What is a Cell Delivery Vehicle?

A wide variety of cells at different stages of differentiation from a range of adult and embryonic sources is being investigated for therapeutic use in the human body. However, transplanted cells face a harsh environment and in general, most cells die shortly after implantation ^{1-5, 7}. This can be attributed to ischemia due to absence of vascularization, the absence of cell attachment sites resulting in cell death (anoikis), the host immune response^{6, 7}, and low cell density ⁸. In the end, few cells engraft and many may veer down undesired differentiation pathways ⁹.

Cell delivery vehicles are designed to provide cell attachment sites to overcome anoikis⁷, protect from host immune cells, and localize the cells to maintain appropriate cell densities. The vehicle must also be non-immunogenic, biodegrade at appropriate rates, and have a physiological pH and temperature to avoid pH shock and hypothermia ^10-12. Ideally, it is desirable for its formulation to be customizable so that appropriate signals can be added (e.g. growth factors, extracellular matrix proteins) to provide boundaries against continued cellular differentiation. With the growing recognition of the benefits of minimally invasive surgical procedures, injectability of the vehicle also becomes an increasingly important attribute.

Several hydrogels are currently used as vehicles in some cases for both preclinical and clinical research. They are differentiated by their origin (natural, synthetic, and semi-synthetic), microstructure (fibrous network), stiffness, degradation time, and potential toxicities. The following table lists some of the more commonly used vehicles*:

Matrix	Origin	Commercial Name
Fibrin	Human Plasma	Tisseel
Collagen I	Human Fibroblast Cells	Vitrocol
Alginate	Seaweed	AlgiMatrix
Crosslinked Hyaluronate	Bacillus subtilis	Hystem
Polyethylene glycol diacrylate (PEGDA)	Synthetic	NA
Self-assembling peptides	Synthetic	HydroMatrix

(*while a basement membrane preparation from EHS mouse sarcoma is used widely for in vivo animal research, its rodent origin present large challenges for obtaining FDA approval for human implantation. It will not be considered in this discussion).

Comparison of the vehicles listed here is shown below. The primary differentiation occurs in their microstructure (i.e. self-organization into a fibrous network), stiffness (i.e. elastic, tensile, or shear modulus measured in kPa), gelation time (from initial preparation as a liquid to final gel), and the pH, temperature, and salt content when the matrix is still a liquid.

Matrix	Micro-structure	Stiffness (kPa)	Gelation time (minutes)	pH	Temperature (°C)	Additional Factors
Fibrin	Fibrous13	Elastic, 0.1314	1-3	Physiological	Physiological	Calcium
Collagen I	Fibrous13	Shear, 0.05516	6016	2.00	2-10°C	NA
Alginate	NA	Tensile, 13-7017	30-4017	Physiological	Physiological	Calcium
Crosslinked Hyaluronate18	NA	Shear, 0.011-3.518	5-20	Physiological	Physiological	NA
PEGDA	NA	Elastic, 1019	519	Physiological	Physiological	Photo-initiator
Self-assembling peptide	Fibrous20	Elastic, 0.00322	30-6021	2.00	Physiological	Salt causes gelation21

Hyaluronate (HA) is a key component of the cell extracellular matrix and has been used for a variety of medical applications¹³. HyStem is thiol-modified HA which is chemically crosslinked in the presence of PEGDA (HyStem).Thiol-modified gelatin can also be included for providing cellular attachment sites (HyStem-C) as can thiol-modified heparin for providing slow growth factor release (HyStem-HP).

In general, HyStem differentiates itself from its competitors in its user-friendly customizability and gentleness to cells when mixed in liquid form.HyStem allows the user to adjust not only its stiffness but also gelation time by varying concentrations of crosslinker and HA. HyStem is at physiological pH and temperature in its liquid form and hence provides a gentle microenvironment for encapsulated cells during transplantation.In contrast, Collagen I and Puramatrix are pH 2 in liquid form whereas fibrin and alginate gel only in the presence of calcium which may adversely affect neuronal cell signal transduction¹⁵.PEGDA requires UV light for crosslinking in the presence of free radical photocrosslinkers¹⁹.UV light is a known genotoxic agent can have deleterious effects on the cell cycle²³.

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