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Introduction

The combination of polymers with nanostructured silicon scaffolds, in particular porous Si (PSi), into a single composite system opens vast opportunities for developing advanced functional materials. These composites display unique properties that are culminated by the characteristics of each building block, to allow the design of highly tunable nanomaterials. Over the past decade, various PSi-polymer composites were introduced and their application as sensors (chapter “Porous Silicon Optical Biosensors”), actuators, optical devices (e.g., chapter “Porous Silicon Optical Waveguides”), drug delivery systems (chapter “Drug Delivery with Porous Silicon”), and tissue-engineered scaffolds (chapter “Porous Silicon and Tissue Engineering Scaffolds”) was demonstrated.

In this chapter, we will describe the basic considerations in designing functional PSi-polymer composites, synthesis strategies, and emerging applications of these nanomaterials. Future prospects and challenges in both fabrication and implementation of PSi-polymer-based devices will be discussed.

Design of PSi-Polymer Composites

Porous Si-polymer composites may be designed in diverse configurations. Figure 1 schematically illustrates the most common structures: PSi infiltrated with a polymer, polymer-coated PSi, polymer-capped PSi, released PSi film supported by a polymer, PSi particles encapsulated by a polymer, and composite microparticles. Each of these structures possesses different properties, which can be further refined by a proper choice of the polymer constituent and the PSi nanostructure.

Fig. 1
figure 1

Common structures of PSi-polymer composites. Insets in A illustrate interfacial chemistry where the polymer is not attached to PSi (right) and is covalently attached to the PSi surface through different linkers. Schematics are not drawn to scale

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The simplest composite structure is that of a polymer-infiltrated PSi substrate (Fig. 1a), wherein the polymer fills the entire porous volume (Segal et al. 2007; Massad-Ivanir et al. 2010; Perelman et al. 2010; Massad-Ivanir et al. 2012a; Krepker and Segal 2013). The polymer is confined within the nanoscale pores, and its interaction with the pore wall can be enhanced by covalent attachment (Massad-Ivanir et al. 2012a; Bonanno and Delouise 2010; Bonanno and Segal 2011; Sciacca et al. 2011). To some extent, polymer-coated PSi (Fig. 1b) has a similar design, where the polymer only forms a uniform layer onto the pore walls, resulting in an open porous structure (Segal et al. 2009; McInnes et al. 2009). Polymer-capped PSi is a more sophisticated architecture (Fig. 1c), in which the polymer only forms a blocking layer on top of the PSi, leaving the greater fraction of the porous volume unoccupied (Wu and Sailor 2009). The fabrication of these composites is more challenging, as the degree of polymer penetration into the pores needs to be precisely controlled. Figure 1d presents a polymeric replica from PSi. These replicas are usually prepared from polymer-infiltrated PSi by selective removal of the Si scaffold (Li et al. 2003; Park et al. 2007). The previously described designs make use of intact PSi substrates. However, the following PSi-polymer composite configurations require mechanical processing of the PSi. A freestanding PSi film supported by polymer is presented in Fig. 1e. In this case, separation of the porous layer from the bulk Si is typically achieved by electropolishing (DeLouise et al. 2005). Further fracturing of these porous films allows us to fabricate particulate composites (McInnes et al. 2012). The resulting PSi micro-/nanoparticles can be embedded in a polymer matrix (Fig. 1f) or encapsulated individually by a polymer layer (Fig. 1g). The latter case is more synthetically challenging.

The diversity in the design of these composites highlights the possibility to select a suitable configuration for tailoring specific mechanical, chemical, and optical properties for a desired function. Rational selection of the polymer component, PSi substrate nanostructure, and fabrication strategy are of utmost importance in tuning the behavior of the resulting composite.

Fabrication Methods of PSi-Polymer Composites

There are many synthetic approaches for integrating polymers with PSi. Figure 2 summarizes the main methods that are practiced for the fabrication of PSi-polymer composites. In general, these techniques can be divided into two main categories. The first is incorporating a preformed polymer with the Si scaffold. The second involves in situ polymerization of monomers within/on the PSi. Herein, we will focus only on the main techniques for the fabrication of PSi-polymer composites.

Fig. 2
figure 2

Common practiced methods for the fabrication of PSi-polymer composites

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Incorporation of a Preformed Polymer with PSi

