A). This is likely because the hydrogels prepared with 4-arm PEG methacrylate became more resistant to deformation due to increased crosslinking density. Further tests varying the MW and weight % (w/v) of 4-arm PEG methacrylate showed that a lower MW, with a constant weight %, resulted in higher stiffness due to a greater number of crosslinking sites (Fig. 4B). Similarly, a higher weight % (w/v) in pre-gel solutions led to stiffer hydrogels (Fig. 4C).
Because of the small dimensions of the suprachoroidal spacer implant candidates (PEG hydrogels made with 0.3-, 0.5-, and 1.0- mm diameter), direct mechanical testing was not possible. Thus, bend strength of PEG implants with different parameters was determined (Table 1). The parameters that seemed to impart the greatest rigidity to the PEG implants were a diameter of 1.0 mm and relative dehydration of the implant. However, neither of these were feasible, as it was observed that upon contact with water, the PEG implants rapidly rehydrated within seconds and lost the mechanical rigidity of the dehydrated implants. Furthermore, 0.5- and 1.0- mm diameters were larger than the bore of the needle. No PEG implants with diameter of 0.3 mm were able register a force before bending.
Table 1 Bending force of select PEG hydrogels with length of 30, 20, 10, and 5 mm, indicating the material’s rigidity. Mean ± SD for triplicates.
3.3. Development of custom-designed injector
The PEG-based implant was delivered into the SCS with our custom-designed injector (Fig. 1). The injector enables the implant to safely and reliably access the SCS using a microneedle with length matched to the thickness of the sclera22,23. A 27-gauge needle (0.406 mm OD, 0.305mm ID) was chosen as this is the largest commonly used needle size for intravitreal injections in the outpatient ophthalmology clinic24. Such a needle size is commonly used and does not require suturing of the sclerotomy to prevent leakage and/or infection. Prior studies using a microneedle to deliver drugs into the SCS were used perpendicular to the sclera, but such an approach would likely kink the implant and/or penetrate the choroid/retina. Thus, an oblique approach was used.
To determine the ideal length and insertion angle of the needle16,22,23, we performed insertion studies in ex vivo rabbit eyes (Fig. 5). Injectors with different needle lengths were applied to sclera and a metal rod was advanced through the bore of the injector. If strong resistance was felt and the injector came off the eye, the tip of the injector had not yet entered the eye (outside the eye, indicated by a yellow V). If the rod could be easily advanced and a slight ‘pop’ was felt (rod going through choroid/retina), the tip of the injector was considered to be in the SCS (indicated by a green O). If the rod could be easily advanced and no ‘pop’ was felt and the rod was visible within the eye, the tip of the injector was considered to be in the vitreous (indicated by a red X). It was observed that a needle length of about 1.0 mm at an oblique angle was able to reliably enter the SCS. Such an oblique angle would enable delivery of the spacer implant without kinking. From this experiment, we derived the optimized parameters for the suprachoroidal injector, i.e., 27G bore, microneedle length of 0.8 mm, 45° needle bevel, and 34° angled inserter (Fig. 1B).
3.4. Optimization of PEG hydrogel implant parameters for atraumatic delivery to the SCS.
To improve the reliability of the implant delivery into the SCS without damaging the choroid or retina, the force required to penetrate the choroid/retina was determined using fresh enucleated rabbit eyes. As a proxy, different sized Prolene sutures (2 − 0, 4 − 0, 5 − 0, 6 − 0, 7 − 0, 8 − 0, 9 − 0, and 10 − 0) were advanced through the bore of the microneedle inserted onto the sclera. If a slight ‘pop’ was felt (suture going through choroid/retina), this Prolene suture was recorded as being able to penetrate through the choroid/retina. If no ‘pop’ was felt with a Prolene suture, it was recorded as unable to penetrate through the choroid/retina. If the suture could not be advanced, another sclerotomy was made and this was not considered a replicate.
Figure 6 shows the probability of these Prolene sutures penetrating choroid/retina as well as the bend force of each suture. Prolene sutures designated 6 − 0 (diameter 90 µm) were unlikely to penetrate choroid/retina, and 7 − 0 (diameter 67 µm) never penetrated in our study. The 7 − 0 Prolene suture had a bend force of 0.262 ± 0.035 gF when held 5 mm from the tip. This force was considered as the upper limit and used to guide the choice of the optimized PEG hydrogel implant parameters. Since the 0.3 mm diameter PEG hydrogels were not able to register a bend force, the final parameters were chosen as follows: 30% (w/v) 4-arm PEG methacrylate, 10 kDa MW, and 0.3 mm diameter.
