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The use of cells has always been an intrigued the scientists as it has the potential to cure and treat many disorders in the biomedical field, scientist have performed experiments and have attempted to artificially clone human organs using stem cells. There has also been an approach to delivers these stems cells in treatment degenerative diseases and also use of cells in repairing as a regenerative way of on an organ, use of cells, as therapeutic agents by process of encapsulating it with a polymer or an hydrogel and thus contributing in life science and health care sector.
Using hydrogel in delivery of cells therapeutically clearly signifies the purpose and the intrigue of the topic. For delivering the cells into a system requires a clear understanding of the biology and molecular working of the system. If cells directly delivered into a living system then the body/biological system, then the immune system springs into action and will take it as a foreign entity and as per programming of the biological system will try to flush it out of the system or destroy it in the system by known of the immunological pathways, for e.g. either by macrophages or apoptosis of that cell (Sawhney & Hubbell, 1992, Auchincloss & Sachs, 1998 and Sayegh & Turka, 1998). Also if not that, then every type of a cell requires an appropriate condition for its survival and viability. If it does not meet that requirement they are destroyed and then removed from the biological system. Hence there is requirement for protecting the cells against these activities and safely delivering the cells to a targeted site or release of cells in the right and appropriate conditions for its survival and growth hydrogels are used.
Hydrogels are of two types naturally occurring and synthetically produced. E.g., PEGA, HEMA, Methaacryclic acid is a synthetically derived hydrogel whereas, Alginate, Hyaluronic acid, Chitosan is naturally occurring hydro gel; these are basically closely arranged polymer chains or networks like a mesh. They have been found to have such properties which enable them as carrier for cells by encapsulation which are (i) high resemblance to extra cellular matrices of cell. (ii) Low cell affinity (iii) high cell constraint. Scientists have taken a liking to hydrogel as carriers and as a medium of encapsulating the cells with some modifications both chemical and physical to achieve their desired results. There have been many positive results when scientist used this technology in treatment of different disorders like Diabetes e.g. (Wang et al., 2009 and Teramura & Iwata, 2009), Parkinson's (Roberts et al., 1996), Alzimerer's diseases (Schmidt et al., 2008). There are many aspects into the process of this technology which needs to be taken in consideration during the process and to give appropriate results, which will be discussed ahead but just to give an idea during the encapsulation process, Properties of hydrogel, Purity of hydrogel, Properties of hydrogel with respect to the immune system, Stability of hydrogel, strength of hydrogel, mathematical calculations required in the design, Controls, Hydrogel-Cell viability effects and relationship, etc.
What kind of cells can be used is a good question to be asked in the encapsulation process and as a drug and active material it's plays a very important role. Deciding the type of cell will surely depend on which diseases or protein one wants to produce? For e.g. embryonic stem cells were used and were found to differentiate into insulin producing cells for diabetes (Wang et al.,2009), also myoblasts cells were used in the encapsulation process (Orive et al.,2009).
For encapsulation process a critical material would be hydrogel. Deciding based on ones requirement choosing an hydrogel and then preparing that hydrogel of right viscosity and rheological properties are very essential, and the reason being to have cell-cell adhesion and for cell viability, as it was seen that if there is decrease is cell adhesion, cell viability is also reduced which is a direct consequence of viscosity (Weber et al., 2008).
Pore size of the hydrogel:
This is so, as the cells are encapsulated in the hydrogel and they would require oxygen and other nutrients for their survival inside the hydrogel. Since hydrogel are nothing but network of polymer chains looking like mesh then, if the pore size is too small to allow these then the cell viability will be reduced and if they are too big to allow antibodies and other immune system generated bodies will attack these cells and destroy them. So a right balance is required which also depend on the encapsulation process and the material of hydrogel i.e., the molecular weight (Schmidt et al., 2008)
E.g. the production of alginate beads having ideal properties for that of cell culture is matrix pore size (Abbah et al., 2008), the diffusion rate for exchange of nutrition (Barbucci and Leone, 2004), and mechanical strength (Maguire et al., 2006). The normal range for a pore size in a normal scaffold is around 50Âµ to 250Âµm (Ren et al., 2007; Huang et al., 2008; Lee and Lee, 2006), this so as techniques and technology may differ.
(1.c) Viscosity measurement of gelling liquid:
Hydrogel's have an ability of swelling and so shall the cell encapsulated by the layer of hydrogel. If the viscosity of the gelling liquid is not correct the strength of the hydrogel bead is affected making to crack or burst either due compression or shearing effect while administering via iv due internal expansion of the cell and early release of cell resulting failure of experiment as cells will be discharged early and not the targeted site.
