# Paper and Fiber-Based Bio-Diagnostic Platforms: Current Challenges and Future Needs
## 2 Major Challenges and Opportunities
### Chemical Characteristics of the Paper/Fiber Materials and the Need for Surface Modification  
A variety of paper types have been used, or integrated into, a wide range of analytical designs to produce bio-sensing platforms (Tables 1 and 2). Porous cellulose is perhaps the most widely used commercial material in a majority of these designs (Table 1). Although the chemical structure of cellulose offers an abundance of hydroxyl (-OH) functional groups, these moieties are not highly reactive towards biomolecular entities. Consequently, in most cases, surface modification is required to modulate the functional nature of the paper materials [2; 4]. Strategic alterations of the surface could also result in the enhanced control of fluidic flow, improved color uniformity (in the case of colorimetric detection), enhanced chemical stability of the surface, the fabrication of microfluidic valves (in the case of \(\upmu\)PADs), as well as the generation of chemically reactive functionalities [2]. The presence of chemically reactive space through functionalization by carboxyl (-COOH) and/or amine (-NH\({}_{2}\)) groups, highly encourages the development of analyte-surface interactions through both physical and covalent immobilization. If active functional groups, such as -COOH and/or -NH\({}_{2}\), are optimally generated on the surface, there is a high chance for involving biomolecules in ionic attractions and hydrophobic interactions, as well as for hydrogen bonding opportunities [5; 94]. These forces and reactions are known to play prominent roles in protein immobilization, which would result in the subsequent attachment of the complementary biomolecules, thereby producing an improved final biological response generated from the bio-receptive surfaces. Apart from the intricacy of the modification strategies, the natural characteristics of the paper materials are frequently compromised during the treatment procedures [2]. Aggressive treatments might also contaminate papers due to exposure to the different chemicals and reagents (Table 3) [2]. In addition, papers are reported to lose their original shape subsequent to chemical treatment, having a tendency to curl or wrinkle after modifications are made. This may make their application difficult for specific types of bio-diagnostic platforms [3]. Furthermore, even if the properties of the paper are not affected by modification techniques, there might be no ability to carefully control the generation of active functional groups and their focused distribution on the paper surface [101; 102]. Optimal generation and distribution of the surface functional groups are crucial in biomolecule immobilization and subsequent detection of the analyte of interest. Overly functionalized surfaces tend to reject/hinder approaching biomolecules due to the steric repulsion, while an insufficient concentration of active functionalities may cause the biomolecules to fall on the surface and lose activity [91; 102; 103]. Therefore, a suitable bio-receptive platform would be credited for its substantial surface area and controlled wettability and porosity, and for an effective chemical design that can offer a protein-friendly surface with high occurrence and reproducibility for bimolecular interaction associated with binding stability. In that perspective, the development of customized paper/fiber materials with improved control over physical and chemical material properties would appear to be a highly desirable solution. Different characterization techniques are typically used to analyze the physical and the chemical properties of the newly developed materials. Water-in-air contact angle analysis can determine the wettability of the paper/fiber samples [5; 6; 104; 105]. The morphology of the papers/fibers can be recorded by scanning electron microscopy (SEM) and the fiber diameter range can be subsequently measured by analyzing SEM micrographs by using different software such as Image ] [5; 6; 37; 106].