Stem cell–materials interactions

Matthias P. Lutolf

Ecole Polytechnique Federale de Lausanne, Switzerland

Jason A. Burdick

University of Pennsylvania, USA

Stem cells hold enormous potential for widespread application in therapeutics, not only to treat patients for disease and trauma, but also as unique tools to generate human tissue models for drug development. As our population grows and ages, it will be increasingly important to accelerate the development of stem cell-based therapies in regenerative medicine. A major roadblock in moving stem cells into these applications is our poor control over their behavior both outside and inside the body. On the one hand, it remains very challenging to increase the number of adult stem cells obtained through tissues and biopsies and control their properties as they divide, ultimately providing a clinically acceptable cell source for implantation. On the other hand, biologists are struggling to generate mature and functional specialized (‘differentiated’) cell types from pluripotent stem cells (i.e. embryonic or induced pluripotent stem cells). What is clear though is that the behavior of stem cells after implantation or through in vitro culture can be guided by signals from their local environment, or niche; thus, it is crucial to better understand their interactions with their niche, all the way from isolation to therapy.

It is now well known that stem cells respond to a complex cocktail of biochemical and biophysical signals in their niche. It was historically believed that soluble cues from the environment were largely responsible for controlling stem cell behavior, but we now have begun to understand that there are many other important niche signals, including the mechanical properties of the microenvironment, engagement of integrins through adhesive ligands, or the presentation of bound matrix factors. These signals also translate to engineered culture environments, ranging from two-dimensional (2D) tissue culture plastic to highly complex 3-dimensional (3D) and dynamic microenvironments. In these cases, it is becoming clear that local properties, such as mechanics and adhesion, can be exploited to guide stem cell behavior (e.g. spreading, migration, self-renewal, differentiation, etc.). Regardless of whether the cell is an embryonic stem cell or one isolated from adult tissue, these signals are now a component of our growing biomaterials tool-box to control stem cell function and will surely be important in the success of new therapies.

The major goal of this themed issue is to provide selected highlights of our growing insight into the interface between stem cells and materials, including examples of new technology and further steps towards clinical application. This themed issue includes a collection of exciting reviews and original research articles by world leaders in stem cells and materials interactions, with expertise ranging from stem cell biology to materials science. Specific focus areas include: (i) cell culture substrates for the control of stem cell behavior; (ii) high-throughput screening approaches to optimize culture environments; (iii) 3-dimensional culture environments to guide stem cell behavior; (iv) patterned hydrogels for spatiotemporal control of stem cell fate; and (v) in vivo application of stem cells in tissue repair.

This themed issue regarding the interface between stem cells and materials includes four reviews that cover important aspects of our current understanding of this interface. Lee-Thedieck and Spatz (DOI: 10.1039/C4BM00128A) introduce an overview of our understanding of the biophysical properties of hematopoietic stem cell niches, particularly regarding physical properties and their role in controlling hematopoietic stem cell fate. Although these rare blood stem cells are already routinely used in therapies, there is still much to be learned regarding their regulation outside of the body. Since the expansion of these stem cells would be therapeutically advantageous, a better understanding of their regulation would greatly impact clinical translation. Next, Dalby and Turner (DOI: 10.1039/C4BM00155A) provide an overview of stem cell niches including intestinal and bone marrow niches, with a particular focus on the role of nanotopography in guiding stem cell behavior. They reflect on the mechanisms of how stem cells respond to topography, particularly related to stem cell differentiation and understanding how biomimetic culture environments can mimic the complexity of the stem cell niche.

Continuing with reviews in this important field, Ito and colleagues (DOI: 10.1039/C4BM00126E) provide insight into stem cell culture substrates that are used for self-renewal and differentiation. Building from the complexity of stem cell microenvironments, many researchers try to incorporate key niche signals such as proteins into cell culture to enhance adhesion and signaling. A growing area is the use of substrates that are derived directly from cells to form bioactive surfaces for stem cells to grow on. Building on this complexity, Putnam (DOI: 10.1039/C4BM00200H) provides a final review addressing the role that tissue vasculature plays in guiding cell fate. The vasculature provides a range of signals to the local environment, including stem cells, and if better understood can be used for applications in biomaterial design and regenerative medicine.

