Engineering Biological Tissues from the Bottom Up
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HIGHLIGHT www.rsc.org/softmatter | Soft Matter Modular tissue engineering: engineering biological tissues from the bottom up Jason W. Nicholab and Ali Khademhosseini*ab DOI: 10.1039/b814285h Tissue engineering creates biological tissues that aim to improve the function of diseased or damaged tissues. To enhance the function of engineered tissues there is a need to generate structures that mimic the intricate architecture and complexity of native organs and tissues. With the desire to create more complex tissues with features such as developed and functional microvasculature, cell binding motifs and tissue specific morphology, tissue engineering techniques are beginning to focus on building modular microtissues with repeated functional units. The emerging field known as modular tissue engineering focuses on fabricating tissue building blocks with specific microarchitectural features and using these modular units to engineer biological tissues from the bottom up. In this review we will examine the promise and shortcomings of ‘‘bottom-up’’ approaches to creating engineered biological tissues. Specifically, we will survey the current techniques for controlling cell aggregation, proliferation and extracellular matrix deposition, as well as approaches to generating shape-controlled tissue modules. We will then highlight techniques utilized to create macroscale engineered biological tissues from modular microscale units. 1. Introduction congenitally defected tissues. The field of Traditional tissue engineering strate- tissue engineering has emerged to fill the gies employ a ‘‘top-down’’ approach, in There is a substantial unmet demand for void where neither native physiology which cells are seeded on a biodegradable tissues to repair injured, degenerated or nor purely artificial implantable materials polymeric scaffold, such as poly(glycolic can sufficiently replace or repair these acid) (PGA)7–10 (Fig. 1). In top-down 1 aHarvard-MIT Division of Health Sciences and damaged tissues. While tissues such as approaches, the cells are expected to Technology, Massachusetts Institute of bone2 or skin3,4 can effectively repair populate the scaffold and create the Technology, CambridgeMA 02139 a small injury given sufficient time, many appropriate extracellular matrix (ECM) b Center for Biomedical Engineering, tissues such as myocardium5 and carti- and microarchitecture often with the Department of Medicine, Brigham and Women’s 6 7 8 Hospital, Harvard Medical School, lage do not regenerate properly without aid of perfusion, growth factors and/or CambridgeMA 02139 intervention. mechanical stimulation.9,11 However, Dr Jason W. Nichol is Dr Ali Khademhosseini is an a Research Fellow in Medicine Assistant Professor of Medicine at Harvard-MIT’s Division of and Health Sciences and Tech- Health Sciences and Tech- nology at Harvard-MIT’s Divi- nology. His research is in tissue sion of Health Sciences and engineering of cardiac and Technology and the Harvard vascular tissues, investigating Medical School. His research is the cell and tissue responses to based on developing micro- and mechanical stimuli. He is nanoscale technologies to currently developing biomate- control cellular behavior with rials with biomimetic mechanical particular emphasis on devel- properties combined with oping microscale biomaterials microscale techniques to control for tissue engineering. He has Jason W: Nichol tissue microarchitecture. Select Ali Khademhosseini published 1 edited book, 65 peer- awards include the Whitaker Foundation Predoctoral Fellowship reviewed papers, 19 book chapters, 100 abstracts, and 14 patent (1999–2004) and the American Heart Association Postdoctoral applications. Select awards include Technology Review Magazine’s Fellowship (2006–2008), Finalist in the ASME Bioengineering ‘‘Top Young Innovator’’ (TR35) award (2007), the BMW Division student paper competition at the MS (2000), and PhD Scientific Award (2007), the ACS Victor K. LaMer Award (2008) (2002, 2003) level, as well as the BMES Poster Award in 2005. and the IEEE-EMBS Early Career Award (2008). 