Solution casting (Fig. 2a) is probably the simplest route for fabricating PSi-polymer composites, and thus, it is widely implemented (Wu and Sailor 2009; Park et al. 2007; McInnes et al. 2012; Gao et al. 2008; Orosco et al. 2006; Shang et al. 2011; De Stefano et al. 2009; Schwartz et al. 2006; Kim et al. 2008; Koh et al. 2008; Sychev et al. 2009; Whitehead et al. 2008; Li et al. 2005). In this case, a polymeric solution is prepared and cast onto the PSi. This procedure is usually followed by a subsequent spin-coating step to remove excess solution and to distribute the polymer evenly on the surface of PSi (Gao et al. 2008; Shang et al. 2011; Schwartz et al. 2006). The extent of polymer solution penetration into pores plays a vital role in determining the final structure of the composite. Generally, polymer infiltration into the porous scaffold depends on several key parameters: the molecular weight (MW) of the polymer, the solution viscosity and its surface tension, pore dimensions and morphology, and the pore wall surface chemistry. In addition, the small size of pores can trap air or other gases, inhibiting the penetration of the polymer solution into the pores. In this case, degassing under vacuum and PSi conditioning with the solvent is recommended. Overall, this technique is relatively straightforward, as it does not involve complex synthetic steps and specialized experimental setups. Moreover, the use of a preformed polymer, which its characteristics are already defined (e.g., MW and chain configuration), offers significant advantages in terms of the properties and behavior of the final composite. Capping layer on PSi film (Wu and Sailor 2009; Gao et al. 2008; Orosco et al. 2006; Shang et al. 2011), PSi infiltrated with polymers (McInnes et al. 2012; De Stefano et al. 2009; Schwartz et al. 2006), polymer replicas of PSi templates (Li et al. 2003; Park et al. 2007; Kim et al. 2008), and a freestanding PSi film supported by a polymer (Koh et al. 2008; Sychev et al. 2009) are the most common designs of PSi-polymer composites prepared by solution or melt casting.

Another simple technique for preparation of PSi-polymer composites is by dispersing PSi particles within a molten polymer or a polymeric solution (Fig. 2c). Further processing of these dispersions is required in order to form coatings (Svrcek et al. 2009), monoliths (McInnes et al. 2012; Mukherjee et al. 2006), and fibers (Fan et al. 2011; Kashanian et al. 2010).

Fibrous PSi-polymer systems can be fabricated by electrospinning (Fig. 2d) which is an established method for the production of large area networks of thin flexible fibers. In this case, the PSi-polymer dispersion or melt is squeezed through a nozzle, to which a strong electrical field is applied. The applied voltage causes a cone-shaped deformation of the drop of polymer solution/melt and a jet is formed. As the spurt makes its way to the counter electrode, the melt solidifies or the solvent evaporates and a polymer fiber is formed (Greiner and Wendorff 2007). The resulting properties of electro-spun fibers are controlled by the process parameters, e.g., electrical conductivity, electrode separation and geometry, temperature, concentration, and the polymer characteristics. Coffer et al. (Fan et al. 2011; Kashanian et al. 2010; Fan et al. 2009) have applied this technique to produce electro-spun fibers of PSi particles encapsulated within a polycaprolactone matrix.

Grafting involves covalent attachment of the polymer to a surface. Grafting provides a versatile tool for surface modification and functionalization in a highly controllable manner. Two main grafting categories can be identified. The first is termed “grafting to,” in which a preformed polymer is attached to the surface. In the second approach “grafting from,” polymerization is initiated from the substrate surface by the attachment of initiating groups (Minko 2008). The latter method will be discussed in the following section, dealing with polymerization techniques.

In the “grafting to” method (Fig. 2b), end-functionalized polymer chains are reacted with complementary functional groups located on the PSi surface to form tethered polymer chains (Minko 2008). The versatile chemistry of Si/SiO2 allows functionalization of the PSi surface with a wide repertoire of reactive groups (Buriak 2002; Ciampi et al. 2008; Kilian et al. 2009a; Alvarez et al. 2009; Jarvis et al. 2012). Some of the most common PSi functionalization routes are presented in Fig. 3.

Fig. 3
figure 3

Commonly practiced surface chemistries for the functionalization of PSi. These end groups can be reacted with a wide variety of polymers

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The advantage of the “grafting to” method is that the end-functionalized polymers with a predetermined configuration (e.g., MW and functional groups) are employed for grafting, and, as a result, well-defined layers can be readily obtained. It should be mentioned that a disadvantage of this method is the limited grafting density that can be achieved (Zdyrko et al. 2006). Thus, this technique was implemented for grafting poly(n-isopropylamide) (polyNIPAM) (Segal et al. 2009), chitosan (Sciacca et al. 2011), and gelatin (Kilian et al. 2009b). “Grafting to” is typically used for fabricating polymer-coated PSi surfaces and nanostructures (see Fig. 1b).

Polymerization Within PSi

In many cases, when using high-MW polymers, these macromolecules are size excluded from the nanoscale pores, or their efficient infiltration into the PSi nanostructure is impaired. Thus, to circumvent these issues, in situ polymerization of low-MW monomers (or oligomers) within the PSi scaffold can be applied (Segal et al. 2007; Massad-Ivanir et al. 2010; Perelman et al. 2010; Krepker and Segal 2013; Massad-Ivanir et al. 2012b; El-Zohary et al. 2013). The PSi nanostructure is commonly filled with a prepolymer solution, which may contain a composition of a solvent, monomers, cross-linking agents, initiators, and catalysts. When the polymerization reaction is initiated, the polymer is formed inside the pores of PSi, resulting in a uniform and high pore filling. In certain cases, polymerization can be initiated from the PSi surface, generally termed as a “grafting from” technique, or the growing polymer chains can be tethered to the pore walls.