(1.d) Cell encapsulation process:
Cell encapsulation can be done in either of two ways one by macro-encapsulation and micro-encapsulation. Macro encapsulation is a large volume device where a large volume of cells are entrapped in one device. Micro encapsulation includes number of small volume devices where small amounts of cell are being encapsulated into a device in form micro sized beads.
These micro sized beads are spherical in nature so as to provide more surface area and also usually he transport occurs by diffusion from these hydro gels small amounts of cells helps in facilitating molecular transport. (Wang et al., 2009, and Teramura & Iwata 2009)
Here further changes and adjustment can be made to the CaCl2 concentration while making the hydrogel encapsulation (Li et al., 2006).
It was demonstrated that encapsulation process of encapsulation of embryonic stem cells (ES cells) in alginate. A simple but careful process was followed on dissolving the alginate in calcium free DMEM. A confluent layer of ES cells were removed by incubation in trypsin and then resuspending it in PBS and washing with PBS. Later cell count and cell viability noted. This led to formation of single small ES cells aggregates which were suspended alginate mixture to have mixture of both cell and alginate and these beads were formed.
The diffusion rate of supply of nutrition and others from hydrogels depends on molecular weight and tends to vary, more the initial concentration of alginate lesser the diffusion (Gelain et al., 2007). Furthermore, it has been show by De Vos and Marchetti (2002), ES cells were differentiated and immune-isolation of these cells inside the hydrogel for transplantation in islet encapsulation.
Recovery of viable cells:
The technology as such is very gentle which can be found out by cell recovery and 90% to 95%of the cells were recovered easily after encapsulation and the result also matches the same amount when compared with people performed similar researches around the globe e.g. (Li et al., 2006 and Maguire et al., 2006).
Suppression of immune system:
As discussed before the immune system requires to be suppressed for the success of this technology,
Which obviously is not a desirable trait? Also the properties of hydrogel have to be checked whether they are stable and strong. Properties like the gel strength which will decide the diffusion and release of the cells. The osmotic properties of the gel required to be examined.
Compression and shear stress:
These hydrogel beads are injected via a syringe in the body and will have to have to face many mechanical strength issues like compression and shear stress and so its very important that they do not crack at undesired location due gel being brittle which result in reduced cell viability or compression due to hydro gel being soft and so a mixture which is stiff and tough in nature is to be taken care to achieve appropriate results.
Degradation of hydrogel:
The hydrogels are easily degraded by proteolytic action and so rapid renal clearance and short plasma half life. Which forces us to use multiple, regular and high doses of the drug administration and results in very low patient compliances and also higher risk of systemic toxicity. And there is requirement of a control release formulation (Lin and Metters, 2006).
Use of hydrogel composite mixture:
There is also a question of which hydrogel is better or a composite mixture can be used. And the answer is still not found for there are lots of pro and cons of each cell and each hydrogel polymer and lot of combination is tried for e.g. when stability studies of alginate and agarose results showed that alginate with presence of calf adrenal chromaffin (CAC) cells proved to have more strength in comparison to agarose (Shoichet et al.,1995).And also when injecting these hydro gel beads care has to be taken that everything is performed in sterile area and especially hydrogels which are naturally occurring are pathogen free.
Cell viability assay:
This is an essential component of the technology and the experimental design as the cell viability assay determines the LIVE/DEAD cells. Qualitative and Quantitative tests both need to be performed. The results will show the fate of cells in the body (Wang et al., 2009).
Protein/gene/hormone expression and analysis:
The effect after release of the cell requires to be calculated and action and therapeutic efficacy is to be checked (Wang et al., 2009). E.g insulin release assay conducted by Elisa and protein content checked by Bradford method (Tal et al., 1985).
There may be a few cautious steps in this technology which should not be taken as weakness; just care has to be taken during the process. And one has to remember every technology has weakness but we use it because we look at the strengths which over power the weakness at any given day. Naming one of the weak points is the controversy of using stems cells especially embryonic stem cells but a responsible, but they have given many positive results.
Problems relating to sterilization of hydrogels like alginate.
Diffusion rate of these hydrogels, mechanical strength and stability when dealing with them.
Suppression of immune system may be required.
Positive are that they are sensitive towards the cells encapsulated and approx 90 to 95% of cells can be recovered back.
And in this technology at many clinical and preclinal stages have to be found to give positive results in for different disease states like in Parkinson's(Karuppagounder S. Senthilkumar et al.2007, Todd Roberts et al. 1996), in Diabetes (Wang et al.,2009 and Teramura & H. Iwata, 2009), in Cancer(Wang et al., 2009), in Alzihemeres. There is still a lot of work required and other approaches should also be not neglected like peptide conjugated ph responsive micelles, use of induced pluripotent cells.
Unlike other approaches in encapsulation like peptide encapsulation, using cell has provided better results and more stability (Orive et al., 2009).