1312 | Soft Matter, 2009, 5, 1312–1319 This journal is ª The Royal Society of Chemistry 2009 2. Control of cell aggregation Controlling cell aggregation is a key component of creating modular tissues with controlled microarchitecture. By restricting the geometry of cell aggrega- tion, engineers can create more controlled environments to study cell behaviors (Fig. 2, panels A–C).15,27–29 This control can be enacted in many ways, such as seeding cells in microwells30 or channels,31 micromolding cells in hydrogels16 or culturing cells in sheets.17,23 2.1 Micromolds as templates to control cell aggregation Adaptation of similar techniques was used to create modular tissues by seeding cells in microwells to form spheroids 15 30 Fig. 1 Bottom-up and top-down approaches to tissue engineering. In the bottom-up approach made by primary or stem cells and there are multiple methods for creating modular tissues, which are then assembled into engineered their own secreted extracellular matrix tissues with specific microarchitectural features. In the top-down approach, cells and biomaterial (ECM). Researchers then created larger, scaffolds are combined and cultured until the cells fill the support structure to create an engineered more intricate modular structures such as tissue. linear channels31 to form more tissue-like structures. One major advantage of this despite advances in surface patterning12,13 for transplantation in diabetic rats.26 technique is the ability to create modular or use of more biomimetic scaffolding, Although these studies were not tissues of specific shape and geometry such as decellularized ECM templates,14 conceived as modular tissue engineering, where cells have created their own top-down approaches often have diffi- each cell-containing microcapsule microarchitecture through ECM remod- culty recreating the intricate microstruc- behaves as a tissue module performing eling and cell migration/aggregation. tural features of tissues. a particular function. Packing the However, one major limitation of this Modular tissue engineering aims to microcapsules created an artificial technique is that some cell types are address the challenge of recreating biological structure, however traditional unable to produce sufficient ECM, biomimetic structures by designing microencapsulation techniques did not migrate or form cell–cell junctions. structural features on the microscale to create tissue-like structures and lacked Researchers in the Morgan group build modular tissues that can be used as the microvasculature and architecture demonstrated that cell aggregation could building blocks to create larger tissues of native tissues. Current techniques be controlled to not only form spher- (Fig. 1). These modules can be created in for creating modular tissues draw from oids,32 but also individual and connected a number of ways, such as through research in microencapsulation and tori15 (Fig. 2C). The resulting connected self-assembled aggregation,15 micro- microfabrication as well as traditional cell tori yielded honeycomb structures, fabrication of cell-laden hydrogels,16 and tissue culture procedures. By creating demonstrating the potential for creating creation of cell sheets17 or direct printing modular tissues with more physiological larger modular tissues of defined geom- of tissues.18 Once created, these modules microarchitectural features, bottom-up etry using cells cultured in replica molds. can be assembled into larger tissues tissue engineering aims to provide more through a number of methods such as guidance on the cellular level to direct 2.2 Sheet-based tissue modules random packing,19–22 stacking of layers17,23 tissue morphogenesis. or directed assembly.24 There is a strong The following review will highlight the Another technique for creating modular biological basis for this bottom-up current techniques for creating modular tissues manipulates the cell culture envi- approach as many tissues are comprised of engineered tissues using bottom-up tissue ronment to create modular cell sheets. repeating functional units, such as the engineering principles. We will describe Through optimizing the conditions lobule in the liver.25 By mimicking approaches to engineering modular leading to cell proliferation, and ECM native microstructural functional units, tissues by classifying the techniques that production in cell culture, researchers bottom-up approaches aim to create more utilize only cells and cell-produced mate- have successfully created large cell sheet biomimetic engineered tissues. rials as well as approaches using cell/ modules. By stacking these cell/ECM The historical roots of the modular biomaterial combinations. Subsequently, sheet modules and allowing the modules approach trace back to microencapsula- we will highlight the approaches used to to remodel and interconnect over time, tion techniques, such as encapsulating direct the formation of larger engineered researchers have been able to create pancreatic islet cells in alginate gels tissues from microscale building blocks. tissues with robust mechanical properties This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5, 1312–1319 | 1313 3. Techniques for assembling engineered tissue building blocks One of the major challenges of bottom-up tissue engineering is to assemble modular tissues with specific microarchitectures into macroscale biomimetic engineered tissues. The challenge is to retain the microarchitecture and