Technology of mammalian cell encapsulation☆
Introduction
Cell encapsulation aims to entrap viable cells within the confines of semi-permeable membranes. The scientific literature suggests that membranes are expected to be permeable for transport of molecules essential for cell survival, but not to allow the transport of molecules larger than a desired critical size. Despite its nonphysiological nature, the permselective capsule environment has been shown to support cellular metabolism, proliferation, differentiation and cellular morphogenesis. The primary impetus behind the recent development of cell encapsulation technologies has been the desire to transplant cells across an immunological barrier without the use of immunosuppressant drugs. The latter, due to their systemic administration, are associated with unwanted side effects due to non-specific suppression of the immune system that leads to a variety of undesired complications (e.g., opportunistic infections, failure of tumor surveillance, as well as adverse effects on the encapsulated tissues) [1], [2], [3]. These complications cannot justify the ethical use of systemic immunosuppressants for a significant fraction of patients who are candidates for cell transplants and provide the impetus to develop alternative methods to overcome the immune rejection process. By surrounding a transplant with a membrane barrier, the access of a host’s immune system to the transplant can be physically prevented. The feasibility of transplanting cells in immunoprotective membranes is under study for the treatment of a wide variety of endocrine such as anemia [4], dwarfism [5], hemophilia B [6], kidney [7] and liver failure [8], pituitary [9] and central nervous system (CNS) insufficiencies [10], and diabetes mellitus [11] (see other chapters in this Volume). Besides transplantation, encapsulated cells are being pursued for a variety of other applications (Table 1). The common thread for all such applications is the need to isolate a desired cell population from an outside environment. Depending on the choice of membrane material and whether membranes are pre-fabricated or fabricated around viable cells, cells will be subjected to different entrapment conditions. The final capsule properties will be variable to suit the needs of a particular application and it is essential, for any encapsulation technology, to tailor capsule properties in a reproducible fashion by the process variables.
An optimal balance has to be maintained among various capsule properties to support cell survival in the context of an expected performance criterion. The mass transport properties of an encapsulation membrane are critical since the influx rate of molecules essential for cell survival and the outflow rate of metabolic end products will ultimately determine the extent of entrapped cell viability (Fig. 1). The metabolic requirements of various cell types are different and, hence, optimal membrane permeability is expected to depend on the choice of cells. Although the role of permeability for particular elements essential for cell survival has been explored (for example, oxygen [27]), no systematic approach has been taken to determine the permeability requirements of particular cell types. An empirical approach has been typically taken to tailor capsule permeability for cell survival. The upper limit of capsule permeability, i.e., molecular mass cut-off (MWCO), will be application dependent. In the case of transplantation, the MWCO is expected to be different whether xenogeneic or allogeneic tissues are destined for engraftment (see Section 5.3 for more details). For bioreactor cultures, the MWCO will vary whether it is desirable to have the cell-derived biomolecules to permeate through the capsule wall. Besides permeability, an important consideration is the availability of an extracellular matrix (ECM) to encapsulated cells [28], [29], especially for anchorage-dependent cells. The ECM not only allows the cells to express their differentiated functions but also helps to organize the cell mass within the capsules for optimal viability [28], [29]. Capsule biocompatibility is critical when encapsulated cells are intended for transplantation since it is the compatibility of a biomaterial with the host that ultimately determines the nature of host response and graft survival. The physicochemical nature of a biomaterial is important to enhance the capsule biocompatibility and exploration of different materials for this purpose has necessitated development of different encapsulation procedures to accommodate the varying physicochemical properties.
This chapter is intended to provide a summary of current cell encapsulation techniques. Since other chapters of this Volume are for specific applications, it was our intention not to emphasize application-specific issues but rather to review the advantages and disadvantages of various cell entrapment processes. A major distinction is in the utilization of ‘macrocapsules’ vs. ‘microcapsules’. In macrocapsules, large groups of cells are enveloped in tube or disc shaped hollow devices while in microcapsules a smaller cell mass is individually entrapped in their own spherical capsule. This manuscript will review techniques of microencapsulation, where cells are entrapped in spherical capsules and beads, or conformally coated. This will be followed by a review of macroencapsulation technology and issues specifically pertinent to macrocapsules. The success of any encapsulation technique will ultimately rely on a systematic evaluation of capsule properties and encapsulated cell performance. Accordingly, various techniques utilized for assessment of capsule properties will be summarized. Where indicated, relationship between the capsule properties and encapsulated cell performance are presented, with a critical emphasis on capsule engineering efforts. Numerous reviews were published over the years on encapsulation technology [30], [31], [32], [33], [34], [35], and the reader is referred to these reviews for different perspectives.
