Imaging of Hydrogel Microsphere Structure and Foreign Body Response Based on Endogenous X-Ray Phase Contrast

Abstract

Transplantation of functional islets encapsulated in stable biomaterials has the potential to cure Type I diabetes. However, the success of these materials requires the ability to quantitatively evaluate their stability. Imaging techniques that enable monitoring of biomaterial performance are critical to further development in the field. Xray phase-contrast (XPC) imaging is an emerging class of X-ray techniques that have shown significant promise for imaging biomaterial and soft tissue structures. In this study, XPC imaging techniques are shown to enable three dimensional (3D) imaging and evaluation of islet volume, alginate hydrogel structure, and local soft tissue features ex vivo. Rat islets were encapsulated in sterile ultrapurified alginate systems produced using a highthroughput microfluidic system. The encapsulated islets were implanted in omentum pouches created in a rodent model of type 1 diabetes. Microbeads were imaged with XPC imaging before implantation and as whole tissue samples after explantation from the animals. XPC microcomputed tomography (mCT) was performed with systems using tube-based and synchrotron X-ray sources. Islets could be identified within alginate beads and the islet volume was quantified in the synchrotron-based mCT volumes. Omental adipose tissue could be distinguished from inflammatory regions resulting from implanted beads in harvested samples with both XPC imaging techniques. Individual beads and the local encapsulation response were observed and quantified using quantitative measurements, which showed good agreement with histology. The 3D structure of the microbeads could be characterized with XPC imaging and failed beads could also be identified. These results point to the substantial potential of XPC imaging as a tool for imaging biomaterials in small animal models and deliver a critical step toward in vivo imaging.

The emerging field of pancreatic tissue engineering: A systematic review and evidence map of scaffold materials and scaffolding techniques for insulin-secreting cells

“…This relies on the basic concept of minimizing distances that limit oxygen and nutrient diffusion while retaining properties that prevent anoikis and immunological host-reactions and has commonly been used for in vivo studies in the reviewed literature. 18,56–59 The importance of diffusion distance for the viability and function of encapsulated ISC was shown experimentally by comparison of glucose-stimulated insulin secretion by free islets, microencapsulated islets in alginate (diameter = 700 µm), and encapsulated islets in larger alginate capsules (diameter = 1800 µm). The smaller microcapsules showed only a slightly delayed and decreased insulin secretion, whereas in the larger capsules the insulin secretion was > 40% decreased and the delay time was increased due to a larger diffusion distance.…”

“…61 The controllable generation of these droplets or the encapsulation of cells in general can be achieved by various techniques such as microfluidic devices (Figure 4(a)), pressure-based air jacket microencapsulation, vibrational jet flow technology, electrostatic droplet generation, or manual syringe-based extrusion into a chemical crosslinking solution (Figure 4(b)). 18,19,56,57,59,62–76 Most droplet-generation methods for cell encapsulation used extrusion-based systems, where a cell-containing hydrogel solution will be extruded through a needle. Depending on the viscosity of the hydrogel, the diameter of the needle, and the flow rate or extrusion pressure, a droplet will be generated.…”

“…Cell-containing microcapsules are most suitable for implantation into existing anatomical cavities such as the peritoneal cavity or into an omental pouch. 59 The lack of retrievability of the microcapsules is a disadvantage of this technique, so different in vivo monitoring methods have been investigated for non-invasive tracking of the microcapsules. 59,69,81,82 The application of new imaging techniques can also provide information about cell morphology, spatial distribution, and oxygen supply within a scaffold or hydrogel encapsulation.…”

“…Scaffold size, mechanical stability, and diffusion distances affect the choice further. Among the in vivo studies included (n = 91) the ISC/scaffold construct was transplanted into subcutaneous locations (n = 31), 27,51,69,92–94,112,115,120,133,135,145,147,156,157,171,212–225 the peritoneal cavity (n = 19), 18,26,56,63,65–68,101,134,135,173,195,226–232 an omental pouch (n = 14), 57,59,81,118,119,163,174,225,233–238 the renal capsule (n = 18), 54,…”

Recent progress in the use and tracking of transplanted islets as a personalized treatment for type 1 diabetes

“…A demonstration of loss of contrast agents following capsule rupture was shown using SPIO nanoparticles [84]. Recently, micro-CT imaging was employed to image the fine structure of microcapsules using their intrinsic contrast properties without the use of contrast agents [91], using CaCl 2 instead of BaCl 2 [83]. Barium is particularly attractive, as its high affinity for alginate increases the mechanical strength of the capsules [83, 92], making them suitable for implantation sites that exhibit high mechanical pressure.…”