Engineered substrates are very important with respect to their interaction with and control over stem cell behavior, particularly in stem cell expansion. Cooper-White and Li (DOI: 10.1039/C4BM00109E) describe the modification of surfaces with self-assembled block copolymers, where bioactive groups (e.g. RGD or IKVAV) could be added sequentially with click chemistry. This approach provided significant control over adhesive group presentation and spacing, which influenced stem cell adhesion, cytoskeletal organization and spreading. The work highlights the importance of controlling ligand presentation in modulating stem cell behavior on 2D films. Moving towards more 3D structures, Hsu and colleagues (DOI: 10.1039/C4BM00053F) describe the culture of mesenchymal stem cells as spheroids by promoting cell–cell interactions through various substrates. They illustrate that the substrate chemistry, particularly if it contains natural polymers such as chitosan and hyaluronic acid, influences spheroid cell migration, chemokine expression, and ultimately cartilage repair. This is critically important as it provides evidence that the chemistry of an ex vivo culture system influences tissue repair outcomes.

To avoid the generally linear and iterative process that is used to develop new biomaterials to interface with stem cells, high-throughput screening is becoming more prevalent in understanding stem cell interactions with their environment as well as to identify new chemically defined biomaterials for stem cell culture. Alexander and colleagues (DOI: 10.1039/C4BM00054D) investigated the interaction of pluripotent stem cells with a great number of polymer formulations using micro-arrays. They processed a large variation of monomers to produce a diverse range of chemicals to identify expansion environments that would not have been predicted without this screening approach. Bradley, de Sousa and colleagues (DOI: 10.1039/C4BM00112E) build from this concept to identify culture substrates that support mesenchymal stem cell growth, as well as maintain an undifferentiated phenotype. Two polymers were identified that met these criteria and have the potential for chemically defined stem cell culture systems.

Moving from 2D culture substrates to 3D environments introduces added complexity into biomaterial systems. Huck, Ma and colleagues (DOI: 10.1039/C4BM00104D) used elegant droplet microfluidics to fabricate microbeads to entrap mesenchymal stem cells in controlled environments. These ‘microniches’ contained hyaluronic acid and fibrinogen, permitted viable cell culture for up to 2 weeks, and could be assessed through staining and imaging to monitor the entrapped cell behavior. Overall, the entrapped cells exhibited heterogeneity, despite the control over the material microenvironment. Heilshorn and colleagues (DOI: 10.1039/C4BM00142G) explored the soluble factor presentation within 3D self-assembled polypeptide hydrogels by entrapping growth-factor containing alginate microgels with adipose-derived stromal cells. Sequential delivery of the growth factors altered stromal cell lipid accumulation, illustrating the importance of both the matrix and the soluble environment in stem cell culture systems. As another example of 3D environments, Levenberg and colleagues (DOI: 10.1039/C4BM00304G) used the culture of endothelial cells on macroporous and biodegradable scaffolds to increase the population of pancreatic progenitors from human embryonic stem cells. After implantation, these constructs maintained blood glucose at normal levels, whereas non-treated animals became diabetic.

To increase the spatial complexity of biomaterials for stem cell culture, a variety of patterning techniques have been developed. Kloxin and colleagues (DOI: 10.1039/C4BM00187G) used thiol–ene chemistry to form hydrogels based on poly(ethylene glycol) and then further patterned them with biochemical groups through orthogonal reactions. Mesenchymal stem cells could be entrapped within the hydrogels, due to the non-toxic chemistry and reactions used. In another approach to patterning hydrogels, Lutolf and colleagues (DOI: 10.1039/C4BM00262H) developed a technique using caging chemistry. Light was used to control the crosslink density through uncaging of reactive groups, including with spatially defined resolution. Stiffness patterns and gradients influenced the migration of the mesenchymal stem cells. In both examples, light was important to spatially control the reactions, introducing the desired complexity.

As an example towards clinical application, Sakiyama-Elbert and colleagues (DOI: 10.1039/C4BM00106K) delivered embryonic stem cell derived neural cells into a spinal cord injury using fibrin gels. Importantly, the cells were enriched to remove undifferentiated pluripotent cells that may be harmful after implantation. The implanted cells were positive for markers of neurons, oligodendrocytes and astrocytes after implantation.

In all of these examples, the biomaterial played an important role in the success of the stem cell behavior, whether it was from unique culture substrates or to provide diverse and sometimes spatially controlled signals within the 3D niche. As the field moves forward, biomaterials will continue to play an important role in the generation of cell types with well-controlled behavior, as well as in the development of 3D tissue constructs that mimic the complexity of tissues.

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