Section snippets
Microencapsulation
Capsules in the 0.3–1.5 mm range have been traditionally refereed to as microcapsules in the cell encapsulation field. Their relatively small size (i.e., large surface area to volume ratio) is considered advantageous from a mass transport perspective. Microcapsules are typically more durable than macrocapsules and difficult to mechanically disrupt. Numerous microencapsulation techniques, fundamentally different in the nature of entrapment mechanism, were developed in independent labs. Besides
Macroencapsulation
Macrocapsules are generally much larger devices compared to microcapsules and typically possess a planar or cylindrical geometry, and a smaller surface-to-volume ratio. Living cells are physically isolated from directly interacting with host tissue by enclosure between two or more selectively permeable flat sheet membranes or within the lumen of a semipermeable hollow fiber. Like their smaller counterparts, these devices rely on the host animal’s own homeostatic mechanisms for the control of
Other encapsulation techniques
A variety of entrapment techniques have been developed to overcome various shortcomings of the micro- and macroencapsulation (Table 2). A silica sol–gel technique has been successfully used to entrap islets in ceramic membranes [192]. This is the first report on the encapsulation of mammalian cells in ceramic membranes. Although detailed investigation of encapsulated cell performance is yet to be performed, this is an intriguing choice of material and it may offer distinct advantages over
Assessment of capsule properties
A critical requirement for the development of a successful cell encapsulation technology is the close relationship between capsule properties and performance of encapsulated cells. The desired capsule properties will obviously depend on a particular application. Even though an expected performance criteria is satisfied (e.g., restoration of normoglycemia in a diabetic model), it is essential to probe the capsule properties that contribute to successful performance. This will facilitate a more
Future considerations
Since the early pioneering period, the technology of mammalian cell encapsulation has developed significantly. We reviewed several approaches to achieve cell encapsulation and possibly to enhance the efficiency and long-term utility of this technology. During recent years, there has been a large increase in the published studies using mammalian encapsulation methods as academic and industrial laboratories drive toward clinical implementation of the technology. In the field of
References (276)
Side-effects of a monoclonal antibody, Muromonab CD3/orthoclone OKT3: bibliographic review
Toxicology
(1995)- et al.
Risk for posttransplant diabetes mellitus with current immunosuppressive medications
Am. J. Kidney Dis.
(1999) - et al.
Functional recovery in hemiparkinsonian primates transplanted with polymer-encapsulated PC12 cells
Exp. Neurol.
(1994) - et al.
In vivo cultivation of tumor cells in hollow fibers
Life Sci.
(1995) Immunoprotection of therapeutic cell transplants by encapsulation
Trends Biotechnol.
(1996)Engineering challenges in cell encapsulation technology
Trends Biotechnol.
(1996)- et al.
Transplantation of encapsulated cells and tissues
Surgery
(1997) - et al.
A new multinozzle encapsulation/immobilization system to produce uniform beads or alginates
J. Biotechnol.
(1998) - et al.
Upscaling the production of microencapsulated pancreatic islets
Biomaterials
(1997) - et al.
Interaction of alginate and pectins with cationic polypeptides
Carbohydr. Res.
(1990)
Insulin independence in a type 1 diabetic patient after encapsulated islet transplantation
Lancet
Prolonged survival of transplanted islets of Langerhans encapsulated in a biocompatible membrane
Biochim. Biophys. Acta
Effect of the alginate composition on the biocompatibility of alginate–polylysine microcapsules
Biomaterials
Alginate polycation microcapsules. I. Interaction between alginate and polycation
Biomaterials
Alginate polycation microcapsules. II. Some functional properties
Biomaterials
Microcapsules through polymer complexation. I. By complex coacervation of polymers containing a high charge density
Biomaterials
Microcapsules through polymer complexation. II. By complex coacervation of polymers containing a low charge density
Biomaterials
Microcapsules through polymer complexation. III. Encapsulation and culture of human Burkitt lymphoma cells in vitro
Biomaterials
A newly developed three-layer agarose microcapsules for promising biohybrid artificial pancreas: rat to mouse xenotransplantation
Cell Transplant.