3.5. Ex vivo delivery of suprachoroidal spacer implant
The ability of the custom-designed injector to deliver the spacer implant into the SCS was determined by noninvasive UBM, which has been used previously to examine suprachoroidal expansion25,26, and dissection17,27. After delivery, the UBM image showed the injected implant as a void within the SCS (Fig. 7A-E). The optimized injector was able to reliably deliver the PEG hydrogel implants of 4 different lengths (15, 30, 37.5, and 45 mm). Out of 38 loaded implants, 34 (89.5%) were delivered within the SCS and the other 4 into the sub-retina. Injectors made with other parameters were not as reliable as the optimized design.
Assuming the shape of the injected PEG implant as an ellipsoid, the volume that the PEG implant occupied in the SCS was estimated. First, the thickness of the implant within the SCS was determined from the UBM images. As examples, one UBM image is shown for each different implant length (Fig. 7B-E). Then, the eye was dissected and illuminated by blue light to better visualize the fluorescein dye added to the implants (Fig. 7F and G). The photos of the dissected eyes were analyzed using the Image J software to precisely estimate the major and minor axes of the ellipse occupied by the implant within the SCS. Finally, the total volume of the SCS implant was determined by calculating the volume of an ellipsoid:
\(V=\frac{4}{3}\pi abc\) , where \(a\) = SCS thickness/2, \(b\) = radius of major axis, and \(c\) = radius of minor axis
The results showed that the thickness, cross-section area, and the total volume of the SCS occupied by the injected implants were proportional to the implant’s lengths (Fig. 8). As the implant length increased from 15 to 45 mm, the SCS thickness increased from 0.276 to 0.525 mm, and the estimated volume of the SCS implant increased from 0.403 to 1.76 mm3 (= µL). Interestingly, the experimentally estimated volume of the implant within the SCS was only 44% of the theoretical volume of the cylindrical PEG implant (averaged from 4 different lengths). It is possible that the implant was partially dehydrated or compressed within the tissues post-injection, or the experimentally determined volumes might have been underestimated because the actual geometry of the implant within the SCS may not be precisely ellipsoidal. Despite this discrepancy, this shows we are able to fine tune the extent of SCS expansion, which is likely to influence the degree of IOP reduction, by varying the implant length. This aspect will be further tested in in vivo studies.
3.6. Clinical insights into suprachoroidal spacer implantation
Previous studies by Chiang et al13 and Chae et al14 demonstrated that suprachoroidal expansion with a fluid formulation can lower IOP. However, controlling the extent and direction of suprachoroidal expansion with a liquid formulation is challenging. Our study shows that varying the implant’s length can achieve different heights and volumes within the SCS. A solid monolithic implant, like ours, may offer easier implantation that has the advantage of remaining in place and also be removal if desired. Additionally, a recent phase II open label clinical trial demonstrated 39% IOP reduction with an implant inserted into the SCS28. The Cilioscleral Interposition Device (Ciliatech, Chavanod, France) is an acrylic-based implant with dimensions (6 mm circumferential, 4 mm anterior-posterior, and 0.2 mm thick plate) similar to our 45mm PEG implant in volume. While the Ciloscleral device requires implantation in the operating room, the hydrogel implant described herein has the potential to be delivered in-office by microneedle injection29.
It is important to note that the suprachoroidal spacer implant described (as well as the Ciliatech and liquid formulation described previously) are not like other “suprachoroidal”/ ”supraciliary” minimally invasive glaucoma surgical devices (MIGS), such as the Cypass®30,31, MINIject32,33, among others34,35. With those devices, the implant is used to create, and stent open a cyclodialysis cleft. This creates a direct communication between the anterior chamber and suprachoroidal space, enabling shunting of fluid without a subconjunctival bleb. Though effective in lowering IOP31, there was loss of corneal endothelium that could eventually require corneal transplantation36. Ultimately, the Cypass® device was voluntarily withdrawn from the market37, and other devices are in various stages of development to fill the void38. Since the suprachoroidal spacer implant and the suprachoroidal MIGS function by different mechanisms of action, it is possible that they may lower IOP synergistically.