The effect of retinal scaffold modulus on performance during surgical handling

Abstract

Emerging treatment strategies for retinal degeneration involve replacing lost photoreceptors using supportive scaffolds to ensure cells survive the implantation process. While many design aspects of these scaffolds, including material chemistry and microstructural cues, have been studied in depth, a full set of design constraints has yet to be established. For example, while known to be important in other tissues and systems, the influence of mechanical properties on surgical handling has not been quantified. In this study, photocrosslinked poly(ethylene glycol) dimethacrylate (PEGDMA) was used as a model polymer to study the effects of scaffold modulus (stiffness) on surgical handling, independent of material chemistry. This was achieved by modulating the molecular weight and concentrations of the PEGDMA in various prepolymer solutions. Scaffold modulus of each formulation was measured using photo-rheology, which enabled the collection of real-time polymerization data. In addition to measuring scaffold mechanical properties, this approach gave insight on polymerization kinetics, which were used to determine the polymerization time required for each sample. Scaffold handling characteristics were qualitatively evaluated using both in vitro and ex vivo trials that mimicked the surgical procedure. In these trials, scaffolds with shear moduli above 35 kPa performed satisfactorily, while those below this limit performed poorly. In other words, scaffolds below this modulus were too fragile for reliable transplantation. To better compare these results with literature values, the compressive modulus was measured for select samples, with the lower shear modulus limit corresponding to roughly 115 kPa compressive modulus. While an upper mechanical property limit was not readily apparent from these results, there was increased variability in surgical handling performance in samples with shear moduli above 800 kPa. Overall, the knowledge presented here provides important groundwork for future studies designed to examine additional retinal scaffold considerations, including the effect of scaffold mechanical properties on retinal progenitor cell fate.

Figure 1.

Photo-rheology enables the collection and analysis of real-time polymerization data. A) Schematic of the rheometer module with UV lamp used for real-time data collection. B) Representative real-time rheology data showing the five-point moving average of storage (G’) and loss (G”) moduli. Shaded area indicates when UV light was on. Dashed line indicates crossover time (t_c). Storage modulus data was fit using least squares method to the empirical model (red line) to determine the final polymerized modulus for each sample (plateau modulus parameter).

Figure 2.

Simplified schematic of the general loading procedure of the scaffold into the surgical tool. Left: the polymer scaffold is initially suspended between the two semi-cylindrical arms. Right: upon closing of the arms, the scaffold is scrolled up within the tool, ejecting as the arms open again.

Figure 3.

PEGDMA samples demonstrate a wide range of polymerized shear moduli dependent on prepolymer formulation. A) PEG Dilutions follow a binary trend in polymerized modulus based on the concentration of PEGDMA 750. B) PEG Blends appear to exhibit a power-law relationship based on the concentration of PEGDMA 8000. N=9 for all formulations, error bars represent 95% confidence interval, *p < 0.05.

Figure 4.

Surgical handling trials suggest a lower mechanical property limit for retinal transplantation scaffolds. A) All 12 PEGDMA samples ranked on a 1–10 score based on handleability in a mock retinal transplantation procedure. B) Select samples were retested to ensure repeatability of the results from Panel A. N=3 for all samples in each trial (except for one with N=2, [#]), data points and bars represent median and range, respectively.

Figure 5.

Ex vivo surgical handling trial validated in vitro results. A) Representative images of retinal transplantation surgery on cadaver pig eyes. Polymer sample is shown being ejected out of the surgical tool and unscrolling in the sub-retinal space. B) Based on results from the first surgical handling trial, samples 30/15/55, 30/20/50, and 20/10/70 were tested in mock retinal transplantation surgeries on post-mortem pig eyes. Samples were rated on the same 1–10 scale used in the first trials. Ex vivo results closely follow those of the first trial. N=3 for all samples (except one with N=2, [#]), data points and bars represent median and range, respectively.

Figure 6.

Compressive modulus measurements validate the assumption that Poisson’s ratio is 0.4–0.5 for these polymer samples, with the compressive modulus being roughly 3 times higher than the shear modulus. Dashed lines indicate where the lower mechanical property limit was observed (orange represents compressive modulus limit; purple represents shear modulus limit). Data points and bars represent mean and 95% confidence intervals, respectively (N=9).