HEMA/MMA microcapsule implants in hemiparkinsonian rat brain: biocompatibility assessment using [3H]PK11195 as a marker for gliosis
Biomaterials
Microencapsulation of isolated pituitary cells by polyacrylamide microlatex coagulation on agarose beads
Biomaterials
A critical review of immunosuppressive regimens
Transplant. Proc.
Secretion of erythropoietin from microencapsulated rat kidney cells
Int. J. Artif. Organs
Delivery of recombinant gene products with microencapsulated cells in vivo
Hum. Gene Ther.
Expression of human factor IX by microencapsulated recombinant fibroblasts
Hum. Gene Ther.
Tissue engineering of a bioartificial kidney
Biotechnol. Bioeng.
Hybrid artificial cells: microencapsulation of living cells
Trans. ASAIO
A bioartificial parathyroid
Trans. ASAIO
Microencapsulated islets as bioartificial endocrine pancreas
Science
Encapsulation of OKT3 cells in hollow fibers
Trans. ASAIO
Cell growth and hemoglobin synthesis in murine erythroleukemic cells propagated in high density microcapsule culture
In Vitro Cell. Dev. Biol.
Entrapment of animal cells for the production of biomolecules such as monoclonal antibodies
Dev. Biol. Stand.
A method for obtaining monodispersed cells from isolated porcine islets of Langerhans
Int. J. Artif. Org.
Culture and growth characteristics of chondrocytes encapsulated in alginate beads
Connect. Tissue Res.
Cartilage formation by fetal rat chondrocytes cultured in alginate beads: a proposed model for investigating tissue–biomaterial interactions
J. Biomed. Mater. Res.
Encapsulated multicellular spheroids of rat hepatocytes produce albumin and urea in a spouted bed circulating culture system
Artif. Organs
Evaluation of angiogenic inhibitors with an in vivo quantitative angiogenesis method using agarose microencapsulation and mouse hemoglobin enzyme-linked immunosorbent assay
Jpn. J. Cancer Res.
A quantitative in vivo method of analyzing human tumor-induced angiogenesis in mice using agarose microencapsulation and hemoglobin enzyme-linked immunosorbent assay
Jpn. J. Cancer Res.
Feasibility of cellular microencapsulation technology for evaluation of anti-human immunodeficiency virus drugs in vivo
J. Natl. Cancer Inst.
Microencapsulation of bovine spermatozoa for use in artificial insemination: a review
Reprod. Fertil. Dev.
The potential impact of sperm encapsulation technology on the importance of timing of artificial insemination: a perspective in the light of published work
Reprod. Fertil. Dev.
Forty years of control of the oestrous cycle in ruminants: progress made, unresolved problems and the potential impact of sperm encapsulation technology
Reprod. Fertil. Dev.
Clonogenic cytotoxicity testing by microdrop encapsulation
Nature
Mathematical modeling of immobilized animal cell growth
Artif. Cells Blood Subs. Immob. Biotechnol.
Chitosan as a matrix for mammalian cell encapsulation
Biomaterials
Morphological assessment of hepatoma cells (HepG2) microencapsulated in a HEMA–MMA copolymer with and without Matrigel
J. Biomed. Mater. Res.
Bioengineering in development of the hybrid artificial pancreas
J. Biomech. Eng.
Microencapsulation of live animal cells using polyacrylates
Adv. Polym. Sci.
Encapsulated cells as therapy
Sci. Am.
Scale-up of gel bead and microcapsule production in cell immobilization
Cited by (0)
- ☆
All authors contributed equally to this work. Correspondence can be directed to any one of the authors.
- 1
Tel.: +1-780-4920-988; fax: +1-780-4928-259.
- 2
Tel.: +31-50-3632-792; fax: +31-50-3632-796.
- 3
Tel.: +1-801-5818-873; fax: +1-801-5855-151.