There are many different polymerization routes that have been employed for the fabrication of PSi-polymer composites, including free-radical polymerization (FRP), photopolymerization (Segal et al. 2007; Massad-Ivanir et al. 2010; Perelman et al. 2010; Krepker and Segal 2013; Massad-Ivanir et al. 2012b), atom transfer radical polymerization (ATRP) (Vasani et al. 2011; Yang and Choi 2010; Pace et al. 2013), ring-opening polymerization (McInnes et al. 2006, 2009, 2012; Yoon et al. 2003), and electro-polymerization (Belhousse et al. 2010; Jin et al. 2009; Chiboub et al. 2010; Fukami et al. 2009; Harraz et al. 2008; Betty 2009; Urbach et al. 2007a, b; Nahor et al. 2011; Badeva et al. 2012; Dian et al. 2013). Polymerization method has a profound effect on the resulting PSi-polymer composite structure and its properties. For example, FRP typically results in time invariant degrees of polymerization and a high polydispersity index (a wide MW distribution). ATRP, on the other hand, allows for achieving a controllable molecular weight and low polydispersity, as the polymer chains preserve their ability to grow for a long time and their degree of termination or chain transfer is very low (Braunecker and Matyjaszewski 2007).

When “grafting from” approach is applied for polymer synthesis, an appropriate initiator is commonly immobilized onto the pore walls followed by polymer chains growth via different possible mechanisms (Jarvis et al. 2012; Vasani et al. 2011; Xu et al. 2004; Chen et al. 2009). The main advantage of the “grafting from” methods is the possibility to form polymer layers of high density in comparison to the “grafting to” approach.

It should be emphasized that in situ polymerization within nanostructures is a complex process in which nano-confinement conditions may affect the polymerization kinetics and the resulting properties of the polymer (Massad-Ivanir et al. 2012a; Alcoutlabi and Gregory 2005; Keten et al. 2010; Kruk et al. 2008; Liang et al. 2000; Uemura et al. 2010; Gorman et al. 2008). Recent studies have shown that the imprisonment of hydrogels in PSi nanoscale pores induces significant changes in the confined polymer properties, e.g., volume phase transition (VPT) kinetics, compared to those observed for the bulk “free” polymers (Segal et al. 2007; Massad-Ivanir et al. 2010; Perelman et al. 2010; Bonanno and Segal 2011). Moreover, significant differences between the thermal degradation behaviors of the confined polymers, poly(acrylamide), and polyNIPAM and neat polymer and thin polymer films deposited onto planar Si surfaces were observed. The confined polymers have inferior thermal stability than the neat polymers. These findings indicate that the in situ polymerization and the polymer confinement conditions have a profound effect on the nanostructure and resulting behavior of the polymeric phase (Massad-Ivanir et al. 2012a).

We expect that investigation and characterization of the properties of different polymeric systems confined within nanostructured porous Si hosts will allow to finely tune the polymer properties by controlling the confinement conditions and interfacial interactions between the polymer and the host material. This will expand the possibility for rational design of new PSi-polymer nanomaterials with tailored properties and functions.

Applications

As PSi-polymer composites exhibit unique properties that are culminated by the characteristics of each building block, they can be rationally designed to display highly tunable properties, e.g., mechanical, chemical, optical, and electrical. Over the past decade, these attractive nanocomposites have been studied as platforms for designing different devices. Applications of these composites range from drug delivery systems, sensors, and actuators to optoelectronics and photovoltaics.

A recent review (Bonanno and Segal 2011) provides an updated overview on the biomedical applications of these materials, highlighting the construction of smart drug delivery systems (Wu and Sailor 2009; Vasani et al. 2011; Godin et al. 2011), improved label-free optical biosensors (Massad-Ivanir et al. 2010; Massad-Ivanir et al. 2012b; Holthausen et al. 2012), or sensors (Bonanno and Delouise 2010) for “point-of-care” applications such as temperature sensors to monitor wound healing (Pace et al. 2012), lab-on-chip systems (Chen et al. 2009), and tissue engineering scaffolds (Coffer et al. 2005). In addition to the vast biological applications, PSi-polymer composites are being investigated as chemical sensors for different targets (Belhousse et al. 2010; Jin et al. 2009; Wang et al. 2012; Levitsky et al. 2007; Pang-Leen and Levitsky 2011), optoelectronics devices, photovoltaics (Svrcek et al. 2009; Badeva et al. 2012; Halliday et al. 1996; Mishra et al. 2008; Nguyen et al. 2003a, b; Gongalsky et al. 2012), and energy storage (Ge et al. 2012). Thus, the versatility of polymers in combination with the unique properties of PSi offers a wealth of opportunities for the design of new functional materials for a range of applications. We have yet to see the widespread translation of these composite-based devices into commercial application, and only the future will reveal the true impact of these materials.